Timber Strength & Spans A4Build_1
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Timber strength and spans How timber is graded Strength grading is needed to ensure timber is strong enough for a particular job, e.g. a floor joist or a roof truss. It also saves money by helping to avoid over-specification. Grading can be carried out visually or by machine. Visual strength grading uses the grader’s experience across a number of different factors to predict the load a piece of timber will be able to carry in service. Machine strength grading is best suited to high volumes of timber where the species and cross section are not changed very often. When a piece of timber is cut, for example when it is divided into smaller pieces, it has to be re-graded. Timber used for structural applications must be graded and clearly marked to show it complies with the correct standards and strength requirements laid down by building codes and regulations. The load a member can carry depends on several factors, including its span, thickness, width and species. Usually the deeper/wider the section, the longer the span. Different species have different strength properties – a factor that needs to be considered when choosing your timber. Produced by TRADA, the Timber Research and Development Association (www.trada.co.uk) in conjunction with wood for good (www.woodforgood.com) This information sheet provides general advice only and is not specific to the requirements of a particular building project. It is the builder’s responsibility to check compliance with building regulations and standards. © TRADA Technology and wood for good.
Transcript of Timber Strength & Spans A4Build_1
Timber strengthand spans
How timberis gradedStrength grading is needed to ensure timber isstrong enough for a particular job, e.g. a floorjoist or a roof truss. It also saves money byhelping to avoid over-specification. Gradingcan be carried out visually or by machine.Visual strength grading uses the grader’sexperience across a number of different factorsto predict the load a piece of timber will beable to carry in service. Machine strengthgrading is best suited to high volumes oftimber where the species and cross section arenot changed very often. When a piece oftimber is cut, for example when it is dividedinto smaller pieces, it has to be re-graded.
Timber used for structural applicationsmust be graded and clearly marked to showit complies with the correct standards andstrength requirements laid down bybuilding codes and regulations.
The load a member can carry depends onseveral factors, including its span, thickness,width and species. Usually the deeper/widerthe section, the longer the span.
Different species have different strengthproperties – a factor that needs to beconsidered when choosing your timber.
Produced by TRADA, the Timber Research and Development Association (www.trada.co.uk) in conjunction withwood for good (www.woodforgood.com)
This information sheet provides general advice only and is not specific to the requirements of a particular building project. It is the builder’s responsibility to check compliance with buildingregulations and standards. © TRADA Technology and wood for good.
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Produced by TRADA, the Timber Research and Development Association (www.trada.co.uk) in conjunction withwood for good (www.woodforgood.com)
This information sheet provides general advice only and is not specific to the requirements of a particular building project.It is the builder’s responsibility to check compliance with building regulations and standards. © TRADA Technology and wood for good.
Sustainable timberTimber is the most sustainable building productavailable. It is naturally renewable - over 97% ofsoftwood timber used in the UK comes fromEurope, where the forest area is increasing by theequivalent of 3 football pitches every hour of theday and night.* For reassurance for softwoodsand hardwoods look for certification labels likeFSC (Forest Stewardship Council) or PEFC(Programme for the Endorsement of ForestCertification). Always ask your local branchabout their responsible purchasing policies.
* IIED & ECCM, Using Wood to Mitigate Climate Change, 2004and UNECE-FAO, State of the World’s Forests, 2007.
Visual strengthgradingVisual strength grading rules define the size, typeand number of strength-reducing characteristicsallowed in each grade (for example, naturalfeatures such as knots, wane and slope of grain,plus splits and shakes which may have developed asa result of drying).
Machine strengthgradingMachine grading is based on the relationshipbetween strength and stiffness. The machinegrades each piece and stamps it with theappropriate mark.
The grader assesses each piece and stamps it withthe appropriate mark.
An additional visual assessment takes account ofstrength-reducing characteristics not automaticallysensed by the machine.
The rules governing strength grading andstructural uses of timber are laid down in Britishand European standards:
BSEN 14081-4: 2005 covers strength gradedstructural timber, machine grading. It refers toBS 4978:2001 for softwood visual grading rules.BS 5268 – The structural use of timber andBS 5268-2 Code of practice for permissible stressdesign, materials and workmanship are theengineer’s design code for timber and itsstructural uses.
The European Standard EN 1995-1-1:2004, orEurocode 5: Design of Timber Structures, isalready in use and will replace BS 5268 when theBritish Standards Institution (BSI) stops supportingthe British code in around 2010.
Wane: uneven edge caused by a residue of bark.
Shakes: fissure caused by the splitting away ofthe wood fibres along the grain.
Certification bodyor logo mark
Companyreference
Species orspecies group
Standard reference Timber condition: DRY Strength class
Certification bodyor logo mark
Companyreference
Species orspecies group
Standard reference Timber condition: DRY Machine graded
What do the strength stamps and grades tell me?Strength grading is a way of assessing the strength of a piece of timber, which depends on its species aswell as its grade. A low grade timber from a strong species may be equal in strength to a high grade timberfrom a weaker species. To make specifying easier, species and grades are grouped into strength classes ofsimilar strength. Strength classes range from C14 to C50 for softwood and D30 to D70 for hardwoods. Thehigher the number, the stronger the timber. The most common grade for softwood carcassing is C16.
