Roof Truss Installation Guide

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Installation Guidelines for Timber Roof Trusses May 2009

Transcript of Roof Truss Installation Guide

Page 1: Roof Truss Installation Guide

Installation Guidelines forTimber Roof Trusses

May 2009

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CONTENTSSECTION 1 SCOPE AND GENERAL

1.1 GENERAL1.2 APPLICATION1.3 DOCUMENTATION1.4 SERVICES1.5 ANCILLARY TIMBER1.6 TRANSPORT AND STORAGE1.7 SAFETY

SECTION 2 SUPPORTING STRUCTURE2.1 GENERAL2.2 LOAD-BEARING WALLS2.3 NON-LOAD-BEARING WALLS

SECTION 3 TRUSS INSTALLATION3.1 GENERAL3.2 SET OUT3.3 LIFTING3.4 FIRST TRUSS3.5 SUBSEQUENT TRUSSES3.6 ERECTION BRACING AND TOLER-

ANCES

SECTION 4 ROOF BRACING4.1 GENERAL4.2 BATTENS4.3 TOP CHORDS4.4 BOTTOM CHORDS4.5 WEBS

SECTION 5 TRUSS CONNECTIONS5.1 GENERAL5.2 HIP ENDS5.3 GIRDERS5.4 VALLEYS5.5 OVERHANGS

SECTION 1 SCOPE AND GENERAL

1.1 GENERALTimber roof trusses are engineered and manufactured toan exacting standard which require special handling, erec-tion and bracing techniques. To ensure that the expectedperformance is achieved, it is imperative that all relevantparties are familiar with the requirements set out in thisdocument.

The guidelines in this document are a sub-setof the full Australian Standard® AS4440-2004 “In-stallation of nailplated timber trusses”, and assuch should be read in conjunction with thisStandard.

1.2 APPLICATIONThis document intends to apply to nailplate timber trusseswithin the following general limitations :-

a) Residential structures and light commercialstructures

b) Maximum roof pitch 45 degreesc) Maximum truss span 16md) Maximum design wind speed of 70m /sec for

limit state design methods.

1.3 DOCUMENTATIONIt is the builder’s responsibility to supply all of the relevantinformation required for the truss designs. It is recommendedthat the fabricator confirms all details prior to manufacture.

1.3.1 LayoutA roof truss layout must be determined prior to erection,with the following points considered :a) Check dimensions of the supporting structure prior to

manufactureb) Check the truss layout to identify the trussesc) Check that the supporting structure is adequate for

the loads to be applied, especially where girder trussesare located, and lintels

d) Check that information on roof bracing, bottom chordbracing (including the need for BC ties, if applicable)and any other applied loading on the roof is provided.

e) Check that information on truss to truss connections,tie-downs, web-ties, scabs etc is provided clearly onthe layout.

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Pryda Australia acknowledges Standards Australia forpermission to reproduce some of the drawings andtechnical content from within AS4440-2004.

Typical Truss Layout

Gable End

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1.4 SERVICES1.4.1 Hot Water ServicesHot water services are best supported directly on internalwalls, but where this is not possible, and the trusses arerequired to carry this load, the trusses either side of theservice may be spaced up to double the normal spacing,up to a maximum of 1800mm, with a double laminatedtruss either side of this service. In this case the servicemust be located midway between these two double trusses,sitting on bearers located within 300mm of a truss bottomchord panel point.1.4.2 Solar Hot Water ServicesFor solar powered hot water services, with internally or ex-ternally mounted water storage tanks, refer to the trussfabricator for appropriate details.

1.5 ANCILLARY TIMBERCheck that adequate ancillary timber is available whererequired (eg for temporary bracing).

1.6 TRANSPORT AND STORAGEDuring transportation, in either the flat or upright positions,the trusses must be fully supported, taking care whilst ty-ing down to avoid putting undue strain on the truss mem-bers. Trusses should be stored on the job site clear of theground and kept flat to avoid distortion. All trusses shouldbe inspected on delivery, and any damaged trusses re-ported to the fabricator immediately to ensure correct rec-tification.

Trusses must not be modified on site without the priorwritten approval from the fabricator.

1.7 SAFETYThe installation of timber roof trusses must adhere to therelevant safety work practices for the general constructionof roofs. This will require that barriers or safety lines beinstalled at the appropriate time. These systems must notmodify the trusses, not put loads onto the trusses, unlessprior written approval is provided by the fabricator.

