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Manual 8 – Termite attack
PROJECT NUMBER: PN07.1052
August 2007
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Manual No. 8: Termite Attack 1
USP2007/045
MANUAL NO. 8
Termite Attack
R.H. Leicester, C-H. Wang, and M.N. Nguyen
April 2008
This report has been prepared for Forest & Wood Products Australia (FWPA).
Please address all enquiries to:
Urban Systems Program
CSIRO Sustainable Ecosystems
P.O. Box 56, Highett, Victoria 3190
Manual No. 8: Termite Attack 2
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Acknowledgments
This Manual is one of a series of Manuals that have been produced as part of a project titled
‗Design for Durability‘. The authors are deeply indebted to the Forest and Wood Products
Australia for their funding and collaboration in this project over the past 10 years. The authors
would especially like to thank Colin MacKenzie (Timber Queensland) for the major role that
he has played in managing and guiding this project to completion. Thanks are also due to Dr.
Laurie Cookson (CSIRO), Dr. John French (Ecospan Consulting Services), Mr. Doug Howick (
AEPMA), Mr. Jim Creffield (CSIRO), Dr. Don Ewart (Granitgard), Mr. Nicholas Cooper
(Systems Pest Management), and Dr. Berhan Ahmed (University of Melbourne) for contributing
extensive data and expertise to the development of the models described in this Manual.
Finally our thanks go to Greg Foliente, Craig Seath, Sandra Roberts and numerous other
CSIRO personnel for their assistance and contribution to this project
© 2008 CSIRO
To the extent permitted by law, all rights are reserved and no part of this publication covered by
copyright may be reproduced or copied in any form without acknowledgment of this reference source.
Manual No. 8: Termite Attack 3
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Contents
EXECUTIVE SUMMARY ........................................................................................................ 6
1 INTRODUCTION .............................................................................................................. 7
1.1 Project Background ........................................................................................................ 7
2 TERMITE TALLY ............................................................................................................ 8
2.1 Zonation ......................................................................................................................... 8 2.2 Analysis of Termite Tally Based on Temperature Zonation .......................................... 9
2.2.1 Effect of Age of House ................................................................................................ 9
2.2.2 Effect of Temperature and Rainfall on the Termite Hazard..................................... 12 2.2.3 Effects of Frame and Floor Types on Termite Incidence ......................................... 15
2.3 Analysis of Termite Tally Data Based on Agro-Ecological Zonation ......................... 17
2.3.1 Zonation Procedure .................................................................................................. 17
2.3.2 Effect of Age of House .............................................................................................. 18 2.3.3 Effect of Frame and Floor Types on Termite Incidence .......................................... 23
2.4 Analysis of Termite Tally Data Based on Housing Clusters ....................................... 23 2.4.1 Zonation Procedure .................................................................................................. 23
2.5 Concluding comments .................................................................................................. 27
3 EXPERT OPINION MODEL .......................................................................................... 28 3.1 The Survey Questionnaire ............................................................................................ 28 3.2 Choice of Parameters ................................................................................................... 31 3.3 The Base Model ........................................................................................................... 38
4 THE PROBABILISTIC MODEL .................................................................................... 41
4.1 Introduction .................................................................................................................. 41
4.2 The Basic Probability Model ........................................................................................ 41 4.2.1 The probability Distributions ................................................................................... 41 4.2.2 The True Risk in the Past ......................................................................................... 42 4.2.3 True Risk in the Future ............................................................................................. 43 4.2.4 Apparent Risk in the Past ......................................................................................... 43 4.2.5 The Apparent Risk in the Future .............................................................................. 44 4.2.6 The Distribution Parameters .................................................................................... 44
Manual No. 8: Termite Attack 4
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4.3 The Practical Model ..................................................................................................... 46
5 COMPUTATION MODEL .............................................................................................. 49
5.1 Concept ......................................................................................................................... 49 5.2 Calibration of the Model .............................................................................................. 49 5.3 The Coefficient of Variation ........................................................................................ 52
6 RISK ASSESSMENT EQUATIONS .............................................................................. 54 6.1 Hazard Parameters ........................................................................................................ 54
6.2 Supplementary Hazard Parameters .............................................................................. 56 6.3 Expanded definitions of hazard parameters ................................................................. 57
6.3.1 Procedure for assessing the hazard h4 due to the quantity of wood occurring in a
garden and under a house .................................................................................................... 57 6.3.2 Definition of ground contact .................................................................................... 58
6.3.3 Hazard level h6 related to type of construction material ......................................... 58 6.3.4 Hazard h4 related to exposure of timber .................................................................. 59
6.4 Computational Procedure ............................................................................................. 59 6.4.1 Computing the mean time to attack mean(t) ............................................................ 59 6.4.2 Computed Risk and Hazard Score ........................................................................... 60
6.5 Computing Risk ............................................................................................................ 65
6.6 Acceptable Risk ............................................................................................................ 66 6.7 Risk Management ......................................................................................................... 67
6.7.1 Cost Assumptions ..................................................................................................... 67 6.7.2 Comparative Costs of Termite Protection Strategies ............................................... 67
6.8 Some Computed Examples .......................................................................................... 67
6.8.1 Applications to Risk Assessments ............................................................................. 67 6.8.2 Applications to Risk Management ............................................................................ 68
6.8.3 Comment ................................................................................................................... 69
7 APPLICATION FOR DESIGN GUIDE .......................................................................... 70
7.1 Procedure to compute risk ............................................................................................ 70 7.2 Hazard score components ............................................................................................. 70 7.3 The Hazard Score Total ................................................................................................ 72
7.3.1 Comment on Hazard Zone A (Tasmania) ................................................................. 72 7.4 Parameters for the risk equation ................................................................................... 73
7.5 Acceptable Risk ............................................................................................................ 73
8 APPLICATION FOR TIMBERLIFE .............................................................................. 75 8.1 Procedure to compute risk ............................................................................................ 75
8.2 Hazard score components ............................................................................................. 75 8.3 The Hazard Score Total ................................................................................................ 77
8.3.1 Comment on Hazard Zone A (Tasmania) ................................................................. 77 8.4 Parameters for Evaluating the Risk Equation .............................................................. 78
8.5 Risk Management procedure ........................................................................................ 78 8.5.1 Cost Components ...................................................................................................... 78 8.5.2 Effective Cost of Termite Protection Strategies ....................................................... 79
REFERENCES ......................................................................................................................... 80
Manual No. 8: Termite Attack 5
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Manual No. 8: Termite Attack 6
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Executive Summary
The purpose herein is to describe the development of a model to predict the
probability of attack of housing by termites. Such a model may be used to estimate
risk as part of an asset management strategy. There are three components to the model.
The first component is a survey by school students initiated by Dr John French and
analysed by Dr Laurie Cookson in 1999. This strategy provided statistical data on
some 5000 houses and will be referred to as the ―CSIRO Termite Tally‖ (Cookson and
Trajstman, 2002). Processed data from this tally is discussed in Section 2. The data
shows a strong effect of age of a house on the probability of attack. It also shows an
effect of the mean annual temperature on the probability of attack.
The second component is the development of a model of termite behaviour based on a
survey of the expert opinion of a limited number of six experts. This ―Expert Opinion‖
model uses a large number of parameters as input to provide a quantitative estimate of
the mean and variability of observed times of termite attack. This model is described
in Section 3.
The third important component of this work was the development of a probabilistic
model, described in Section 4. The form of the probability model is based on the data
obtained in the Termite Tally. It is completely defined by a single parameter, i.e. the
mean time to a termite attack. In this model an important distinction is made between
the observed or apparent attack rate and the true attack rate. In the application of the
model, it is assumed that a house occupant will be aware of only the most recent
history of termite attack on his house. The historical memory of the occupant in this
study is taken to be to be 20 years. This historical memory needs to be taken into
account when using field data on termite attack, such as the data from the CSIRO
Termite Tally mentioned above.
In Section 5, the data from both the Expert Opinion model and the probabilistic model
are combined to give a termite attack model that is suitable for practical use. This
model is calibrated with the data from the CSIRO Termite Tally.
In Section 6 the termite attack model is used to develop a simple hazard score system
that can be used to compute risk. With this capability, various risk management
strategies can be formulated and implemented quite simply. Some examples of these
are given. For example, Section 6.6 provides the conditions required to obtain a risk
that would be considered an ‗acceptable risk‘ in Australia; Section 6.7 gives a method
for computing the cost of implementing a termite management strategy, including the
costs that would be incurred if attack were to occur; Section 6.8.2 gives examples of
the costs associated with a variety of risk management strategies.
Manual No. 8: Termite Attack 7
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1 INTRODUCTION
Equation Section (Next)
1.1 Project Background
The purpose herein is to report on progress in the development of a model to predict the
probability of attack of housing by termites. Such a model may be used to estimate risk as part
of an asset management strategy, i.e. to anticipate the risks and costs associated with termite
attack and various mitigation programs. There are three components to the model.
The first is a survey by school students initiated by Dr John French and analysed by Dr
Laurie Cookson in 1999. This strategy provided statistical data on some 5000 houses and will
be referred to as the ―CSIRO Termite Tally‖ (Cookson and Trajstman, 2002). Processed data
from this tally is discussed in Section 2.
The second study of value is the model of termite behaviour based on an opinion survey
of a limited number of experts. It will be called the ―Expert Opinion model‖. This model
provides a quantitative estimate of the mean and variability of observed times of termite
attack.
The third important component of this work was the development of a probabilistic
model, described in Section 4.
The data from both the Termite Tally and the Expert Opinion model are then used to
calibrate the reliability model. In this model an important distinction is made between the
observed or apparent attack rate and the true attack rate. It is assumed that a house occupant
will be aware of only the most recent history of termite attack on his house. The historical
memory of the occupant in this study is assumed to be about 20 years.
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2 TERMITE TALLY
Equation Section (Next)
2.1 Zonation
The raw data from the Termite Tally has been analysed to provide information in terms of
probabilities. In order to do this it is necessary to first group houses in terms of a specified set
of parameters; then for each group, the probability of a termite incidence can be approximated
by noting the proportion of that group that has been attacked in the past. These probabilities
can then be related to other parameters that are averaged for the group.
In this report, the following three types of groupings have been used:
temperature zones
agro-ecological zones
housing clusters.
The agro-ecological zones are based on a previous study by Cookson (1999). The
housing clusters used correspond to locations where the Termite Tally shows more than 78
houses within a circle of 100 km diameter.
The data within the temperature and agro-ecological zones can be further subdivided
according to the age of the homes.
It is important for the reader to understand the notation used. The most important of
these is the notation used for probability, which may also be interpreted as risk. This notation
is as follows:
P(location, event, reliability type, t) = probability that an event at a given location has
occurred before time t. In this report the reliability type of the
probability figure will be assumed to have two possible values; one
will be a perfect value denoted as an exact value; the other will be an
imperfect estimate based on the observations of the building
occupant. The time t denotes the time in years after the construction
of the target house.
The subscripts indicating locations are as follows
house = target house for which the risk of termite attack is to be assessed,
Manual No. 8: Termite Attack 9
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garden = garden surrounding the target house,
suburb = suburb within which the target house is located.
The subscripts indicating events are as follows:
attack = probability that termites have attacked a house
nest = probability that a nest with a mature colony has been established
within the specified location
termite = probability that termites have occurred within the specified location
The subscripts used to define the type of probability figure are as follows
true = the true or exact probability that an event has occurred
obsv = the estimated probability that an event has occurred, based on
observations of by the occupant of a house.
2.2 Analysis of Termite Tally Based on Temperature Zonation
The continent is divided into three zones as follows:
Zone 1: Tmean < 18C
Zone 2: 18C Tmean < 25C
Zone 3: Tmean 25C
where Tmean denotes the mean annual temperature. A hazard map based on temperatures is
shown in Figure 2.1. The incidence of termites in Tasmania is taken to be zero.
Figure 2.1. Termite hazard map based on temperature zones.
