Building technology

Chapter-4 Roofs

4.1 Roofs & their types4.2 Timber roofs (Single/double/ multiple timber roofs)4.3 Steel trusses and their components (Angle & tubular truss)4.4Roof coverings

4.1 Roofs & their types

ROOF• Roof may be defined as a covering provided

over the top of a building • to protect the building from the atmospheric

effects (like rain, snow, sun and wind )

Two main component of building roof

i. Structural element • Design for supporting roof covering• Should be strong so that it should carry the

load• May be trusses, portals, beams etc

Two main component cont….

ii. Roof covering• Layer of covering on the structural element• Should be water proof , fire resistance• May be tiles, galvanized corrugated, iron

sheets, slates etc

The functions of any roof

• To keep out rain, wind, snow and dust• To prevent excessive heat loss in winter• To keep the interior of the building cool in

summer• Designed to accommodate all stresses

encountered

The functions cont….

• Designed to accept movement due to changes in temperature and moisture content

• For strength and stability of building• For durability and free from maintenance

The functions cont….• For fire safety• Provide resistance to the passage of sound• Safety to occupants• Aesthetic beauty

Technical Terms of Roof

Technical Terms of Roof cont…

1. Span: The clear distance between the supports if

an arch, beam or roof truss2. Rise of roof: It is the vertical distance between the wall

plate and the top of the ridge


• Pitch of roof: - inclination of the sides of roof to the

horizontal is termed as pitch of roof - expressed either in terms if degree (angle)

or as a ratio of rise to span


• Ridge: -the spine of the roof and is essentially a

pitching plate for the rafters which are nailed to each other through the ridge board


• Hip: - the ridge formed by the intersection of two

sloping surfaces, where the exterior angle is greater than 180°


• Valley: - a reverse of a hip - formed by the intersection of two roof

surfaces, making an external angle less than 180°


• Common rafters: - main load bearing members of a roof are

common rafters - span between wall plates at eaves level and

the ridge


• Jack rafters: - are the rafters shorter in length which run

from hip or valley to the eaves.


• Hip rafters: -the sloping rafters which form the hip of a

sloped roof -They run diagonally from the ridge to the

corners of the walls to support roof coverings -They receive the ends of the purlins and

ends of jack rafters


• Valley rafters: -These are the sloping rafters which run

diagonally from the ridge to the eaves for supporting valley gutters

-They receive the ends of the purlins and ends of jack rafters on both sides


• Wall plates: -long wooden members which are provided

on the top of the stone or brick wall -for the purpose of fixing the feet of the

common rafters - Wall plates actually connect the walls to the

roof


• Dragon ties: - the tie placed across the corners and over

the wall plates and helps to provide resistance the truss of a hip rafter


• Ceiling joist: - the wooden member that functions as tie to

the feet of the rafters and provides support to the ceiling boards on the undersides and any cistern housed within the roof void


• Purlin: - Are horizontal wooden or steel members,

used to support common rafters of a roof when span is large

-Purlins are supported on trusses or walls


• Struts: - Are the compressive members that transfer

load of purlin to a suitable load bearing support within the span of roof


• Eaves: -Lower edge of roof• Template: - Bedding block provided at the end of truss• Verge: - Edge of roof coverings, slates, CGI sheets

or tiles on the roof.


• Cleats: -Are small pieces of wood fixed on rafter to

prevent purlin to slide down.• Battens: - Are the wooden members fixed on rafter

to provide support to the roof coverings: slates tiles etc.

Types of Roof

• Pitched or Sloped roof • Flat roof• Shelled roof• Doomed roof

Pitched or Sloped roof :

• Roof with sloping surface• considered suitable for buildings in areas

where rainfall/snowfall is very heavy• Normally the sloped roof has the slope from

10⁰ to 70⁰

Pitched or Sloped roof cont…

• Pitched roofs are basically of the following forms:

Lean-to-roofGable roof Hip roof Gambrel roofMansard or curb roofDeck roof

Pitched or Sloped roof cont…

Flat roof

• Are considered suitable for buildings in plains or in hot regions where rainfall is scanty

• Are either horizontal or practically horizontal with slope less than 10

• Even a perfectly horizontal roof has to have some slope at top so that rain water can be drained off easily and rapidly

Flat roof cont…

Shelled roof

• Type of roof that has thin section

• May be constructed in curved surface such as parabola, hyperbola or segment of curve

Shelled roof

Domed roof:

• Curved roof having the roof area more than semicircle

• Has largest roof area compare to all types of roofs

• Can be constructed by the use of R.C.C or arch technology

Domed roof cont…

• Care must be taken in the construction as it requires high skill

4.2 Timber roofs (Single/double/ multiple timber roofs)

• Single roofs: -Those which consist of only the rafters which

are supported at the ridge and at the eaves - Such roofs are used only when the span is limited to 5 meters -otherwise the size of the rafters will be uneconomical.