SECTION 2 SUPPORTING STRUCTURE

2.1 GENERALTo ensure a satisfactory roof line on completion, the sup-porting structure must be plumb, and the load bearing topplates level. It is particularly important to maintain levelwhen the trusses are supported on more than two walls. Ifa camber had been inadvertently provided at an internalsupporting wall, insert packers prior to loading to ensurefully bearing at this support.

Care must be taken to ensure that the supporting structureis adequately designed and stable in its own right.All trusses are to be fixed to the supporting structure withappropriate fixings, typically Pryda Multigrips, etc. Theselection of the fixing depends on the magnitude of thewind uplift. Refer to Section 5 for details.

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Hip End

Dutch Hip EndNotes:

1. S/F in the above illustrations refers to structuralfascia, which is mandatory at corners.

2. The immediate support structure of trussesshould be checked for adequacy.

3. Refer Section 5 for all connection details andSection 4 for bracing information.

Multigrip (MG)

Cyclone Strapfixed to side eg:QHS6Cyclone Strap wrapped

under eg: QHS6(wr)

4 nails

4 nails

2 nails

4 or 6 nailsper leg asspecified

4 nails per legto underside

2 nails per leg

2 nails per leg

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2.2 LOAD-BEARING WALLSThis is where the full load from the roof trusses is supportedon walls - generally the exterior perimeter walls, but mayoccasionally also include some internal walls.

Refer to AS1684 –2006 “Residential Timber-framed Con-struction” for the plate, stud and lintel sizes and installationdetails. Heavy, concentrated loads such as from girdersand truncated girders are best supported directly over studs,with special attention given to the load pathway all the waydown the structure to the foundations.

2.2.1 LintelsLintels must be correctly sized and supported in order tolimit deflection. Lintels supporting heavy, concentrated loadswill require a specific engineering design.

2.3 NON-LOAD-BEARING WALLSNon-load-bearing walls shall not carry any truss loading atany time, and shall not be packed to touch the undersideof the truss. It is common to set non-load-bearing wallslower than the supporting walls by an amount equal to thedepth of the ceiling battens plus 10mm. The truss is stillrequired to stabilize the top of the wall, and this is done byusing Pryda Partition Hitches which are nailed near the topof the vertical slots. Do not embed the nail heads fully home,as we must allow the truss to settle downwards as timepasses and the camber comes out of the truss.

If internal walls are required for support, the truss it-self and the layout shall be marked accordingly, andthis intention must be made very clear. The support-ing structure, including footings, should be checkedby the builder to allow for this load.For a wall that is not designated as a bracing unit,the top plateof such walls shall be stabilised at 1800mm centresmaximum..

b) Non-bracing wall

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2.3.1 Fixing to top plates of non-load-bearing wallsa) Bracing wall

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The following recommendations are guidelines only, as thedetails are the responsibility of the roof truss erectors. Alsorefer to the Australian Standard® AS4440-2004 “Installa-tion of nailplated timber trusses” for greater detail.

3.2 SET OUTPrior to lifting any truss into place, it is often convenient tomark out the truss locations on the top plate, using thesupplied truss layout for reference.

Girder and truncated girder trusses should be set out firstas they have specific, fixed locations. Dual purpose trussessuch as truncated girder / girders need special attentionas they may appear similar to other trusses which mustnot be used accidentally instead.

Standard trusses must then be set out, taking care not toexceed the design spacing. Generally they would be evenlyset out over runs of similar trusses. But it is also acceptableto space them at the design spacing and have a closinggap smaller than this - adjacent to a more heavily loadedtruss, if possible.

A gauging rod is very useful for setting out trusses.

3.3 LIFTINGTrusses must always be lifted in a manner that minimiseslateral bending stresses. It is preferable that they be keptstrapped in bundles until they are erected, as this reducesthe chances of damage.

Trusses may be lifted either by crane, or manually. Thechoice will depend on the truss weight and wall height.They should be maintained in a vertical plane as much apossible when being handled individually.

Trusses lifted by crane require slings or spreader bars asshown in the diagrams. Where trusses are place in bundlesdirectly onto the top plate, it is imperative that they aresupported by internal walls and that the supporting structureis stable in it’s own right.

When erected manually, they may be slid flat over theside walls on skids spread at 3m intervals, then rotatedvertically into position – supported at the apex and panelpoints to ensure that they do not distort or sag betweensupports during this process.