2.2.1 Effect of Age of House
Figures 2.2, 2.3 and 2.4 show the effect of age of house on the possibility of observation of
termites both in the house and in the garden for the three primary temperature zones. Each
Manual No. 8: Termite Attack 10
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plotted data point is based on a data taken from the Termite Tally; the average sample size is
165 with a minimum value of 51.
There is obviously a very strong effect of age of house on the incidence of termite
attack on a house. In fact, it was found that in the data of the Termite Tally, the probability of
termite attack on a house is more strongly correlated with the age of the house than with any
other parameter. It is of interest to note that the fitted lines for Zones 1 and 2 are roughly
parallel to each other.
0 20 40 60 80 100 120
House age (years)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
P(h
ouse
, att
ack, obsv, t)
zone 1 (T
mean < 18 o C) zone 2 (T
mean = 18 - 25C zone 3 (T
mean > 25 o C)
Inside Termite Incidence (by Temperature Zones)
R 2 1 = 0.918
R 2 2 = 0.855
zone 1
zone 2
zone 3
Figure 2.2. Effect of house age on the apparent incidence of termite attack
(temperature zonation).
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0 20 40 60 80 100 120
House age (years)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7P
(ga
rde
n,
term
ite
, o
bsv,
t)
Outside Termite Incidence (by Temperature Zones)
zone 3
zone 2
zone 1
zone 1 (Tmean < 18oC)
zone 2 (Tmean = 18 - 25oC)
zone 3 (Tmean > 25oC)
Figure 2.3. Effect of house age on the apparent incidence of termites in the garden
(temperature zonation).
0 20 40 60 80 100 120
House age (years)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Termite Incidence (by Temperature Zones)
house house garden garden
zone 1
zone 1
zone 3
zone 3
P(h
ouse
, att
ack, obsv, t)
P(g
ard
en, te
rmite,
obsv, t)
zone 2 zone 2
Figure 2.4. Effect of house age on the apparent incidence of termites
(temperature zonation).
P(h
ou
se
, a
tta
ck,
ob
sv,
t) P
(ga
rde
n,
term
ite
, o
bsv,
t)
Manual No. 8: Termite Attack 12
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2.2.2 Effect of Temperature and Rainfall on the Termite Hazard
In the development of a hazard maps, it is of interest to examine whether temperature and
rainfall have a significant relationship to the incidence of termites to be found in gardens. To
do this the data of the Termite Tally was broken up into temperature-rainfall clusters as
shown in Table 2.1, the cluster boundaries being chosen so that the sample size within each
cluster is greater than 90. The data within each cluster was then averaged and plotted as
shown in Figures 2.5 & 2.6.
Figure 2.5 shows that there is a modest correlation between mean annual temperature
and probability of finding termites in the garden. Figure 2.6 shows that the addition of rainfall
consideration does not improve the accuracy of prediction; this is because, as illustrated in
Figure 2.7, the termite data has been chosen in locations for which there is a reasonable
correlation between rainfall and temperature. In Figure 2.8, data points related to rainfall have
been plotted, and it is seen that in fact there is very little relationship, if any, between rainfall
and the probability of finding termites in a garden.
Table 2.1. Temperature-rainfall divisions and sample size in each division
N=142
0
14
15
16
17
18
19
20
21
23
30 N=91
N=159
N=355
N=94 N=288
N=156 N=156 N=253 N=217
N=290 N=782 N=413
N=316
N=504
N=178
N=188
500 0 1000 1500 2000 2500 3000
Rainfall range (mm)
Tem
pera
ture
range
(C
)
Manual No. 8: Termite Attack 13
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12 16 20 24 28
Temperature (C)
0.1
0.2
0.3
0.4
0.5
0.6
0.7 P
(gard
en
, te
rmite
, o
bsv, t)
Termite Incidence by Temperature-Rain
R2 = 0.505
Figure2.5. Effect of temperature on the apparent incidence of termites in the garden
(temperature-rainfall zonation using all data).
0.1 0.2 0.3 0.4 0.5 0.6 0.7 Predicted probability of apparent termite occurrences
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Measu
red p
robab
ility o
f ap
pare
nt
term
ite o
ccurr
ence
Termite Incidence by Temperature-Rain
2 R2 = 0.514
Figure 2.6. Use of temperature and rainfall data to predict the apparent incidence of
termites in the garden (temperature-rainfall zonation using all data).
Manual No. 8: Termite Attack 14
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12 14 16 18 20 22 24 26 28 Mean annual temperature (C)
400
800
1200
1600
2000 M
ean
an
nua
l ra
infa
ll (m
m)
Termite Incidence by Temperature-Rain
R2 = 0.609
Figure 2.7. Relationship between temperature and rainfall (temperature-rainfall zonation
using all data).
400 600 800 1000 1200
Mean annual rainfall (mm)
P(g
ard
en,
term
ite,
obsv,
t )
Termite Incidence by Temperature-Rain
0.15
0.25
0.35
0.45
T = 18 - 20 C T = 16 - 18 C regression
Figure 2.8. Effect of rainfall on the apparent incidence of termites in the garden
(temperature-rainfall zonation using all data).
Manual No. 8: Termite Attack 15
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2.2.3 Effects of Frame and Floor Types on Termite Incidence
It is noted that while the incidence of internal attack is roughly similar within each of the
three temperature zones, the data in Figure 2.4 shows that the external hazard (as indicated by
the probability of finding termites in gardens) varies considerably with the temperature zone.
One reason for this may be that the type of construction, degree of inspection and extent of
barrier protection varies from zone to zone, the greater protective measures being taken in the
higher hazard zones.
In the following, the effects of construction frame types and floor types on inside
termite incidence are examined in each temperature zone. Three frame types and three floor
types are considered:
Frames: timber steel, and masonry;
Floors: timber, concrete, and combined timber and concrete.
Table 2.2 and Table 2.3 show the percentages of each frame type and floor type used in
each temperature zone. There is no obvious trend in the type of construction used within the 3
zones, except perhaps that for Zone 3, which is essentially Darwin, there is relatively less
timber construction than within Zones 1 and 2.
Table 2.2. Percentage of frame types in the temperature zones
Zone 1 Zone 2 Zone 3
timber 72 75 30
steel 4 6 31
Masonry 24 19 38
Table 2.3 Percentage of floor types in the temperature zones
Zone 1 Zone 2 Zone 3
timber 55 48 29
timber + concrete 9 6 2
concrete 36 46 69
The mean house ages for each of the temperature zone-construction type clusters are
given in Tables 2.4 and 2.5 for the case of frames and floors respectively. The incidence of
building termite attack is divided by these average house ages to provide an apparent average
rate of attack for each cluster, and results of this are shown in Figures 2.9 and 2.10; as may be
expected, the annual frequency of attack increases with the hazard zone classification.
Manual No. 8: Termite Attack 16
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Table 2.4. Mean house ages (years) of various frame types in the three
temperature zones
Zone Number Steel Masonry Timber
Zone 1 11.9 38.3 27.7
Zone 2 10.8 19.8 30.6
Zone 3 12.7 14.6 27.8
Table 2.5. Mean house ages (years) of various floor types in the three
temperature zones
Zone Number Timber Concrete Timber + Concrete
Zone 1 37.5 15.6 40.9
Zone 2 38.2 14.4 37.8
Zone 3 24.6 16 18
1 2 3 Temperature zones
0.000
0.005
0.010
0.015
Estim
ate
d a
vera
ged
ann
ual p
rob
ab
ility
of
term
ite a
ttack
steel masonry timber
Internal Termite Incidence for Various Frame Types
Figure 2.9. Effect of zone on the apparent termite for various frame types
(temperature zonation).
Manual No. 8: Termite Attack 17
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1 2 3 Temperature zones
0.000
0.005
0.010
0.015
Estim
ate
d a
vera
ged a
nnu
al pro
ba
bili
ty o
f te
rmite a
ttack
timber concrete timber + concrete
Internal Termite Incidence for Various Floor Types
Figure 2.10. Effect of zone on the apparent termite for various floor types
(temperature zonation).
2.3 Analysis of Termite Tally Data Based on Agro-Ecological Zonation
2.3.1 Zonation Procedure
The agro-ecological zonation of termite hazard was developed by Cookson (1999) and is
based on agro-ecological regions of Australia as defined by the Commonwealth of Australia
(1991) and illustrated in Figure 2.11. These regions are then broken down into sub-regions as
shown in Table 2.6 and accordingly numbered. After dropping sub-regions 1 and 17 where
there are virtually no termite incidences found, all other sub-regions are grouped into 4 zones
as follows:
Zone 1 (low hazard): 14;
Zone 2 (medium hazard): 10, 11, 15, 18;
Zone 3 (high hazard): 5-8, 13, 19-21; and
Zone 4 (very high hazard): 2-4, 9, 12, 22.
A termite hazard map, based on this zonation is shown in Figure 2.12.
Manual No. 8: Termite Attack 18
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Table 2.6 Termite Incidence in Agro-Ecological Regions
Agro-
ecological
region
Agro-ecological
sub-regions
Sample
number
Mean age
(years)
Outside
incidence of
termites
(%)
Inside
incidence of
termites
(%)
1 (part) 1, Tasmania 98 37.8 1.0 0.0
1 (part) 17, Melbourne,
west of 145oE
202 40.5 6.9 8.9
1 (part) 14, Melbourne,
east of 145oE
591 29.5 11.5 11.3
1 (part) 18, Wollongong,
south of 34.16oS
126 32.1 26.4 24.6
1 (part) 13, Sydney 603 39.8 33.5 22.1
1 (part) 19, Newcastle,
north of 33.33oS
115 18.8 27.8 13.9
1 (part) 12, Perth 421 26.8 49.2 14.0
2 (part) 21, NSW portion 574 20.6 23.9 15.5
2 (part) 2, Brisbane 394 26.9 44.7 23.6
2 (part) 22, Bundaberg,
north of 26.5oS
162 21.8 23.5 13.6
3 3, Cairns +
Rockhampton
114 26.1 42.1 28.1
4 4, Townsville +
Weipa
62 22.6 45.2 12.9
5 5, Toowoomba 260 33.1 26.1 14.6
6 6, Bathurst 241 32.6 31.5 17.0
7 (part) 7, Dubbo +
Bendigo
348 33.7 31.6 17.2
7 (part) 20, Adelaide +
SA portions
241 35.8 36.1 20.7
7 (part) 15, WA portion 49 30.8 32.7 16.3
8 8, Mount Isa +
semi-arid
51 36.5 23.5 19.6
9 9, Darwin 85 14.4 67.0 17.6
10 10, Canberra
+Bega
363 26.9 20.9 11.6
11 11, Arid interior 22 28.0 27.2 18.2
2.3.2 Effect of Age of House
Figures 2.13, 2.14 and 2.15 show the effect of the age of house on the probability that termite
incidences have been observed either within the house or within the garden. The average
sample size used for each data point is 147, with a minimum value of 29. It is seen that again
there is a very strong relationship between probability of termite incidence and age of house
for each zone. There appears to be little difference between Zones 2 and 3 and these are
combined further analyses.
Figures 2.15 and 2.16 show again the probabilities of observation of termite incidences
in the houses and gardens, respectively, following the mergence of Zones 2 and 3.
Manual No. 8: Termite Attack 19
19
Figure 2.11. Agro-ecological regions of Australia.
Port Hedland
Mount Gambier
Albany
Darwin
Hobart
Broome
Alice Springs
Geraldton
Perth
Adelaide
Cairns
Townsville
Rockhampton
Brisbane
Sydney
Newcastle
Canberra
Bega
Albury
Mildura
Dubbo
Narrabri
Mount Isa
Kalgoorlie
Melbourne
Charleville
1 Wet temperate coast
2 Wet sub-tropical coast
8 Semi-arid tropical and subtropical plainlands
3 Wet tropical coast and tableland
9 North-western wet / dry tropics
6 Sub-tropical highlands
5 Sub-tropical slopes
7 Temperate semi-arid slopes and plains
4 North-east wet / dry tropics
10 Temperate highlands
11 Arid interior
Manual No. 8: Termite Attack 20
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Figure 2.12. Termite hazard map based on agro-ecological regions.