Single roofs cont…

Single roofs are of five types: • Lean-to-roof• Couple roof• Couple close roof• Collar beam roof• Collar and scissor roof


• Lean-to-roof: -The simplest type of sloping roof in which

rafters slope to one side only -Suitable for maximum span of 2.5m -Are provided for sheds, out-houses attached

to main building, verandahs etc

Lean-to-roof cont…


• Couple roof: -Roof is formed by couple or pair of rafters

which slope to both the sides of the ridge of the roof

-Used when the span is limited to 3.6 meters

Couple roof cont…


• Couple close roof : -Similar to the couple roof , except that the

ends of the couple of common rafters is connected by horizontal member called tie beam to prevent the rafters from spreading and thrusting out of the wall

-A couple-close roof is economically suitable for spans up to 4.2 m

Couple close roofs cont…


• Collar beam roof: -When the span increases, or when the load is

more, the rafters of the couple close roof have the tendency to bend -Avoided by raising the tie beam and fixing it at one-third to one-half of the vertical height from wall plate to the ridge -This raised beam is known as the collar beam (collar tie)

Collar beam roofs cont…-This roof is suitable for spans up to 5 meters


• Collar and scissor roof:

-Similar to the collar roof, except that two collar beams crossing each other to have an appearance of scissors is provided as shown in the fig

Timber roofs cont…

• Double or Purlin roofs -A double roof is the one in which purlins are

introduced to support the common rafters at intermediate point

-Such roofs are used when the span exceeds 5meters

- The function of purlin is to tie the rafters together and to act as an intermediate support to the rafters

Double or Purlin roofs cont…

Triple-membered or framed or trussed roofs:

• Consists of three sets of members: i) common rafters ii) purlins and iii) trusses

• The purlins which give an intermediate support to the rafters, are themselves supported on trusses which are suitably spaced along the length of a room

• Provided when the span of the room is greater than 5 meters and when the length of the room is large, i.e. where there are no internal walls or partitions to support the purlins

Triple-membered or framed cont…..

• Various types of Triple membered or framed or trussed roof are :

1) King-Post Truss 2)Queen-Post Truss 3)Combination of King-Post and queen post

Trusses4)Mansard Truss 5)Truncated Truss6)Bel-fast Truss 7)Steel Trusses8)Composite Trusses

Triple-membered or framed cont….

1)King-Post Truss:• A King-Post consists of following components:• Two inclined principal rafters• Lower tie beam• Two struts • King post

King-Post Truss cont….

• Principal rafters support the purlins• Purlins support the closely-spaced common

rafters which have the same slope as the principal rafters

• Common rafters support the roof covering as usual.

• Suitable for spans varying from 5 to 8meters

King-Post Truss cont….


2)Queen-Post truss : • Differs from a king post truss in having two

vertical posts rather than one• Vertical posts are known as queen-post, the

tops of which are connected by a horizontal piece known as straining beam

• Are suitable for spans between 8 to 12 meters.

Queen-Post truss cont….

Queen-Post truss cont…


3)Combination of King-Post and Queen-Post Trusses:• Queen-Post trusses are suitable for spans up to

12meters• For greater spans the queen-post truss can be

strengthened by one or more upright member called princess-post to each side

• ( fig a) and fig (b) shows the resulting combination of King-Post and queen-post trusses, which are suitable up to 18m span.

Combination cont….


4) Mansard roof truss: • Combination of King-Post and queen-Post

trusses• A two-storey truss, with upper portion

consisting of king-post truss and the lower portion queen post truss

Mansard roof truss cont…

Mansard roof truss cont….


5)Truncated truss:• Similar to Mansard truss, except that its top is

formed flat with a gentle slope to one side.

Truncated truss cont….


6) Bel-fast roof truss ( Bow string truss) • In the form of a bow, consists of thin sections

of timber, with its top chord curved• If the roof covering is light, this roof truss can

be used up to 30m span.

Bel-fast roof truss cont….


7) Composite roof trusses: • Roof trusses made of two materials such as

timber and steel • Some common types of composite roof

trusses, using fittings such as C.I. head, C.I shoe, steel angle bolts and straps etc

Composite roof trusses cont….


8) Steel Truss :• Used for span more than 10m• Commonly used these days since they are more

economical, easy to construct or fabricate , fire proof , more rigid and permanent

• Consists of rolled steel sections welded and bolted in gusset plates and may be tube (pipe) or angles

4.3 Steel trusses and their components (Angle & tubular truss)

• Steel roof Truss – Angular:• Angle sections and plates are commonly used for

fabrication though channel sections and T-sections can be also used

• Designed that the members carry only direct stresses, (i.e., either compression or tension) and no bending stress are induced

• Principal rafter as well as the main tie are generally made of two angle sections placed side by side, while the struts and ties are generally made of single angle sections

Steel trusses cont….