When positioning multiple span or cantilever trusses,take care that they are the correct way around. Suchtrusses will have markings on the bottom chordshowing the point of internal support.

SECTION 3 TRUSS INSTALLATION

3.1 GENERAL

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3.4 FIRST TRUSS3.4.1 Gable roofsOn gable roofs, start with the gable truss which is locatedover or just inside the end wall. Then brace it back to theground, or to some other stable part of the structure.

Some gable end trusses are designed to sit on theend wall. In these cases, it must be supported at everybottom chord panel point along its length, as it cannotact as a clear span truss.

On hip and Dutch gable roofs, start with the truncated girder,apex girder or Dutch hip girder truss and brace it back tothe corner of the building as shown. It is important that thistruss be correctly plumbed and aligned, as other trussesmust fit exactly up against it.

Where it is not feasible to install bracing of the first truss inthe manner described, the first two or three trusses can beerected and cross-braced between them to form a stableunit.

3.5 SUBSEQUENT TRUSSESAs each truss is installed, fix it to the top plate at therequired location, usually indicated by setout marks, or byusing a setout rule. Use a gauging rod and ties for spacingthe trusses, and a string line along the apex to ensurecorrect alignment.

It is important that trusses are lined up along the apex,not the heels.

Any multiple ply trusses must be fixed together priorto being put unto position. If the truss fabricator hasnot done this at the factory, it is his responsibility toprovide the information for fixings trusses together,and the erecting crews responsibility to ensure thatthis fixing is properly carried out.

3.4.2 Hip and Dutch gable roofs

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3.6 ERECTION BRACING AND TOLERANCESAs the trusses are erected, they must be bracedlongitudinally. This is to provide stability to the trusses duringthe erection process, and the bottom chord ties should bemaintained in place after full installation is completed.

However, the temporary top chord ties may be removedonce the roof battens are adequately fixed in place.

Refer to AS 4440-2004 for the full details for temporarybracing, however the following is a brief summary.

3.6.1 Top chordsTemporary braces at each top chord panel point.(maximum 3.0m apart).50 x 25 F5 or MGP10 for heavy, tiled roofs70x35 F5 or MGP10 for light, steel roofs

3.6.2 Bottom chordsTemporary braces at all mid-panels (maximum 3.5m apart),but not required for creepers, jacks, hip trusses, Dutch hipgirders and TG trusses with stations up to 3.6m. Use 70 x35 F5 or better, fixed with 2/65 nails or 1/65 screw pertruss crossing.

Where the bottom chord is not laterally restrained by theceiling, or by battens, ie exposed trusses or suspendedceiling, then the truss designer’s requirements must bestrictly followed.

3.6.3 TolerancesTrusses shall be installed straight and vertical and in theircorrect positions.

Bow – trusses shall be installed with bow not exceedingthe smaller of span/200 nor 50mm.

Plumb – trusses shall be installed so that no part of thetruss is out of plumb by more than the smaller or rise/50 or50mm.

These tolerances will produce a good roof line, and theperformance of the trusses will deteriorate rapidly if theseare exceeded, producing excess deflections and overstressin the truss.

SECTION 4 ROOF BRACING4.1 GENERALThe following recommendations allow for bracing of the roofsystem only and assume that the walls are stable in theirown right.

Bracing to the trusses is essential to prevent buckling ofmembers (chords and some webs), and to provide overallstability to the roof under all relevant loading conditions,including wind uplift where members may reverse from beingin tension to being in compression.

Care should be taken to ensure that all supporting structurebracing is in place prior to the trusses being installed.

4.2 BATTENSThe bracing of top chords is achieved via the overlyingbattens or purlins. Battens are to be nailed to both outerlaminates of any multiple ply trusses eg. double girders.Splice locations are restricted:-• Not more than one third of the battens should be

spliced at a single truss, and there must not betwo splices adjacent to each other at any truss.

• Areas in the vicinity of the ends of gable roofs,should be as free of splices as practical.

• Splices are not permitted at girder trusses unlessapproved by the fabricator in writing. This restrictiondoes not apply to truncated girder trusses, nor togirder trusses that are designed to have roof planebracing that is independent of the battens.

• Battens fixed to multiple ply girder trusses mustbe nailed into each outer ply forming the girdertruss assembly.