0 20 40 60 80 100 120 House age (years)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
P(h
ou
se
, att
ack,
on
sv,
t)
zone 1 zone 2 zone 3 zone 4
zone 1
zone 2
zone 3 zone 4
Inside Termite Incidence (by Agro-ecological Zones)
Figure 2.13. Effect of house age on the apparent incidence of termite attack
(agro-ecological zonation).
Port Hedland
Mount Gambier
Albany
Darwin
Hobart
Broome
Alice Springs
Geraldton
Perth
Adelaide
Cairns
Townsville
Rockhampton
Brisbane
Sydney
Newcastle
Canberra
Bega
Albury
Mildura
Dubbo
Narrabri
Mount Isa
Kalgoorlie
Melbourne
Charleville
Zone 4
Zone 3
Negligible
Zone 2
Zone 1
Manual No. 8: Termite Attack 21
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0 20 40 60 80 100 120 House age (years)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
P(g
ard
en
, te
rmite
, o
bsv,
t)
zone 1 zone 2 zone 3 zone 4
zone 1
zone 2
zone 3
zone 4
Outside Termite Incidence (by Agro-ecological Zones)
Figure 2.14. Effect of house age on the apparent incidence of termites in the garden
(agro-ecological zonation).
0 20 40 60 80 100 120 House age (years)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
P(h
ou
se
, att
ack,
ob
sv,
t)
zone 1 zones 2 & 3 zone 4
Inside Termite Incidence (by Agro-ecological Zones)
zone 1
zones 2 & 3
zone 4 R 2 1 = 0.873
R 2 2&3 = 0.836
R 2 4 = 0.931
Figure 2.15. Effect of house age on the apparent incidence of termite attack (agro-
ecological zonation, zones 2 and 3 merged).
Manual No. 8: Termite Attack 22
22
0 20 40 60 80 100 120 House age (years)
0.0
0.1
0.2
0.3
0.4
0.5
0.6 P
(gard
en
, te
rmite
, o
bsv, t)
zone 1 zones 2 & 3 zone 4
Outside Termite Incidence (by Agro-ecological Zones)
zone 1
zones 2 & 3
zone 4
R 2 1 = 0.673
R 2 2&3 = 0.462
R 2 4 = 0.598
Figure 2.16. Effect of house age on the apparent incidence of termites in the garden
(agro-ecological zonation, zones 2 and 3 merged).
0 20 40 60 80 100 120 House age (years)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
house garden
Termite Incidence (by Agro-ecological Zones)
zone 1
zones 2 & 3 zone
4
zone 1
zones 2 & 3
zone 4
Figure 2.17. Effect of house age on the apparent incidence of termites
(agro-ecological zonation).
P(h
ou
se
, a
tta
ck,
ob
sv,
t) P
(ga
rde
n,
term
ite
, o
bsv,
t)
P(h
ou
se
, a
tta
ck,
ob
sv,
t) P
(ga
rde
n,
term
ite
, o
bsv,
t)
Manual No. 8: Termite Attack 23
23
2.3.3 Effect of Frame and Floor Types on Termite Incidence
As for the case of temperature based zonation, Figure 2.17 shows that while the hazard as
indicated by the termite incidence in gardens varies markedly from one zone to another, the
internal attacks on houses are remarkably similar in the various zones. Again it is worth
investigating the possibility that this is due to the implementation of better building,
inspection and barrier systems in the higher hazard zones.
Tables 2.7 and 2.8 are a summary of the statistics of building systems obtained from the
Termite Tally. Again it is noted that the highest risk zone contains a smaller proportion of
timber construction.
Table 2.7 Percentage of frame types in the agro-ecological zones
Zone 1 Zone 2 Zone 3 Zone 4
timber 88 76 74 58
steel 1 7 5 7
Masonry 11 16 21 35
Table 2.8. Percentage of floor types in the agro-ecological zones
Zone 1 Zone 2 Zone 3 Zone 4
timber 72 57 56 34
timber + concrete 4 9 9 8
concrete 24 34 35 58
2.4 Analysis of Termite Tally Data Based on Housing Clusters
2.4.1 Zonation Procedure
In this procedure, the data was examined to find clusters of houses. A cluster was deemed to
occur when at least 78 houses were found in a circle of 100 km diameter. The locations of the
clusters found are shown in Figure 2.19. Some relevant characteristics of the houses in these
clusters are given in Table 2.9.
Manual No. 8: Termite Attack 24
24
Table 2.9. Termite incidences and climatic data of the housing clusters
Location Longitude
(deg)
Latitude
(deg)
Sample
size.
Incidence
of termites
indoors
Incidence
of termites
in the
garden
Mean
age
(years)
Mean
annual
temp
(C)
Mean
annual
rain-
fall
(mm)
Darwin 130.832 -12.461 79 0.177 0.671 14.12 27.2 1761.8
Sydney 151.221 -33.87 589 0.219 0.334 40.119 17.8 1270.6
Newcastle 151.538 -33.209 109 0.119 0.248 18.959 18.1 1096.8
Armidale 151.873 -30.441 90 0.078 0.222 39.061 16.7 839.4
Taree 152.687 -31.84 307 0.137 0.202 16.82 18 942
Wollongong 150.878 -34.411 91 0.341 0.297 45.505 19.1 1365.7
Canberra 148.816 -34.918 146 0.096 0.171 24.814 13.5 711
Mudgee 149.146 -32.25 99 0.101 0.232 34.929 16.9 637.1
Melbourne 144.948 -37.812 653 0.123 0.112 33.444 15.1 852.6
Bendigo 143.901 -36.434 78 0.141 0.385 31.115 16.2 524.3
Brisbane 153.022 -27.468 311 0.235 0.434 28.406 20 1228.8
Adelaide 138.599 -34.927 174 0.195 0.356 35.039 16.2 465.9
Perth 115.862 -31.95 343 0.146 0.499 27.693 17.8 803.1
110 120 130 140 150
-40
-35
-30
-25
-20
-15
-10
Locations of Clusters
Figure 2.18. Locations of housing clusters.
lati
tud
e (d
eg.)
longitude (deg.)
Manual No. 8: Termite Attack 25
25
As with Appendix A, the purpose here is to examine whether temperature and rainfall
have a significant correlation with the incidence of termites as part of a procedure to develop
a hazard map.
Figure 2.19 shows that there is a modest correlation between temperature and the
incidence of termites in the garden. Figure 2.20 shows that there is some improvement if the
effect of rainfall is taken into consideration. Figure 2.21 shows that there is a good correlation
between rainfall and temperature; this is probably the reason why there is no great
improvement to be obtained by adding rainfall to temperature as a prediction parameter.
Figure 2.22 shows that there is very little relationship between rainfall and termite incidence,
even if separated into specific temperature zones.
12 16 20 24 28 Mean annual temperature ()
0.1
0.3
0.5
0.7
Termite Incidence by Clusters
R 2 = 0.626
Figure 2.19. Effect of temperature on the apparent incidence of termites in the garden
(housing cluster data).
P(g
ard
en, te
rmite,
obsv, t)
Manual No. 8: Termite Attack 26
26
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Predicted probability of apparent termite occurrences
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7 M
easure
d p
roba
bili
ty o
f a
ppare
nt te
rmite o
ccurr
ences
R 2 = 0.7171
Termite Incidence by Clusters
Figure 2.20. Use of temperature and rainfall data to predict the apparent incidence
of termites in the garden (housing cluster data).
12 14 16 18 20 22 24 26 28 Mean annual temperature (C)
300
600
900
1200
1500
1800
Mea
n a
nn
ua
l ra
infa
ll (m
m)
Termite Incidence by Clusters
R 2 = 0.681
Figure 2.21. Relationship between temperature and rainfall (housing cluster data).
Manual No. 8: Termite Attack 27
27
400 600 800 1000 1200 1400 Mean annual rainfall (mm)
0.20
0.25
0.30
0.35
0.40
0.45
0.50 P
(gard
en, te
rmite,
obsv,
t)
T = 18 - 20 C T = 16 - 18 C regression
Termite Incidence by Clusters
Figure 2.22. Effect of rainfall on the apparent incidence of termites in the garden (housing
cluster data).
2.5 Concluding comments
A feature of some interest is that there are very little differences between the incidence of
termite attack on houses between the various zones, whereas there is a considerable difference
in the termite hazard from one zone to the next, as indicated by the occurrence of termites
within gardens. This could be due to the fact that in high hazard areas more effective building
practices, pest control and inspection procedures are used. An attempt was made to examine
the data of the Termite Tally for information related to building practices, but the results were
inconclusive. The only definite statistic obtained was that timber is a dominant construction
material in all zones except the highest hazard zone.
One minor disadvantage associated with the use of agro-ecological zones is that it
involves soil types as a regional parameter and it may be desirable to use soil properties as a
local rather than a regional parameter as is currently done with the termite attack model.
The Termite Tally does not provide data that is in a form that is suitable for assessing
the effect of soil type on termite hazard. However, by evaluating the average soil properties in
each of the agro-ecological zones it may be feasible obtain a rough indication of their effect.
Manual No. 8: Termite Attack 28
28
3 EXPERT OPINION MODEL
Equation Section (Next)
3.1 The Survey Questionnaire
The following is an example of the survey questionnaire that was used.
ESTIMATES OF TERMITE ACTIVITY INTENTION OF THIS QUESTIONNAIRE
The purpose of this questionnaire, is to obtain information to predict the time taken by termites to attack and
destroy a timber element within a building. To do this, a limited number of experts on termites will be asked
to give their opinions on the following matters:
(a) A list of parameters that will affect the rate of attack (if you wish, suggest alternative parameters to
the ones proposed here).
(b) Grouping of these parameters into high and low risk effects.
(c) An estimate of the time taken by termites to undertake certain events.
Note 1. The format chosen for the questions is to enable the response to be used to calibrate a
mathematical model of termite activity.
Note 2. It is expected that for a given situation, there will be a wide range of field experiences and
expectations.
Note 3. Examples of answers are given in order to assist understanding the format of the questionnaire.
However, these answers were provided by a non-expert and should not influence your response.
Note 4. To ensure that your response complies with the intent of the questionnaire, please ensure that a
member of the research team involved (i.e. Bob Leicester, Greg Foliente, Craig Seath, Colin
McKenzie) is present to assist you when you undertake this task.
Note 5. We will leave a spare copy of this questionnaire with you. You may use this to
provide alternative estimates (at a later date) if you wish.
CONTACT INFORMATION
Dr Bob Leicester ([email protected])
Dr Greg Foliente ([email protected])
CSIRO Building, Construction and Engineering
PO Box 56, Highett Victoria 3190
Phone: (03) 9252 6000 Fax: (03) 9252 6246
Manual No. 8: Termite Attack 29
29
INSTRUCTIONS FOR ANSWERING QUESTIONNAIRE In the following, you are invited to estimate the time required for termites to undertake various
events related to the target house shown in Figures 3.1 and 3.2. We ask you to estimate two
times as follows:
Ttypical this would be a typical length of time that you would expect for the
event to take place
Tunlucky this would be a quick time; you would have to be unlucky to
experience this rate of attack.
(In the statistical interpretation of the data, it will be assumed that Ttypical denotes the
average time and Tunlucky denotes the quickest 10-percentile time)
The five events for which you will be required to provide time estimates are as follows,
establishment of nest
travel to a building
penetration or bypass of barriers
destruction of building timber
You will be asked to consider the effect of various parameters on the rate of termite activity.
Each parameter will be grouped into 3 categories designated as follows:
L = low termite activity, M = medium termite activity, H = high termite activity.