• Members are jointed together, using a gusset plate, either through rivets or by welding

• Minimum pitch should not be less than three times the rivet diameter, while the maximum pitch is limited to 15cm for compression members and 20 cm for tension members

• Generally, 15 mm diameter rivets are used for small spans and 20mm rivets are used for large spans


• At least two rivets should be used at each joint• Gusset plate should not be less than 6mm,

though its thickness is designed on the basis of forces carried by members to be jointed

• At the foot of the truss, short angles are fitted on both the sides of the gusset plate, which are connected to the bearing plate


• The bearing plate is jointed to concrete bed through rag bolts

• At the apex, suitable ridge section is fitted


• Steel roof Truss – Tubular:

Steel roof Truss – Tubular cont….

• Tubular Steel roof trusses are used for large span constructions such as factories, industry work sheds, shopping malls, huge exhibition centers, multiplexes etc

• Are generally used for spans as large as 25-30m

• Are quite popular since they are light weight and economical


• Since external loads are transferred to a truss at the joints, various members of a truss are subject only to direct stresses, either tensile or compressive

• However, purlins are designed as flexural members

• Various members of a roof truss are jointed together usually by welding


• usually fabricated in three parts, transported and then assembled at the site

• As shown in fig a simple triangular truss is fabricated in three parts: Part ABC, Part EBD and Part CD


• In order to support purlins, rectangular plates (with holes) are also welded to the principal rafter at the joints (such as F and G) , and these pieces are kept projected

• The tubular purlins are provided with holes at the ends with internal threads

• Studs are placed through the holes in the projected rectangular plates and tightened in the internally threaded ends of purlins

4.4 Roof coverings

I. ThatchII. Wood SinglesIII. Tile roofing IV. CGI-sheetV. AC-sheet (Asbestos Cement)VI. SlatesVII.Light weight roofing

Roof coverings cont….

i)Thatch: • Organic material available in nature and used

for the roof.• Sometime, the rice, and wheat straw are also

used in the roof• But the thatch is special grass available in wild

land


• Simply tied tightly in a bunch in to the battens in closely distanced about 30cm) layers

• Thickness may be from 10cm to 15cm• Used largely in rural areas and slope of this

roof is normally more than 40°• Thatch is combustible, absorbs moisture and is

liable to rot or decay


ii) Wood Singles: • Shingles are thin slabs of wood used to cover

roofs• The use of shingles is restricted to hilly areas

where local timber is easily available at low cost


iii) Tile roofing :• Tiles are manufactured from clay and concrete

to a wide range of design • colors suitable for the pitches from 20°to45°

and work upon the principle of either double or simple lap

• The pitch of the tile is always less than pitch of rafters


• The standard size of plain tile is 265X165 mm and thickness of minimum 10mm

• Tiles are somehow raised to drain out the rain water and raised part is called camber

• It prevents the rainwater being drawn up between the tiles by capillary action, as it would be if the tiles were absolutely flat


• The tiles while fixing on the roof have to be overlapped and hung in to the battens in a definite interval

• Overlapping is called LAP and the interval is called GAUGE

• Gauge= (Length of tile-Lap)/2• Example. For 265mm length and lap of 65mm,

gauge, G=(265-65)/2=100mm


iv) Galvanized Iron corrugated sheets (C.G.I sheets):

• CGI sheets are very much popular these days in different region of Nepal

• Relatively cheap as well and very easy to fix on the roof

• The rafter is fixed to wall plate and the ridge and purlin is laid on it


• The C.G.I sheets are nailed or bolted to purlins• Eaves board is nailed to the ends of rafter• The slope of roof for CGI sheets should be

more than 25° and lapping 15cm on edge & 1 & ½ corrugation at side


v) Asbestos cement sheets ( A.C sheets) • AC sheets are produced by the mixture of

cement, asbestos fibers and water• Cheap, light, impervious, durable and fire

resistant• Available in 2 to 3 meters in length and has

the same procedure as CGI sheet to fix on the roof


vi) Slate roofing: • Slates may be of different types and may be

fixed on the roof in different ways as well• For the method of fixing, slates are nailed to

the battens that are fixed on planks or direct on rafters

• Slates are overlapped by 50-75%.


vii)Light weight roofing :• For wide span industrial structures, it is

desirable to reduce the weight of roof so that structural framing can be economized

• Conventional roofing materials (such as tiles, slates etc) are heavy and required heavy framing to support them


• The light weight roofing materials are of two types:

a) Sheeting• Aluminum sheets• Asbestos cement sheets


b) Decking • Wood wool• Straw board • Aluminum alloy and steel decking.

•THANK YOU

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