Note: For battens in sheet roofs, provide special splicedetails as recommended by Pryda (refer Technical UpdateNo. 12)

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4.3 TOP CHORDSAll trussed roofs require diagonal bracing to the top chords,which is typically at an angle of 30-45 degrees to the ridgeline, measured on plan. Braces should be installed suchthat each main truss has a brace on it.

Bracing is best located near the ends of buildings, and willbe installed on both sides of the ridge line. Some typicalexamples are shown here, but full details are given inAS4440-2004.

4.3.1 SpeedBraceThe bracing shown here is Pryda Speedbrace which hasbeen specifically designed for this purpose and should befixed with Pryda 35x3.15 nails.

4.3.2 CantileversIt is essential that the force in the top chord bracing betransferred to the supporting structure. In cantilever trusses,this is achieved through the use of special details as shownin the diagrams.

Speedbrace

Speedbrace

Speedbrace

Refer to AS4440 forfixing to brick-wallplate

Bend speedbrace over andfix with three nails to face oftop chord

Triple Grip,one to each sideof truss

Refer AS4440 for fixingto brick-wall plate

Triple Grip,one to each sideof truss

Speedbrace

Truss Boot

Speedbrace continuousto truss heel

Refer to AS4440 for endfixing details

Refer to AS4440 for endfixing details

Refer to AS4440 for fixingto brick-wall plate

Speedbrace

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Anchorage Point:Bend speedbrace to side of top plate andunder plate. Fix with five nails to topplate. Nails shall not be closer than10mm to the edge of the timber

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4.3.3 Typical Bracing Details 4.3.4 Gable end roofs – spans up to 8mRefer to AS4440-2004 for details for specific roof shapes.

4.3.5 Gable end roofs – spans 8m to 13mRefer to AS4440-2004 for details for specific roof shapes.In some instances double bracing is required.

4.3.6 Gable end roofs – spans 13m to 16mRefer to AS4440-2004 for details for specific roof shapes.

°

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Single or double speedbracesee AS4440

Double Speedbrace

Vertical bracing,see AS4440

SPAN LIMITS for STEEL BRACE USE (for Span range 8m to 16m)

SINGLE STEEL BRACE DOUBLE STEEL BRACE PITCH (deg) N1/N2 N3/C1 C2 C3 N1/N2 N3/C1 C2 C3 < 15 16.0 16.0 15.0 12.5 16.0 16.0 16.0 16.0

15 - 20 16.0 16.0 13.0 10.5 16.0 16.0 16.0 15.0 21 - 25 16.0 14.0 11.5 9.5 16.0 16.0 16.0 13.5 26 – 30 15.5 12.5 10.5 8.5 16.0 16.0 14.5 12.0 31 - 35 14.0 11.5 9.5 N/S 16.0 16.0 13.0 11.0 36 - 45 11.5 9.5 8.0 N/S 16.0 13.5 11.0 9.0

Note: 1. The above table is based on a typical standard truss shape, without stub heights. 2. It is assumed that a ceiling diaphragm exists at the bottom chord level.

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4.3.7 Hip end roofs – GeneralThe portion of the roof between hip ends, ie along the lengthof the ridge, shall be braced as per gable roofs.

4.3.8 Hip end roofs – jack trussThe jack trusses within the hip end itself, including Dutchgable ends, shall be braced as follows.

4.3.9 Asymmetric or dual pitch roofsConsider each side of the ridge as a separate case, anduse gable end details.

4.3.10 Bell cast roofsThe Speedbrace must be spliced at the breakline. (breakingpitch).

4.4 BOTTOM CHORDSGenerally ceiling battens or ceiling fixed directly to the un-derside of the bottom chords are sufficient for bottom chordbracing. The ceiling itself acts as a diaphragm to transferwind and bracing loads to cross walls.

All bottom chord ties must be braced or fixed to thesupporting structure.

a) For trusses over 12m span, or trusses where there isno ceiling, additional bottom chord bracing will be re-quired.

b) Additional bottom chord bracing is also required wherethere are insufficient internal walls to brace the exter-nal load-bearing walls , or where large cantilevers areused.

c) Additional bottom chord ties and bracing are requiredwhen ceiling is connected through metal furring chan-nels that are only clipped onto the bottom chord. Thetruss layout should indicate details of this.

In each of these cases, the requirement must be checkedby an experienced truss designer, and the details suppliedby the fabricator.