Examine these parameters, rate them, and if necessary make your own suggestions for
modifications to the parameters provided.
You will also be required to classify the importance (in your opinion) of the parameters as they
affect the time estimates. This importance rating will be on a scale of 0 to 10, with ―0‖ denoting
no importance and ―10‖ denoting extreme importance. An important parameter is one for which
you would expect to see a considerable difference in termite activity depending on whether the
parameter is a high risk one or low risk one. An unimportant parameter is one which (in your
opinion) will not have any effect on termite activity, regardless of whether it is a high risk one
or a low risk one.
DEFINITIONS
AWPC Australian Wood Preservation Committee see ―Protocols for Assessment of
Wood Preservatives‖, (H. Greaves, Chairman) Melbourne 1997, 24 pages.
AS 3660 Standards Australia ―Protection of Buildings from Subterranean Termites‖,
Sydney, 1995, 53 pages.
AS 1604 TimberPreservative-treatedSawn and Round. Standards Australia, 1993,
Sydney, 36 pages
Your name is……
Do you wish your name to be kept confidential?
Manual No. 8: Termite Attack 30
30
Stage 4Destruction
Stage 3
(entry)
Stage 2
Stage 1Nest
Figure 3.1. Illustration of termite travel stages.
House50 m
50 m
50 m
50 mTermite
free land
Built
up
Suburbs
Figure 3.2. Hypothetical scenario
for time zero zeroestimateshouse
and land at time zero.
Manual No. 8: Termite Attack 31
31
Estimates of these four event times were obtained via a limited survey of expert
opinion. Although the original plan had been to obtain estimates from at least 20 experts, it
was found difficult to find experts in termite behaviour who were comfortable with
quantifying their opinions in the form requested. Accordingly it was decided to enlist the
assistance of a limited number of Australia‘s leading experts. Not all of these experts were
willing to provide opinions on all questions asked so in the end all opinions obtained were
melded into a single composite response, for which the authors of this report take full
responsibility. The experts who took part in this exercise were Dr John French, Dr Berhan
Ahmed, Mr Jim Creffield and Mr Doug Howick all of whom either are or were at one time
research scientists within CSIRO, Dr Don Ewart (Development manager, Granitgard) and Mr
Nicholas Cooper (Manager, Systems Pest Management).
3.2 Choice of Parameters
The parameters chosen by the experts are listed in the Table 3.1 below.
Table 3.1. List of event times and associated parameters
Event time Influencing parameter t1 the time taken for the establishment of a mature colony within a distance of 50 m from the target house
P1: geographical location P2: age of surrounding suburbs P3: number of potential nest sites
t2 the time taken for the termite foraging galleries to progress to a house 20 m away from the nest site
P4: geographical location P5: soil condition P6: food source
t3 the time taken for termites to penetrate or bypass a chemical or mechanical barrier, if any
P7: geographical location P8: period between inspections P9: maintenance parameter
t4 the time taken (after penetrating the barrier) to reach and cause failure of a timber member
P10: geographical location P11: ground-contact building element P12: period between inspections P13: type of material attacked P14: timber environment
Descriptions of these parameters are given in the following. The hazard zones shown in
Figure 3.3 was based on a combination of consideration of the two hazard maps discussed in
Section 2.
For each parameter Pj, there is an associated parameter factor kj. This factor kj is given
the value of +1, 0 or 1 depending on whether the parameter has been chosen to correspond to
low, medium or high hazard situations respectively. The parameters chosen are described in
Tables 3.2 – 3.11.
Manual No. 8: Termite Attack 32
32
Figure 3.3. Termite hazard zonation.
Table 3.2. Parameter k1, k4, k7, k10
k1, k4, k7, k10 Building location*
+1
0
1
Zone 1
Zone 2
Zone 3
*see Figure 3.1
Table 3.3 Parameter k2
k2
Age of suburb (yrs)
+1
0
1
<6
630
>30
Manual No. 8: Termite Attack 33
33
Table 3.4 Parameter k3
k3
Number of potential nesting sites
+1
0
1
<2
25
>5
EXAMPLES OF POTENTIAL NEST SITES
The following refers to potential nest sites for harbouring mature
colonies which are not more than 50 m from the building.
Tree (diameter larger than 300 mm)
Tree stump or untreated pole (diameter larger than 200 mm)
Untreated landscape timber (e.g. sleepers, retaining walls)
(length >1.0 m, height >0.5 m)
Woodheap
(height >0.5 m, ground contact area 0.5 x 0.5 m, length of
periods that bottom layer
woodheap is untouched >1 year)
Compost heap
Wood ‘stepping stones’
Subfloor storage (height >0.5 m, ground contact area >0.5 x 0.5 m, length of
period which it is
untouched >1 year).
Solid infill under a verandah
Any part of a building with water leaking under it.
Manual No. 8: Termite Attack 34
34
Table 3.5. Parameter k5
K5 Soil type
+1
< fissured clay, sandstone
fertile soil
coarse sand
0 sound clay; loam; silt
1 moist soil mixed with composted material
poor lateritic soils
Table 3.6 Parameter k6
K6
Typical distance between substantial
food sources (m)
+1
0
1
<20
520
>5
A typical example of a substantial food source would be a piece of
timber equal to or greater than 200 x 100 x 50 mm in-ground
contact
Table 3.7 Parameter k8, k12
k8, k12
Period between inspections (yr)
+1
0
1
<2
25
>5
Manual No. 8: Termite Attack 35
35
Table 3.8 Parameter k9
K9
Period between chemical retreatments
+1
0
1
Tm
2Tm
>8Tm
Tm = period recommended by chemical producer
Manual No. 8: Termite Attack 36
36
Table 3.9 Parameter k11
k11
Ground contact elements
+1
House supported by exposed concrete piers
or steel stumps more than 2 m high
0
Intact concrete slab on ground;
House on stumps less than 600 mm high
with ant caps and made of concrete or
treated timber* or heartwood of durable
species**
1
Floor connected to ground by stair cases of
untreated softwood, untreated non-durable
timber**, untreated sapwood of durable
timber;
Attached patio with solid infill
Concrete slab-on-ground with large cracks
and/or unprotected pipe penetrations
Floors connected to ground by elements
containing hidden cavities (e.g. masonry
construction, deeply grooved elements,
members in imperfect contact).
Brick veneer house
Leakage of moisture to ground
Timber floor less than 600 mm off the
ground
treated timber refers to timber treated according to AS 1604
and/or complying with AWPC recommendations
** for a listing of durable species, see timber of durability
class I and II in AS 1604
Manual No. 8: Termite Attack 37
37
Table 3.10 Parameter k13
k13
Type of material attacked
+1
Treated timber*
Untreated heartwood of durability Class 1
Hardwoods
0
Untreated heartwood of durability Class 2
hardwoods
Untreated heartwood of all softwood species
1
Untreated hardwoods of durability
Classes 3 and 4
Untreated sapwood of all species
Composite wood boards
treated timber refers to timber treated in accordance AS 1604 and/or
complying with AWPC recommendations
** durable species refers to species of durability class I and II
according to AS 1604.
Table 3.11 Parameter k14
k14
Exposure of timber
+1
High human activity
High up a building
Humidity <30%
0
Exposed to rain
1
No disturbance and dark (e.g. wall stud,
double leaf masonry wall, roof member.)
Exposed to sources of moisture so as to be
periodically wet
Abandoned houses
Humidity >90%
Manual No. 8: Termite Attack 38
38
3.3 The Base Model
The base model has been derived on the basis of expert opinion. It applies to a house
surrounded by 50 m of termite-free land as shown in Figs. 3.1 and 3.2. The distance of 50 m
was chosen because this is about the limit of the foraging distance of most termite species.
The model used endeavours to estimate four sequential event times t1 – t4 as defined in Table
3.12 and illustrated in Figure 3.4. Relevant data on these four event times were obtained via a
limited survey of expert opinion.
Stage 4Destruction
Stage 3 Stage 2
Stage 1Nest
Figure 3.4 Illustration of termite progress.
In the survey, a set of parameters affecting each event time was obtained from experts.
The set chosen is listed as P1, P2, …, P14, as tabulated in Table 3.12. For each parameter, the
experts were asked to list the importance of the parameter with regard to its influence on the
relevant event time; this importance was rated on a scale of 110, with 10 being the most
important; examples of the importance parameters chosen are also given in Table 3.12.
Table 3.12 List of event times and associated parameters
Event time Influencing parameter Importance factor
t1 the time taken for the establishment of a mature colony within a distance of 50 m from the target house
P1: geographical location P2: age of surrounding suburbs P3: number of potential nest sites
8 5 9
t2 the time taken for the termite foraging galleries to progress to a house 20 m away from the nest site
P4: geographical location P5: soil condition P6: food source
8 6 7
t3 the time taken for termites to penetrate or bypass a chemical or mechanical barrier, if any
P7: geographical location P8: period between inspections P9: maintenance parameter
4 10 7
t4 the time taken (after penetrating the barrier) to reach and cause failure of a timber member
P10: geographical location P11: ground-contact building element P12: period between inspections P13: type of material attacked P14: timber environment
8 5 9 7 7
Manual No. 8: Termite Attack 39
39
For each of the times t1 – t4, the experts were asked to assess both ―typical‖ and
―unlucky‖ values. They were asked to do this when all parameters were set at their high risk
settings, resulting in times tH, and low risk settings, resulting in times tL. A set of values,
based on the responses received, is shown in Table 3.13, where the times t3 have the
following notation
t3B : time that a physical barrier is crossed or breached
t3C : time that a repellent chemical barrier is crossed or breached
t3D : time that a toxic chemical barrier is crossed or breached
t3E : t3 = 0 when there is no barrier
Table3.13 Estimates by experts of time parameters
TIME
Estimate of time t (yrs)
j For high risk parameters tH For low risk parameters tL
tH(typical) tH(unlucky) tL(typical) tL(unlucky)
t1 8 3 30 10 0.0295
t2 4 0.5 6 1.5 0.00964
t3B 4 1 70 15 0.0710
t3C 8 2 100 40 0.0556
t3D 6 1 60 20 0.0513
t3E 0 0 0 0 –
t4 1 0.5 80 20 0.0569
From the data in Table 3.13, the mean and coefficients of variation of times t1 – t4 can
be estimated. The following is an example applied to the time t that is defined by 3 parameters
a, b and c as follows
t = A(1 + kaa)(1 + kbb)(1 + kcc) (3.1)
Hence approximate equations for the mean value are
tH(typical) = mean(A). (1 – jIa) (1 – jIb) (1 – jIc) (3.2)
tL(typical) = mean(A). (1 – jIa) (1 – jIb) (1 – jIc) (3.3)
where j is a subjective dispersion factor related to the experts making the assessment, i.e.
a=jIa, b=jIb, c=jIc (3.4)
Solving (3.2) and (3.3) simultaneously leads to the values of j shown in Table 3.13. Then
using equation (3.4) leads to the following mean values of t1 – t4.
Manual No. 8: Termite Attack 40
40
mean (t1) = 16.7(1 + 0.236 k1) (1 + 0.148 k2) (1 + 0.266 k3)
mean (t2) = 4.9 (1 + 0.0771 k4) (1 + 0.0578 k5) (1 + 0.0675 k6)
mean (t3B) = 27.4 (1 + 0. 497k7) (1 + 0.710 k8) (3.5)
mean (t3C) = 37.9 (1 + 0.222 k7) (1 + 0.556 k8) (1 + 0.389 k9)
mean (t3D) = 24.2 (1 + 0.205 k7) (1 + 0.513 k8) (1 + 0.359 k9)
mean (t3E) = 0.0
mean (t4) = 14.5 (1 + 0.455 k10) (1 + 0.284 k11) (1 + 0.512 k12) (1 + 0.398 k13)
(1 + 0.398 k14)
Similarly the spread of the time estimates can be used to provide a rough estimate of the
coefficient of variation of t1 – t4 as follows:
( ) ( ) ( ) ( )
( ) ( )
H H L L
H L
t typical t unlucky t typical t unluckycov(t)= 0.5
t typical t typical
(3.6)
this leads to the following estimates of the coefficient of variation:
cov (t1) = 0.646
cov (t2) = 0.813
cov (t3B) = 0.768
cov (t3C) = 0.675 (3.7)
cov (t3D) = 0.750
cov (t3E) = 0.0
cov (t4) = 0.625
Manual No. 8: Termite Attack 41
41
4 THE PROBABILISTIC MODEL
Equation Section (Next)
4.1 Introduction
For engineering purposes, it is useful for a termite attack to be considered to be a probabilistic
event. This report describes the development of a model to predict the risk of attack on a
house in Australia. Such a model is useful for assessing (in a quantified manner) the value of
various protection strategy proposals.