12163 TRAInSheetSheetTimberStghSpansA4Build:1 4/11/08 13:58 Page 2
Produced by TRADA, the Timber Research and Development Association (www.trada.co.uk) in conjunction withwood for good (www.woodforgood.com)
This information sheet provides general advice only and is not specific to the requirements of a particular building project. It is the builder’s responsibilityto check compliance with building regulations and standards. © TRADA Technology and wood for good.
Grades and strength classesThe table below shows how some of the softwood species and grades most commonly used in UK constructionare grouped into strength classes.
Timber marked as DRY is graded at an averagemoisture content of 20% and should betransported, stored and installed in the building ina manner that does not allow this moisture contentto be exceeded.
Note: Although moisture content of 20% or less ispermissible for internal use, further shrinkage islikely as the timber dries in service. For example,timber joists used in intermediate floors may dry toaround 12% moisture content.
The species is shown on the grading stamp by acode such as WPNN (British Corsican/Scots Pine),PNSY (European Redwood), or WPCA (EuropeanWhitewood).
GS – General structural and SS Special structural arethe visual grades of the timber assigned by thegrader.
A typical supplier’s stamp may look like this:
The Chain of Custody (COC) number demonstratesthat the timber is from a legal and sustainablesource. North American graded timber is acceptedin Europe under BS EN14081 and may be markedJ&P (joist and plank grade) or NAMSR (NorthAmerican machine stress-rated lumber).
SOURCE AND SPECIES GRADING RULES STRENGTH CLASSES
C14 C16 C18 C22 C24 TR26 C27
BRITISH
British Pine BS4978 visual GS SS
BS EN14081 parts 1-4 machine • • • • • •
British Spruce BS4978 visual GS SS
BS EN14081 parts 1-4 machine • • • •
Douglas Fir BS4978 visual GS SS
BS EN14081 parts 1-4 machine • • • •
Larch BS4978 visual GS SS
BS EN14081 parts 1-4 machine • • • • • •
SOURCE AND SPECIES GRADE RULES STRENGTH CLASSES
C14 C16 C18 C22 C24 TR26 C27
EUROPEAN
Redwood/Whitewood BS4978 GS SS
BS EN14081 parts 1-4 machine • • • • • •
Licencenumber
Standardnumber
Lic No 1691BS EN 14081
UK104KD C16
B/SBM TRADA
TT-CoC-1502FSC “Supplier”
Machinenumber
UK CoCnumber
Species ‘BritishSpruce’ abbreviation
Kilndried
Strengthclass
Certification bodyand mark
SupplierFSClogo
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Use span tables to determine the size of a timbermember of a particular strength class required fora given span. They should also tell you what themaximum spacing should be between eachsection or timber member.
The following examples which apply to solidtimber members show how to use span tables fordomestic floor joists at 400mm centres.
How do I use spantables in relationto strength classes?
Changing to 600mm centres will change the spandistances for the same timber sizes.
So if you wanted to span 4 metres at 600mmcentres, you could use 38 x 220mm C24 instead of50 x 220 C16, a lighter weight option.
Tip: Span tables showing timber sizes andstrengths can be obtained from www.trada.co.uk.
Spans for engineeredwood products andI-joistsThe introduction of specially engineered timberproducts, such as glulam beams, I-joists and metalweb joists provides solutions across a wide rangeof depths and spans, and shrinkage afterinstallation is less likely.
When using these products please refer to themanufacturer’s own literature and span tables.
Further informationand adviceThe structural use of timber is a specialist topic.This leaflet only looks at general situations.For further information visit www.trada.co.ukor www.woodforgood.com or contact yourlocal branch.
aWOLSELEY company
For further information telephone 0800 529 529 or visit our website at www.timbercenter.co.ukAll products featured in this publication are subject to availability and may vary at participatingbranches. Prices and information contained in this publication are considered to be correct at the timeof going to print, but Build Center reserve the right to change those prices and information at anytime without notice. Pictures and photographs are used for approximate representation only. E&OE.
For more information or details of yournearest Timber Center
call: 0800 529 529or visit www.timbercenter.co.uk
Ref: 5378
Produced by TRADA, the Timber Research and Development Association (www.trada.co.uk) in conjunction withwood for good (www.woodforgood.com)
This information sheet provides general advice only and is not specific to the requirements of a particular building project.It is the builder’s responsibility to check compliance with building regulations and standards. © TRADA Technology and wood for good.
Size of joist C16 clear C24 clear(mm) maximum maximum
span (m) span (m)
38 X 97 1.84 2.03
47 X 97 2.10 2.31
38 X 220 4.46 4.75
47 X 220 4.82 5.07
Size of joist C16 clear C24 clear(mm) maximum maximum
span (m) span (m)
38 X 97 1.31 1.66
47 X 97 1.68 1.90
38 X 220 3.67 4.17
47 X 220 4.20 4.58
Choose and Use is a series of information sheets for builders producedby TRADA, The Timber Research and Development Association.
They offer up-to-date advice on how to select the right timber and timberproducts for different applications.
You can often save time and money by choosing the correct timber material ortimber products as well as ensuring you comply with current BuildingRegulations and Building Codes. For more information about specific productsvisit www.trada.co.uk or contact your local supplier.
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