Bracing requirementsfor jack trusses asper AS4440

Refer to AS4440 splicedetail at break

Speedbrace

Speedbrace

Speedbrace

Speedbrace

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4.5 WEBS4.5.1 Long websSome webs must be braced if required by the truss designer.Generally this applies to long webs which are incompression during some part of the life of the structure.Typically this is a 70x35 F5 or MGP10 web tie locatedmid-length of the web. By itself it does nothing, so theseweb ties must be cross-braced back to part of the structurethat can provide adequate resistance.

As an alternative to web ties, Pryda Web Stiffener, Tee-Stiffeners or scabs may be specified.

4.5.2 End websAll trusses with end vertical webs not intersecting withanother truss, will need end web bracing similar to the topchord bracing. Diagonal bracing from the top chord to thesupporting structure should be provided at each end ofeach run of trusses as shown.

5.1 HIP ENDSThe details shown here are for a maximum design wind of50 m/sec (non-cyclonic). Refer to AS 4440-2004 for fulldetails including up to 70 m/sec cyclonic.The hip end connections provided by Pryda Build softwareare similar to those in AS4440-2004, but they may not beexactly the same, as every connection is designedspecifically for the conditions at each site.The requirements called up by Pryda Build take precedenceover AS4440 or any other guidline. A document (SoftwareUpdate No. 7) is available to produce to certifiers if required.

Hip truss to truncated trusses

Top chord - one framing anchor bent tosuit, with 3.15 x 35mm reinforced-headnails into the side of each top chord for

Jack truss to truncated girder truss

Top chord - one framing anchor bent tosuit, with 3.15 x 35mm reinforced-headnails into the side of each top chord fortruncated girder.

NOTE: For design wind speed speedup to 30 m/s, tile roofs, truncated girderwith spans up to 8000mm and station upto 2400mm, detail C1 may be used.

Bottom chord - three effective flathead65mm nails each side of jack truss

Extended jack truss top chord totruncated standard trusses

Two 65mm skew nails into the side ofeach top chord

Creeper truss to hip truss(maximum jack station 1800mm)

Top Chord - three effective flat-head65mm nails through jack truss top chordinto hip truss to chord

Bottom chord - three effective flat-head65mm nails through jack truss bottomchord to hip truss bottom chord

Creeper truss to hip truss(maximum jack station 3000mm)

Top chord - fix as detail D1 plus one mitreplate with 3.15 x 35mm reinforced-headnails to each top chord

Bottom chord - three effective flat-head

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Detail A1

Detail B1

Detail C1

Detail D1

Detail E1

Speedbrace fixed withtwo nails

Bend speedbrace over timberblock and fix with five nails

Bend speedbrace toside of top plate andunder (if necessary).Fix with five nails totop plate. Nails shallbe not closer than10mm to the edge ofthe timber.

Refer to AS4440 fixingto brick-wall plate.

Description Connection Details

SECTION 5 TRUSS CONNECTIONS

PRYDA WEB STIFFENER

TIMBER TEE-STIFFENER

90 x 35 F5 TeeStiffener fixedto web using 3.15dia x 65 nails at 200 c/c

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5.2 GIRDERSWhere a girder truss is utilised to support the standardtrusses perpendicular to it, they must be connected to thegirder with special devices which :-

a) Support the loadsb) Prevent separation of the girder bottom chord

away from the standard trussesc) Prevent rotational force being applied to the

standard truss.

5.2.1 Main girdersStandard trusses may be fixed to the girder truss by aPryda products. The fabricator will generally supply theappropriate bracket for each connection, or otherwise specify

5.2.2 Dutch Hip girdersThe waling plate must be fixed to the Dutch Hip girder trussin accordance with the details supplied by the fabricator.

5.3 VALLEYSWhere overlying members sit on the top chord of a truss,the nominated top chord restraint must still be maintained.This can be achieved with careful detailing as shown here.

5.4 OVERHANGSIt is important that overhangs are supported as intended bythe truss designer. Refer to AS4440-2004 for full details.

5.4.1 Boxed Eaves

5.4.2 Verge Overhangs

For bolting details,refer to Prydaspecifications

Girder bracket / Truss boot

Refer to Prydaspecifications fornailing or screwfixing

Fixed with nailsor screws asspecified

Screw fixed Truss BootUsing No. 12 Type 17 screws(with appropriate screw lengths)Single Ply = 35 long screwsDouble Ply = 65 long screwsTriple Ply = 65 long screws + cluster ofnails (or bolts)

Heavy-duty bolted TrussBootsUsing M16 bolts with washers