4.2 The Basic Probability Model
4.2.1 The probability Distributions
The probability density function of the time for a house to be attacked by termites is assumed
to be of the type shown in Figure 4.1. The form of this function was chosen to fit the data
found in the Termite Tally. The equation for the density function is assumed to be
btap (4.1)
where a and b are the distribution parameters, and t is the time since time zero, the time at
which the house was constructed. The value of a may be either positive or negative, as shown
in Figure 4.1. The notation of ta and tmax will be used to denote the lower and upper end of the
probability density function.
For the case a 0,
ta = 0 (4.2a)
and for the case a < 0,
ta = – (a/b) (4.2b)
for both cases the integration max
a
=1t
tp dt leads to
Manual No. 8: Termite Attack 42
42
2 2
max a a/ / 2 / 2/t a b a b a b t t b (4.3)
The mean value of the time of attack, denoted by mean(t), is then derived from
max
a
2 2 3 3max a max a
mean
/ 2 / 3
t
tt pt dt
a t t b t t
(4.4)
The variance 2
t of the time of attack is given by
max
a
22 2
23 3 4 4max a max a
mean
/ 3 / 4 mean
t
tt
pt dt t
a t t b t t t
(4.5)
The coefficient of variation V(t) of the time to attack is then given by
V(t) = (t) / mean(t) (4.6)
t max
t
a
p
(a) for positive a
p = a + bt
ta = 0
t max
t
p
(b) for negative a
p = a + bt
t a
age of house (yrs)
age of house (yrs)
t max
t
a
p
(a) for positive a
p = a + bt
t max
t
p
(b) for negative a
p = a + bt
t a
age of house (yrs)
age of house (yrs)
Figure 4.1. Probability density functions of the time of a termite attack.
4.2.2 The True Risk in the Past
The probability Ptrue (to) that a house has been attacked before time to is evaluated as follows:
For to < ta,
Ptrue (to) = 0 (4.7)
For ta < to < tmax,
o
a
true o
2 2o a o a/ 2
t
tP t pdt
a t t b t t
(4.8)
Manual No. 8: Termite Attack 43
43
For to > tmax,
Ptrue (to) = 1 (4.9)
4.2.3 True Risk in the Future
The true risk in the future will be defined as the true probability that a house will be attacked
between time t1 and t2, where t1 denotes the time taken since the construction of the house.
This true risk will be denoted as Ptrue,future (t1 t2). To take into account the fact that houses may
have been attacked in the past, the assumption will be made that
Ptrue,future (t1 t2) = Ptrue (t2 – t1) (4.10)
where Ptrue (t2 – t1) is evaluated according to equations (4.7)–(4.9), but with mean(t) chosen as
discussed in Section 4.2.1 to account for the fact that the suburb is effectively t1 years older at
the start of the risk estimate than at the time when the house was built.
4.2.4 Apparent Risk in the Past
This model is required for the purposes of interpreting data obtained from interviewing
people. For practical purposes, the model needs to take into account the fact that reported data
comes from people with a memory of tmean years. The probability that there has apparently
been an attack in buildings of age to will be denoted by Papp (to).
For the case of to < (ta + tmem),
Papp (to) = Ptrue (to) (4.11)
For the case of (ta + tmem) < to < tmax
o
o mem
app o 0
t
t tP t p dt A Bt
(4.12a)
where
A = atmem – (b /2) 2memt (4.12b)
B = btmem (4.12c)
For the case of to > tmax it will be assumed that equation (4.12) still holds true up to a value of
Papp (to) = 1.0, even though this is mathematically not correct. However, the discrepancy will
be assumed to be due to the fact that some of the previously attacked buildings will have been
repaired and will eventually be attacked a second time.
In the Termite Tally, the average time a house had been occupied was 11 years. Taking this
into consideration, plus the results of processing the data in the Tally, it was decided to use
tmem = 20 years in application of the termite attack model.
Manual No. 8: Termite Attack 44
44
4.2.5 The Apparent Risk in the Future
The apparent risk in the future will be defined as the apparent probability that a house will be
attacked between the time t1 and t2, where t1 and t2 denote the time taken since construction of
the house. This probability will be denoted by Papp,future (t1 t2). To take into account the fact
that some of the houses under consideration may already have been attacked in the past, the
assumption will be made that
Papp,future (t1 t2) = Papp (t2 – t1) (4.13)
where Papp,future (t2 – t1) will be evaluated according to equations (4.11) and (4.12) but using a
value of mean (t) chosen as discussed in Section 4.2.1. To account for the fact that the suburb
is effectively t1 years older than when the house was first built.
tmem tmax
t
P
true risk of attack
apparent risk of attack
0
1.0
ta + tM tmax
t
P
P = A + Bt
true risk of attack
P(house, attack, true, t)
apparent risk of attack
P(house, attack, obsv, t)
0
1.0
ta
age of house (yrs)
age of house (yrs)
Pro
bab
ilit
y o
f at
tack
P
rob
abil
ity
th
at a
ttac
k h
as o
ccu
rred
(a) for positive a
(ii) for negative a
0
ta + tmem tmax
t
P
true risk of attack
apparent risk of attack
0
1.0
ta
age of house (yrs)
(b) for negative a
tmem tmax
t
P
true risk of attack
apparent risk of attack
1.0
age of house (yrs)
(a) for positive a
Pro
bab
ilit
y t
hat
att
ack
has
occ
urr
ed
P
rob
abil
ity
th
at a
ttac
k h
as o
ccu
rred
Figure 4.2. Schematic illustration of the cumulative distribution functions of the attack time.
4.2.6 The Distribution Parameters
It should be noted that two parameters, a and b, are required to define the probability
distribution function. However, after examining the data from the Termite Tally, it was
decided to take b to be a fixed value b = 0.0002. Hence only one parameter is required to
define the probability distribution function. It was therefore an obvious choice to use mean(t)
as the defining parameter. The relationship between a, mean(t) and V(t) are shown in Figures
4.3 and 4.4. The relationship between the mean value and their risk of attack within 50 years
is shown in Figure 4.
The equation relating mean(t) and the probability of attack within 50 years, denoted by
risk50 can be closely approximated by the equation
risk50 = 0.000138 mean(t) × mean(t) – 0.029749 mean(t) + 1.618 (4.14)
For risk50 = 0.2, a typical value of risk that occurs in Australia, the corresponding mean attack
time is mean(t) = 44 years. Equation (4.14) is shown plotted in Fog. 4.5.
Manual No. 8: Termite Attack 45
45
-0.05
0
0.05
0.1
0 100 200 300
mean attack time [yrs]p
ara
mete
r 'a
'
Figure 4.3. Relationship between the mean time of attack and the model parameter ‗a‘.
Coefficient of variation
0
20
40
60
0 50 100 150 200
mean attack time [yrs]
Co
eff
icie
nt
oi
va
ria
tio
n [
%]
Figure 4.4. The relationship between the mean and coefficient of variation of the time of
attack.
Manual No. 8: Termite Attack 46
46
Relationship between mean
attack time and true risk of
attack
0
0.5
1
0 50 100 150
mean attack time [yrs]
tru
e r
isk
of
att
ac
k in
50
ye
ars
Figure 4.5. Relationship between mean attack time and the risk of attack.
4.3 The Practical Model
For practical application, the base model must be modified so that the target house is closer
than 50 m to the adjoining suburbs. In addition, the model must allow for the possibility that
there may be mature nests nearby at time zero, the year in which the house is constructed.
This configuration is illustrated in Figure 4.6.
Figure 4.6. House and land surrounded by existing buildings and nest sites
at time zero.
Manual No. 8: Termite Attack 47
47
Figure 4.7. Venn diagram of probabilities of the occurrence of termites at time zero.
The Venn diagram of probabilities at time zero is shown in Figure 4.7. The total
probabilities will be divided into three subgroups defined as follows:
P1 = Probability of termites in the garden
=Pgarden (4.15)
P2 = Probability of termites in the adjoining suburbs but not in the garden
=Psuburb–PsuburbPgarden/suburb (4.16)
P3 = Probability of no termites either in the garden or the adjoining suburbs
=1–Psuburb–Pgarden+PsuburbPgarden/suburb (4.17)
where Pgarden and Psuburb denote the probabilities that there are termites in the garden and the
suburb respectively at time zero, and Pgarden/suburb denotes the probability that there are
termites in the garden, whenever termites are found in the suburb.
Note that
P1+P2+P3=1.0 (4.18)
Note also that for the necessary condition P3 0, it is necessary that Pgarden/suburb Pgarden
when Psuburb = 1.
The mean time taken to destroy a house, based on expert opinion, is then assumed to be
given by
all probabilities
(area = 1.0)
probability of
termites in suburbs
but not in garden
probability of
termites in garden
probability of no
termites in garden
or suburbs
Manual No. 8: Termite Attack 48
48
1 2 3 4
2 2 3 4
3 1 2 3 4
mean mean 0.5
10mean
20
mean
expertt P t t t
dP t t t
P t t t t
(4.19)
where d denotes the shortest distance from the fence to the house or 10 m, whichever is less
(see Figure 4.6).
Examination of data in the Termite Tally indicates that a suitable equation for
estimating Psuburb is
suburb suburb
suburb
suburb
100, 100 years;
1 100 years.
t tP
t
(4.20)
where tsuburb denotes the age of the suburb at time zero, the year in which the target house was
built.
Also, in the absence of any available data, it will be assumed that
Pgarden = 0.5Psuburb (4.21)
Pgarden/suburb = 0.5 (4.22)
Manual No. 8: Termite Attack 49
49
5 COMPUTATION MODEL
Equation Section (Next)
5.1 Concept
The concept behind the computation model is that it commences with an estimate of the mean
attack time using the model developed by expert opinion in Section 3. This mean attack time
is then used to obtain the parameters of the probabilistic model which in turn is used to
estimate the risk of termite attack. However before this can be done, the predictions of the
expert opinion model must be calibrated against real data. To do this, information from the
Termite Tally described in Section 2 will be used.
The data from the Termite Tally indicates that for the probability distribution function
of attack times, suitable calibration choices are for a parameter b = 0.0002, and the mean time
to attack is given by a calibration factor β to be discussed, i.e.
mean(t)=β mean(texpert) (5.1)
5.2 Calibration of the Model
From the data of the Termite Tally as shown in Figures 2.3 and 2.16, it is assumed for
calibration purposes that on average
P(garden,nest,obsv,0)=0.25 (5.2)
Furthermore the following assumptions will be made
P(garden, nest, true, 0) = 2 x P(garden, nest, obsv, 0) = 0.5 (5.3)
P(suburb,nest,true,0)=P(garden,nest,true,0)=0.5 (5.4)
Substitution of these values and d = 2 into equation (4.19) leads to
mean(ttotal) = 0.25mean(t1)+0.9mean(t2)+t3+t4 (5.5)
In equation (5.5) the value of mean (ttotal) is an estimate of the average time to attack,
assessed entirely on the basis of expert opinion and corresponds to mean(texpert) in equation
(1). To allow for the fact that there may be a bias error by the experts in these estimates, a
calibration factor will need to be introduced, so that the best estimate of mean attack time,
mean (tmodel) is given by
mean(tmodel)=mean(ttotal) (5.6)
One estimate of the mean(tmodel) for average conditions can be obtained from the
Termite Tally where the data is grouped according to temperature zones. For this case, the
Manual No. 8: Termite Attack 50
50
data for Zone 2 may be considered to be average and the values of the constants A and B used
in equations (4.12) are 0.08 and 0.004 respectively. This leads to a value of mean(tmodel) =
44.1 years.
A second estimate of a typical mean(tmodel) can be obtained from the Termite Tally
where the data is grouped according to agro-ecological zones. For this case, the data for the
combined Zones 2 and 3 may be taken as average. For this case, the values of A and B are
found to be 0.04 and 0.004 respectively. This leads to a value of mean(tmodel) = 50.2 years.
The data from the Termite Tally also indicates that for the probability distribution
function of attack times, suitable calibration choices are for a parameter b = 0.0002.
The model matches the Termite Tally data for average hazard zone conditions when the value
of mean(t)=44 yrs is used. Figure 5.1 shows a plot of a distribution with mean(t)=44 years,
and Figure 5.2 shows the same plot compared with the findings of the Termite Tally.
Taking into account the scatter of the Termite Tally data, the model appears to give as
good a fit as can be expected for average conditions. Reasons for the kink and the dotted
extension of the predicted graph in Fig 5.2 can be seen in Fig. 5.1.
0
0.5
1
1.5
0 50 100 150 200
age of house (years)
pro
ba
bilit
y t
ha
t h
ou
se
ha
s
be
en
att
ac
ke
d
true risk
apparent risk
mean(tmodel) = 44.1 years
Figure 5.1. Computed risk for the calibration case;
{true risk = P(house, attack, true, t); apparent risk = P(house, attack, obsv, t)}.
Manual No. 8: Termite Attack 51
51
Figure 5.2. Calibration of expert opinion with data from the Termite Tally.
A matter to be decided in the choice of mean(texpert) for calibration purposes is to decide
to what extent were termite barriers used for the houses that featured in the termite survey.
Data taken from Table 10 of the report by Cookson and Trajstman (2002) shown in Table 5.1
below would tend to indicate that the average house was probably protected only by ant-caps.
In the following Table 5.2, the three estimates were obtained by taking all k-values to be
either -1, 0, +1 respectively.
Table 5.1 Estimate of termite barriers at the time of the Termite Tally
(After Cookson and Trajstman 2002)
Treatment Percentage of houses*
Soil poisoning 25
Ant caps only 20
*does not include 25% who were uncertain of treatment
The questionnaire did not ask whether the protection methods were installed before or after termite attack, so
we cannot determine directly which protection methods were in place t the time of the survey
Table 5.2. Suburban model (Expert Opinion model)
Mean time for activity (years)
Termite activity High est. Mean est. Low est.
Component
0.25*mean(t1) 8 4 2
0.9*mean(t2) 5 5 4
mean(t3) 75 30 6
mean(t4) 83 15 1
Total time mean(ttotal)
no termite barrier present 96 24 7
with termite barrier present 171 54 13
Manual No. 8: Termite Attack 52
52
Table 5.3 shows a comparison between estimates of the mean time of termite attack
based on both Expert Opinion model and Termite Tally data. On the basis of this data, it
would appear that a suitable calibration factor would be in the range 1.0-1.5. For the
purpose of this report the value = 1.5 has been chosen, i.e.
mean(t)=1.5 mean(texpert) (5.7)
Table 5.3. Attack time of average models
Data source Mean value
(yrs)
Coefficient of
variation
Expert Opinion model
(no termite barrier present)
Expert Opinion model
(with termite barrier present)
Termite Tally (temperature zone 2)
Termite Tally (agro-ecological zones 2 & 3)
23.8
53.8
44.1
50.2
0.42
0.43
0.46
0.43
5.3 The Coefficient of Variation
An additional check on the uncertainty predictions of the model can be obtained by
comparing the coefficients of variation as predicted by the Expert Opinion and the Reliability
model. It is also to be noted in Table 5.3 that the computed coefficients of variation for all
cases are quite similar.
It is found that the coefficient of variation of the attack time corresponds well with the
reliability model if the true coefficients of variation of each of the time components t1 t4 are
given by
cov(t) = 0.7 cov(texpert) (5.8)
where cov(t) is the value of cov given by using equation (3.7) and (4.19).
A comparison between the variability of the model and the expert opinion is shown in
Figure 5.3. The two models are in reasonable agreement. No attempt was made to check the
uncertainty shown by the data from the Termite Tally, but this may be possible.
Manual No. 8: Termite Attack 53
53
Figure 5.3. The coefficient of variation for a given value of the mean attack time (The thin lines
correspond to the various choices of the type of barrier system used).
factor 0.7 on expert estimates of cov
0
0.2
0.4
0.6
0.8
1
0 50 100 150 200
mean(t) [yrs]
co
v(t
)
expert opinion
model
reliability model
Manual No. 8: Termite Attack 54
54
6 RISK ASSESSMENT EQUATIONS
Equation Section (Next)
6.1 Hazard Parameters
As indicated, the model makes a quantified risk estimate on each specific house based on a
number of parameters related to that house. The input parameters chosen for practical
application are as follows:
termite hazard zone
age of surrounding suburbs
distance of house from boundary
wood in the garden and under the house
type of ground contact for the house
environment of vulnerable timber
However, for practical application some approximations are introduced. First, each hazard
parameter is given a hazard score, depending on whether it is considered to describe a high,
medium or low hazard. These hazard scores are denoted by h. They have been derived
empirically to fit exact equations. Then for each particular house, the hazard scores are added
so as to obtain a hazard score total, denoted by H. The scores for each particular hazard are
given in Table 6.1. The classification of a hazard level is denoted by c. The zone classification
for hazard h1 is shown in Figure 6.1. The descriptions for hazards h4 – h7 are given in Section
6.3. Once the hazard score has been evaluated for each of the parameters h1 – h7, the hazard
score total H is obtained as a summation of these scores as indicated in Table 6.2.
Table 6.1. Hazard scores for termite attack
c1 Location Zone(1)
Hazard score
h1
1 B 0
2 C 2
3 D 4
(1) See Figure 6.1; the hazard for zone A is considered to be negligible.
Manual No. 8: Termite Attack 55
55
c2 Age of suburb(2)
Hazard score
h2 1 <10 yrs 0
2 10-70 yrs 2
3 >70 yrs 4 (2)
A suburb refers to an area in which at least 20% of the land is covered by buildings.
c3 Distance to nearest
boundary
Hazard score
h3 1 >8 m 0
2 2—8 m 0.5
3 <2 m 1.0
c4 Quantity of wood in
garden and under
house(3)
Hazard score
h4
1 low 0
2 medium 0.5
3 high 1.0 (3)
See Section 6.3.1.
c5 Hazard related to
ground contact(4)
Hazard score
h5 1 low 0
2 medium 1
3 high 2 (4)
See Section 6.3.2.
c6 Hazard related to
type of construction
materials(5)
Hazard score
h6
1 low 0
2 medium 1
3 high 2 (5)
See Section 6.3.3.
c7 Hazard related to
exposure of
material(6)
Hazard score
h7
1 low 0
2 medium 1
3 high 2 (6)
See Section 6.3.4.
Manual No. 8: Termite Attack 56
56
Table 6.2. Evaluation of hazard score total
Hazard factor Hazard score
Location zone h1
Age of suburb h2
Distance to boundary h3
Wood in garden h4
Ground contact h5
Construction material h6
Timber exposure h7
Hazard Score Total H:
6.2 Supplementary Hazard Parameters
Table 6.3. Inspection parameter I
I Hazard
Level Period between inspections (yrs)
1 low <1
2 medium 1 – 5
3 high >5
Table 6.4. Maintenance parameter M
M Hazard
level
Period between chemical treatments
(yrs)*
1 low Tm
2 medium 2 Tm
3 high >8 Tm
* Tm denotes the period between re-treatments as recommended by the
chemical manufacturer.
Manual No. 8: Termite Attack 57
57
6.3 Expanded definitions of hazard parameters
Figure 6.1. Termite hazard zonation.
6.3.1 Procedure for assessing the hazard h4 due to the quantity of wood occurring in a
garden and under a house
Table 6.5 shows in quantitative terms some typical distributions of wood corresponding to
low, medium and high hazard levels of termite attack. For other distributions of wood,
suitable estimates may be made through interpolation of these values.
Table 6.5 Definition of hazard h4 assessment due to occurrence of wood in the garden and
under the house
Hazard
class
Number of potential nesting
sites(1)
Typical distance between substantial
food source (m)(2)
Low <2 >20
Medium 2–5 5–20
High >5 <5
(1) Examples of potential nesting sites
The following refers to potential nest sites for harbouring mature colonies which are not more than 50 m from
the building.
Tree (diameter larger than 300 mm)
Tree stump or untreated pole (diameter larger than 200 mm)
Untreated landscape timber (e.g., sleepers, retaining walls, length > 1.0 m, height > 0.5 m)
Woodheap (height >0.5 m, ground contact area 0.5 x 0.5 m, length of periods that bottom layer woodheap is
untouched > 1 year)
Compost heap
Wood ―stepping stones‖
Subfloor storage (height >0.5 m, ground contact area >0.5 x 0.5 m, length of period which it is untouched
>1 year).
Solid infill under a verandah
Any part of a building with water leaking under it
(2) Example of a substantial food source
A typical example of a substantial food source would be a piece of timber equal to or greater than 200 50 mm
as the surface lying in ground contact.
Manual No. 8: Termite Attack 58
58
6.3.2 Definition of ground contact
Table 6.6 gives examples of building construction that leads to high, medium and low hazard
of termite attack related to ground contact characteristics
Table 6.6 Examples of hazard h5 assessment due to the nature of the
ground contact of a house
Hazard class
Ground contact elements
Low
House supported by exposed concrete piers or
steel stumps more than 2 m high
Medium
Intact concrete slab on ground; House on stumps less than 600 mm high with ant caps
and made of concrete or treated timber(1)
or heartwood
of durable timber(2)
High
Construction does not comply with AS 3660.1
Building not inspective according to AS 3660.2
Concealed entry zones of any type
Floor connected to ground by stair cases of
untreated softwood, untreated non-durable
timber(3)
, untreated sapwood of durable timber;
Attached patio with solid infill
Concrete slab-on-ground with large cracks and/or
unprotected pipe penetrations
Floors connected to ground by elements
containing hidden cavities (e.g. masonry
construction, deeply grooved elements, members
in imperfect contact).
Brick veneer house Leakage of moisture to ground
Timber floor less than 600 mm off the ground
(1) treated timber refers to timber treated according to AS 1604.1–2002 [5].
(2) for a listing of timber durable classes 1 and 2, see AS 5604–2003 [8].
(3) for a listing of non-durable timber of classes 3 and 4 see AS 5604–2003 [8].
6.3.3 Hazard level h6 related to type of construction material
Table 6.7 gives examples of high, medium and low hazard of termite attack related to the type
of material used for construction.
Manual No. 8: Termite Attack 59
59
Table 6.7 Examples of hazard h6 assessment related to the type of
construction material used
Hazard class Type of construction material attacked
Low
Treated timber(1)
Untreated heartwood of durability class 1(2)
Medium
Untreated heartwood of durability class 2(2)
High
Untreated hardwoods of durability classes 3 and 4(2)
Untreated sapwood of all species
Composite wood boards
(1) treated timber refers to timber treated in accordance AS 1604–2002 [5].
(2) for naturally durable timber classes, see AS 5604–2003 [8].
6.3.4 Hazard h4 related to exposure of timber
Table 6.8 gives a method for assessing the hazard h7 due to the nature of exposure of timber.
Table 6.8. Examples of hazard h7 related to exposure of timber
Hazard class Exposure of timber
low
High human activity
High up a building
Humidity <30%
medium Exposed to rain
high
No disturbance and dark
(e.g. wall stud, double leaf masonry wall, roof
member.)
Exposed to sources of
moisture so as to be periodically wet
Abandoned houses or
mostly vacant holiday houses
Humidity >90%
6.4 Computational Procedure
6.4.1 Computing the mean time to attack mean(t)
To compute the risk of a hazard attack, the mean attack time mean(t) is first necessary to
compute mean(t) using equations (3.5), (4.19) and (5.7). This value of mean(t) is then be used
to evaluate both the true and apparent risk of termite attack .
Manual No. 8: Termite Attack 60
60
To do the computations, the input data is used to derive the hazard classification
parameters c1 – c7 as defined in Table 6.1. In addition, the inspection parameter I and the
maintenance parameter M as defined in Tables 6.3 and 6.4 respectively.
From the above input data, the factors k1 – k14 required for equation (3.5) are derived as
follows:
k1 = 2 – c1
k2 = 2 – c2
k3 = 2 – c4
k4 = 2 – c1
k5 = 0
k6 = 2 – c4
k7 = 2 – c1
k8 = 2 – I
k9 = 2 – M
k10 = 2 – c1
k11 = 2 – c5
k12 = 2 – I
k13 = 2 – c6
k14 = 2–c7 (6.1)
In addition to k1 – k14, there are two additional input parameters required), i.e. tsuburb and
d, These are indirectly defined by the hazards h2 and h3 respectively. The actual values are
taken to be given by values shown. in Table 6.1..
6.4.2 Computed Risk and Hazard Score
The value of mean(t) needs to be computed for each of 3 values for each of the seven input
hazard parameters h1 – h7. This gives 37 = 2187 values of mean(t) for each choice of barrier
type, inspection quality and maintenance quality. For each of these values the corresponding
value of the hazard score total H can be evaluated according to Table 6.2. Two examples of
these computations are shown in Figures 6.2 and 6.3. It was noted that for all cases a
correlation value of R2 > 0.82 was obtained, indicating that the hazard score total is a good
predictor of mean(t).
Manual No. 8: Termite Attack 61
61
Figure 6.2. Relationship between mean time of attack mean(t) and the hazard score total H
for the case of no termite barrier and medium frequency of inspection.
Figure 6.3. Relationship between mean time of attack mean(t) and the hazard score total H
for the case of a toxicant chemical barrier, medium maintenance frequency and high
inspection frequency.
As indicated in Section 4.2.6, the value of mean(t) is directly related to the risk of
attack. In this study, the risk of attack within a 50 year period is used for illustrative purposes.
Slices of computed data were used to assess the relationship between hazard score total H and
mean attack time mean(t), and thereby the relationship between hazard score total H and the
true risk of attack within 50 years, denoted by risk(50).
The limits of mean(t) used to choose a data slice are given in Table 6.9. For each data
slice, the mean value of the hazard score total H obtained from the data slice is given in Table
6.10. Using this data, graphs such as those shown in Figures 6.4 and 6.5 can be plotted, which
no barrier, medium inspection
0
50
100
150
0 5 10 15 20
Hazard score total H
M
ea
n(t
),
yrs
toxicant chemical, medium
maintenance, high frequency of
inspection
0
100
200
300
0 10 20
Hazard score total H
Mean
(t),
years
Manual No. 8: Termite Attack 62
62
show a roughly linear relationship between H, the hazard score total, and risk(50), the risk of
failure within 50.
On the basis of these graphs, it was decided to write the relationship between risk and
hazard score total as follows:
risk(50) = 20 + m*[H – H(20)] (6.2)
H = H(20) + [risk(50) – 20]/m (6.3)
where risk(50) denotes the probability of a termite attack within 50 years, and H(20) denotes
the hazard score total for which risk(50) = 20%
Computed values of H(20) and m are given in Table 6.11. Note that an average value of
m is taken for each type of barrier. These values were used to compute the values of risk.
Table 6.9. Description of data slices used to assess the relationship between
hazard score total and risk of attack
Data slice
No.
Risk(50)
(%)
Notation
for Hazard
score total
H
mean(t)
(yrs)
Data slice limits on
mean(t)
(yrs)
Lower limit Upper limit
1 20 H(20) 72 70 74
2 30 H(30) 63 61 65
3 40 H(40) 55 53 57
4 50 H(50) 47 45 49
Manual No. 8: Termite Attack 63
63
Table 6.10 Hazard score totals for various values of risk of attack within 50 years
Barrier Inspection
quality
Maintenance
quality H(20)* H(30) H(40) H(50)
Physical 1 9.407 10.95 12.127 13.649
barrier 2 8.573 8.538 9.58 10.369
3 4.039 5.285 6.245 7.074
Toxic 1 1 na**
na**
na**
na**
Chemical 2 1 13.299 na**
na**
na**
3 1 6.764 8.095 8.986 10.583
1 2 na**
na**
na**
na**
2 2 10.362 11.5 13.153 na**
3 2 5.908 6.695 8.065 8.692
1 3 10.917 11.963 13.553 na**
2 3 7.963 9.007 10.176 11.628
3 3 4.292 5.528 6.574 7.765
Repellent 1 1 13.443 na**
na**
na**
chemical 2 1 9.439 11.209 11.698 14.071
3 1 5.639 6.583 7.866 8.554
1 2 11.124 12.092 13.73 na**
2 2 8.211 9.299 10.571 11.821
3 2 4.61 5.833 6.777 8.025
1 3 9.068 9.803 11.253 12.269
2 3 6.934 7.762 8.789 9.881
3 3 3.939 4.884 5.908 7.028
No barrier 1 5.871 6.69 7.137 8.014
2 4.402 5.362 5.957 7.035
3 2.417 3.348 4.213 5.451
*H(20) denotes the total hazard score that will result in a risk of 20% that a termite attack will
occur within 50 years
** The notion ―na‖ denotes that even for the maximum hazard score, the target risk should
not be attained
Manual No. 8: Termite Attack 64
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Figure 6.4. Risk for the case of no barrier.
Figure 6.5 Risk for the case of a steel mesh barrier.
Physical barrier
0
5
10
15
0 20 40 60
Risk(50) (%)
Hazard
sco
re t
ota
l H
high
med
low
inspection
quality
NO BARRIER
0
2
4
6
8
10
0 20 40 60
Risk(50) %
Hazard
sco
re t
ota
l H
high
med.
low
inspection quality
Manual No. 8: Termite Attack 65
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Table 6.11. Parameters for the Risk equations
Barrier
type
Maintenance
quality
Inspection
quality H(20)* m**
Physical
high 9.407 10
barrier med 8.573 10
low 4.039 10
Toxic
high
high no limit 8
Chemical med 13.299 8
low 6.764 8
medium
high no limit 8
med 10.362 8
low 5.908 8
low
high 10.917 8
med 7.963 8
low 4.292 8
Repellent high
high 13.443 8.5
chemical med 9.439 8.5
low 5.639 8.5
medium
high 11.124 8.5
med 8.211 8.5
low 4.61 8.5
low
high 9.068 8.5
med 6.934 8.5
low 3.939 8.5
No barrier
high 5.871 11.5
med 4.402 11.5
low 2.417 11.5
* H(20) denote the total hazard parameter that will cause a risk of attack of 20% in 50 years
** m denotes the inverse slope of the risk-hazard relationship of the type shown in Figures 6.4 and
6.5.
6.5 Computing Risk
The computational procedure is based on values of hazard parameters h1 – h7 shown in Table
6.1 and the supplementary parameters I (inspection) and M (maintenance) given in Tables 6.3
and 6.4 respectively.Unless otherwise stated, all risks herein will refer to the risks of a termite
attack occurring within a 50 year period.
It was shown in the previous Section that there is a near linear relationship between the
hazard score total H and the risk of an attack occurring within 50 years, denoted by Risk(50),
which will be written as follows:
Risk(50) = 20 + m*[H – H(20)] (6.4)
where H(20) denotes the value of H when Risk(50)=20%. Values of m and H(20) for various
protection strategies are given in Table 6.6
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6.6 Acceptable Risk
One application for risk estimates is in the limitation of risk to a specified value. For example
such a limitation may be used to draft building regulations that are specific to a region of
Australia. Also, since the risk estimate may be made on a house by house basis, it may also be
used by a pest control operator who wishes to assess the risk of a house that he is treating and
then to take action so as to limit his liability exposure. Table 6.7 shows the level of
maintenance and inspection that is required to maintain the condition Risk(50)<20%.
Table 6.7. Maintenance and Inspection Quality Requirements
Barrier
type
Maintenance
quality
Inspection
quality
Limit on total
hazard score*
Physical high 9.4
med 8.6
low 4.0
Toxic high high no limit
Chemical med 13.3
low 6.8
medium high no limit
med 10.4
low 5.9
low high 10.9
med 8.0
low 4.3
Repellant high high 13.4
chemical med 9.4
low 5.6
medium high 11.1
med 8.2
low 4.6
low high 9.1
med 6.9
low 3.9
No barrier high 5.9
med 4.4
low 2.4
*hazard limit denotes the hazard value for which
the probability of attack within 50 years is 20%
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6.7 Risk Management
6.7.1 Cost Assumptions
In assessing the cost of various protection strategies, the following assumptions are made for
costs over a 50 year period.
The cost of good quality inspection is assumed to be $500 every 2 years. Combining
this information with the classifications given in Table 4 leads to the following total costs
over a 50 year period:
cost for high quality inspection = $25,000
cost for medium quality inspection = $6,000
cost for low quality inspection = $2,000
The cost of good quality maintenance for chemical treatments is assumed to be $2,000
every 5 years. Combining this information with the classifications given in Table 5 this leads
to the following total costs over a 50 year period:
cost for high quality maintenance = $20,000
cost for medium quality maintenance = $10,000
cost for low quality maintenance = $2,000
The cost of installing a physical barrier such as Granitgard or Termimesh is taken to be
$1,000.
Should a termite attack occur, the cost of the potential damage is classified as follows:
cost of low damage = $2,000
cost of medium damage = $5,000
cost of high damage = $20,000
6.7.2 Comparative Costs of Termite Protection Strategies
The effective cost of a protection strategy is taken to be given as follows:
Cost of strategy = cost of installation of physical barriers + cost of maintenance of
chemical barriers + cost of inspection +
(probability of attack)*(costs incurred if an attack occurs) (6.5)
6.8 Some Computed Examples
6.8.1 Applications to Risk Assessments
Equation (6.4) above may be used directly to assess the risk of attack within a 50 year period.
As an example, the risk for average conditions is shown in Table 6.8. The computed results
show that the risk of attack in a 50 year life covers a wide range from 1% in the case of a
toxic chemical barrier to 61% in the case of no barrier at all. This type of information is useful
Manual No. 8: Termite Attack 68
68
to the building user if he happens to be risk averse to termite attack and therefore would like
to keep the risk below a certain level, regardless of the cost required to do this.
Table 6.8. Risk of attack for average conditions of hazard
inspection and maintenance
Barrier type Risk of attack in 50 years
(%) None 61
Physical 14
Repellent chemical 18
Toxic chemical 1
Another example would be to use the risk computation to assess the termite protection
strategies to maintain risks at an acceptable, as was done for Table 6.7.
6.8.2 Applications to Risk Management
Some illustrative examples of the cost of various protection strategies are given in Tables 6.8-
6.10.
Table 6.8 shows that when no barrier is used, and the potential damage is $5000, a low
frequency of inspection is the lowest cost option. Table 6.9 shows that for the average
situation the physical barrier appears to offer the lowest cost option. Table 6.10 shows that for
situations involving low hazard and low potential damage, the no barrier and low inspection
option offer the lowest cost solution; whereas for a high hazard and high potential damage
situation, the use of a toxic chemical barrier and a low or medium inspection and maintenance
schedule offers the lowest cost strategy.
Finally, it should be noted that use of the highest quality inspection and maintenance
regime would involve a cost of $45,000 over a 50 year period. While such actions will reduce
the risk of a termite attack to negligible proportions, even in the highest hazard situations, the
cost is still larger than the amount of $20,000 which is assumed to be the value for a high
damage potential (even if there are two attacks within the next 50-year period); i.e. while the
use of high quality inspection and maintenance may be justified in terms of giving peace of
mind to a risk averse clients, it cannot be justified in terms of cost effectiveness.
Table 6.8. Cost of protection strategy for average condition when no barrier is used.
(Potential damage to a home is $5,000)
Quality of
inspection
Cost of protection strategy (x $1,000)
Low hazard Medium hazard High hazard
High 25.0 27.2 29.5
Medium 6.8 9.1 11.4
Low 4.4 6.7 9.0
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Table 6.9. Cost of a protection strategy for average conditions of hazard, inspection,
maintenance and potential damage
Barrier type Cost of protection strategy
(x $1,000)
None 9.1
Physical 7.7
Repellant chemical 16.9
Toxic chemical 16.1
Table 6.10. Comparison of the cost of protection strategies for high
and low risk situations
Barrier type Level of quality
control
Cost of strategy
[$1,000]
High hazard and
high potential
damage
Low hazard and
low potential
damage
Toxic chemical High
(inspection and
maintenance)
45 45
Medium
(inspection and
maintenance)
22.6 16.0
Low
(inspection and
maintenance)
20.8 4.9
No barrier High
(inspection) 43.1 25.0
Medium
(inspection) 27.5 6.3
Low
(inspection) 28.5 3.3
6.8.3 Comment
The above are just some examples of the applications that can be made with a Model that
produces quantified assessments of the risk of a termite attack. The Model should be viewed
as a method for placing all known expert opinion and field data into a unified theory. It is a
simple matter to incorporate other expert opinion or new field data into the Model as these
become available. Without the benefit of this type of Model to quantify risk, it is not possible
to optimise risk management strategies.
Manual No. 8: Termite Attack 70
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7 APPLICATION FOR DESIGN GUIDE
Equation Section (Next)
7.1 Procedure to compute risk
First the Hazard score H is evaluated using Tables 7.1-7.8. Then, using these hazard scores,
the value of risk(50), the probability of an attack in 50 years is evaluated using the following
equation
risk(50) = 20 + m*[H – H(20)] (7.1)
where risk(50) denotes the probability of a termite attack within 50 years, and H(20) denotes
the hazard score total for which risk(50) = 20%.
Details of the derivation of the procedure, and explanations of the various parameters
cited can be found in ―Manual No. 8 Termite Attack‖ by R.H. Leicester, C-H Wang and M.N.
Nguyen (April 2008).
7.2 Hazard score components
Table 7.1 Hazard score for location Zone
Location Zone* Hazard score
B 0
C 2
D 4
Table 7.2 Hazard score for age of suburb*
Age of suburb Hazard score
<10 yrs 0
10-70 yrs 2
>70 yrs 4
* Suburb refers to area within which at least 20%
of the land is covered by buildings.
Manual No. 8: Termite Attack 71
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Table 7.3 Hazard score for distance to nearest property boundary
Distance to nearest
boundary
Hazard score
>8 m 0
2—8 m 0.5
<2 m 1.0
Table 7.4 Hazard score for wood in Garden (and under house)
Quantity of wood in garden and
under house*
Hazard score
low 0
medium 0.5
high 1.0
Table 7.5 Hazard score for contact of house with ground
Hazard related to ground contact* Hazard score
low 0
medium 1
high 2
Table 7.6 Hazard score for type of construction material
Hazard related to type of
construction materials*
Hazard score
low 0
medium 1
high 2
Table 7.7 Hazard score for exposure conditions of timber
Hazard related to exposure
of material*
Hazard score
low 0
medium 1
high 2
Manual No. 8: Termite Attack 72
72
7.3 The Hazard Score Total
Table 7.8 Evaluation of hazard score total
Hazard factor Hazard score
Location zone
Age of suburb
Distance to boundary
Wood in garden
Ground contact
Construction material
Timber exposure
Hazard score total H =
7.3.1 Comment on Hazard Zone A (Tasmania)
Currently, Tasmania does not have subterranean termites, which damage houses and
accordingly termite management measures are not warranted.
Manual No. 8: Termite Attack 73
73
7.4 Parameters for the risk equation
The parameters m and H(20) to compute the risk of termite attack according to equation (7.1)
can be from Table 7.9
Table 7.9. Parameters for the Risk equations
Barrier
type
Maintenance
quality
Inspection
quality H(20)* m**
Physical
high 9.407 10
barrier med 8.573 10
low 4.039 10
Toxic
high
high no limit 8
Chemical med 13.299 8
low 6.764 8
medium
high no limit 8
med 10.362 8
low 5.908 8
low
high 10.917 8
med 7.963 8
low 4.292 8
Repellent high
high 13.443 8.5
chemical med 9.439 8.5
low 5.639 8.5
medium
high 11.124 8.5
med 8.211 8.5
low 4.61 8.5
low
high 9.068 8.5
med 6.934 8.5
low 3.939 8.5
No barrier
high 5.871 11.5
med 4.402 11.5
low 2.417 11.5
* H(20) denote the total hazard parameter that will cause a risk of attack of 20% in 50 years
** m denotes the inverse slope of the risk-hazard relationship.
7.5 Acceptable Risk
One application for risk estimates is in the limitation of risk to a specified value. For example
such a limitation may be used to draft building regulations that are specific to a region of
Australia. Also, since the risk estimate may be made on a house by house basis, it may also be
used by a pest control operator who wishes to assess the risk of a house that he is treating and
then to take action so as to limit his liability exposure. Table 7.10 shows the level of
maintenance and inspection that is required to maintain the condition Risk(50)<20%.
Manual No. 8: Termite Attack 74
74
Table 7.10. Maintenance and Inspection Quality Requirements
Barrier
type
Maintenance
quality
Inspection
quality
Limit on total
hazard score*
Physical high 9.4
med 8.6
low 4.0
Toxic high high no limit
Chemical med 13.3
low 6.8
medium high no limit
med 10.4
low 5.9
low high 10.9
med 8.0
low 4.3
Repellant high high 13.4
chemical med 9.4
low 5.6
medium high 11.1
med 8.2
low 4.6
low high 9.1
med 6.9
low 3.9
No barrier high 5.9
med 4.4
low 2.4
*hazard limit denotes the hazard value for which
the probability of attack within 50 years is 20%
Manual No. 8: Termite Attack 75
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8 APPLICATION FOR TIMBERLIFE
Equation Section (Next)
8.1 Procedure to compute risk
First the Hazard score H is evaluated using Tables 8.1-8.8. Detailed definitions of the hazards
have been given in Section 6.3. Then, using these hazard scores, the value of risk(50), the
probability of an attack in 50 years is evaluated using the following equation
risk(50) = 20 + m*[H – H(20)] (8.1)
where risk(50) denotes the probability of a termite attack within 50 years, and H(20) denotes
the hazard score total for which risk(50) = 20%.
The risk can also be combined with the costs associated with the termite management
strategy and the cost of failure, should such a failure occur according to the equation (8.2)
given in Section 8.5
Details of the derivation of the procedure, and explanations of the various parameters
cited can be found in ―Manual No. 8 Termite Attack‖ by R.H. Leicester, C-H Wang and M.N.
Nguyen (April 2008).
8.2 Hazard score components
Table 8.1 Hazard score for location Zone
Location Zone* Hazard score
B 0
C 2
D 4
Table 8.2 Hazard score for age of suburb*
Age of suburb Hazard score
<10 yrs 0
10-70 yrs 2
>70 yrs 4
* Suburb refers to area within which at least 20%
of the land is covered by buildings.
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Table 8.3 Hazard score for distance to nearest property boundary
Distance to nearest
boundary
Hazard score
>8 m 0
2—8 m 0.5
<2 m 1.0
Table 8.4 Hazard score for wood in Garden (and under house)
Quantity of wood in garden and
under house*
Hazard score
low 0
medium 0.5
high 1.0
Table 8.5 Hazard score for contact of house with ground
Hazard related to ground contact* Hazard score
low 0
medium 1
high 2
Table 8.6 Hazard score for type of construction material
Hazard related to type of
construction materials*
Hazard score
low 0
medium 1
high 2
Table 8.7 Hazard score for exposure conditions of timber
Hazard related to exposure
of material*
Hazard score
low 0
medium 1
high 2
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77
8.3 The Hazard Score Total
Table 8.8 Evaluation of hazard score total
Hazard factor Hazard score
Location zone
Age of suburb
Distance to boundary
Wood in garden
Ground contact
Construction material
Timber exposure
Hazard score total H =
8.3.1 Comment on Hazard Zone A (Tasmania)
Currently, Tasmania does not have subterranean termites, which damage houses and
accordingly termite management measures are not warranted.
Manual No. 8: Termite Attack 78
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8.4 Parameters for Evaluating the Risk Equation
As an example, Table 8.9 shows the inspection and maintenance regimes that must be put in
place to ensure that the risk of termite attack is no greater than 20% in a 50 year period.
Table 8.9. Parameters for the Risk equations
Barrier
type
Maintenance
quality
Inspection
quality H(20)* m**
Physical
high 9.407 10
barrier med 8.573 10
low 4.039 10
Toxic
high
high no limit 8
Chemical med 13.299 8
low 6.764 8
medium
high no limit 8
med 10.362 8
low 5.908 8
low
high 10.917 8
med 7.963 8
low 4.292 8
Repellent high
high 13.443 8.5
chemical med 9.439 8.5
low 5.639 8.5
medium
high 11.124 8.5
med 8.211 8.5
low 4.61 8.5
low
high 9.068 8.5
med 6.934 8.5
low 3.939 8.5
No barrier
high 5.871 11.5
med 4.402 11.5
low 2.417 11.5
* H(20) denote the total hazard parameter that will cause a risk of attack of 20% in 50 years
** m denotes the inverse slope of the risk-hazard relationship of the type shown in Figures C3–C5.
8.5 Risk Management procedure
8.5.1 Cost Components
In assessing the cost of various protection strategies, the following assumptions are made for
costs over a 50 year period.
Manual No. 8: Termite Attack 79
79
The cost of good quality inspection is assumed to be $500 every 2 years. Combining
this information with the classifications given in Table 4 leads to the following total costs
over a 50 year period:
cost for high quality inspection = $25,000
cost for medium quality inspection = $6,000
cost for low quality inspection = $2,000
The cost of good quality maintenance for chemical treatments is assumed to be $2,000
every 5 years. Combining this information with the classifications given in Table 5 this leads
to the following total costs over a 50 year period:
cost for high quality maintenance = $20,000
cost for medium quality maintenance = $10,000
cost for low quality maintenance = $2,000
The cost of installing a physical barrier such as Granitgard or Termimesh is taken to be
$1,000.
Should a termite attack occur, the cost of the potential damage is classified as follows:
cost of low damage = $2,000
cost of medium damage = $5,000
cost of high damage = $20,000
8.5.2 Effective Cost of Termite Protection Strategies
The effective cost of a protection strategy is taken to be given as follows:
Cost of strategy = cost of installation of physical barriers + cost of maintenance of
chemical barriers + cost of inspection +
(probability of attack)*(costs incurred if an attack occurs) (8.2)
Manual No. 8: Termite Attack 80
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References
Cookson, L.J. and Trajstman, A. 2002. Termite Survey and Hazard Mapping. CSIRO Forestry
& Forest Products Report No. 137.
http://www.ensisjv.com/ResearchCapabilitiesAchievements/WoodProductsProcessingandProt
ection/WoodProcessing/TermiteHazardMapping/tabid/369/Default.aspx
Commonwealth of Australia, Ecologically Sustainable Development Working Groups. Final
Report — Agriculture. Australian Government Publishing Service, Canberra, 240 pp.,
1991.