95994 Mitek Catalog for pdf - MiTek UK & Ireland · How to use this Manual This manual is provided to give information about the use of trussed rafters for roof construction and is

95994 Mitek Catalog for pdf 15/6/01 3:20 pm Page 1

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How to use this ManualThis manual is provided to give information aboutthe use of trussed rafters for roof construction and isintended to be of interest to Trussed RafterSpecifiers, Trussed Rafter Designers and Contractorsusing Trussed Rafters.

The manual is arranged in three sections:

SECTION 1Information for specifiers

SECTION 2Technical information

SECTION 3Information for site use

Trussed rafters are now used for the overwhelmingmajority of domestic roofs constructed in the UK andEire, and have an exceptional performance recordsince their introduction to the Building andConstruction Industries.

A nationwide network of Authorised Trussed RafterFabricators can provide a competitive, economicsolution to even the most complex of your roofingneeds. Using the MiTek Engineering Design System,these companies provide high quality, purposeengineered units to satisfy the need of an ever morecomplex market.

We are confident that this document will demonstratethe advantages to you in selection and buildingsuccessfully with trussed rafters.

SECTION 1 Information For Specifiers1.1 What is a trussed rafter?1.2 Design responsibilities1.3 Exchange of information1.4 Limits of use for trussed rafters1.5 Basic Design principles1.6 Guide to setting out and dimensioning1.7 Practical roof solutions1.8 Glossary of terms used

SECTION 2Technical Information2.1 Codes and standards2.2 Timber2.3 Connector plates2.4 Design method2.5 Roof and trussed rafter bracing2.6 Loading and load cases2.7 Selecting trussed rafter profiles2.8 Framing common roof shapes2.9 Connection details2.10 Problems to be aware of

SECTION 3Information for Site Use3.1 Do's and Dont's on site3.2 Supporting water tanks3.3 Hatch and chimney openings3.4 Two-Tier construction3.5 Hip ends and corners3.6 Valley intersections3.7 Bracing trussed rafters and roofs3.8 Loose timber connection details3.9 Bearing details3.10 Ventilation and condensation3.11 Site storage and handling3.12 Erection procedures3.13 Risk assessments & method statements3.14 Construction check list3.15 Nailing and bolting3.16 Attic Frames-2 & 3 part construction3.17 Attic Frames-bracing3.18 Attic Frames-environment3.19 General construction details3.20 Builders metalwork

This document is for general guidance only and the information contained in it is provided in good faith but without liability.MiTek Industries Limited gratefully acknowledge the provision of information from the Trussed Rafter Association.

MiTek Industries Limited


Section1.1 Information for Specifiers

2

What is a Trussed Rafter?Trussed rafters or roof trusses are now specified forthe majority of domestic roofs constructed in the UKand Eire. The trussed rafter is an individuallydesigned component, engineered to provide astructural frame for the support of roof or similarstructures.

Pre-fabricated from high quality, stress gradedtimbers and joined with steel nailplate fasteners, thetrussed rafter offers:

A flexible, practical and fully engineered solution toyour roofing requirements.

Economy of materials, as trussed rafters can use upto 40% less timber than a traditionally formed roof.

How to Specify Trussed Rafters

MiTek trussed rafters are available from anationwide network of Authorised Fabricators. A fulllist of these companies is available on request fromMiTek.

Please note that, unless a specific contract exists tothe contrary, the Fabricators liability is limited to thedesign and supply of the individual trussed raftercomponents only. The responsibility for the design ofthe roof structure as a whole lies with the BuildingDesigner (or Roof Designer if one has beenappointed). Please refer to section 1.2 on DesignResponsibility. There are, however, companieswithin the network of Authorised Fabricators thathave the necessary experience and resources toundertake design responsibility for the roof structureas a whole.

To obtain competitive quotations for the design andsupply of trussed rafters, contact one or more of theauthorised Fabricators. They will be pleased to assistyou in assessing your requirements.

Reduced labour costs on site, due to the amount ofpre-fabrications, releasing site joiners for morecomplex areas.

Quick erection of the roof structure enabling othertrades to commence quickly.

Reduction in site waste, loss and pilferage ofvaluable materials.

Space saving on site, with no need for timber storageor carpentry work areas.

Competitive pricing from a nationwide network ofAuthorised Trussed Rafter Fabricators.

The Fabricator will require sufficient, clearinformation regarding the roof structure to determinethe required trussed rafter profiles, dimensions,spacings, quantities, loadings, methods of supportand any special features to be taken into account;together with the requirements for timber treatment,extra items required (eg Builders metalwork),delivery date and delivery address. A check-list ofthe information required, together with a list of theinformation to be provided by the Fabricator to you,follows later in section 1.2.

Using the MiTek Design system, the required trusseswill be designed for your individual application andthe Fabricator will provide you with the detailsrequired to support the quotation.

If the quotation is successful, and subject to alldimensions being checked prior to the manufactureof the units, your trussed rafters will be supplied tosite ready for speedy erection of the roof supportstructure.


Information for Specifiers

3

Section1.1

The Project StepsTo illustrate the line of action for a project where trussed rafters are to be used, the following diagram may beof assistance:

Notes:On specific projects, the building designer may alsoencompass the function of Roof designer. This willgenerally be the case for small projects, where often,the Builder or his Architect will be the onlyprofessional people involved.

It is also possible for the roles of the Roof designer and the Trussed Rafter Fabricator to be combined, inthe case where, by specific contract, the fabricatortakes the responsibilities for the design of the wholeor part of the roof structure. This arrangement willhowever be generally undertaken by the Fabricatoron a professional, fee-paying basis.

Clienthas building to construct

Appoints

Architect, Engineer or

Contractor to act as

Building Designer

Building DesignerDecides on required roofscape, anddecides trussed rafters provide the

economic solution

Approaches

If quotation successful, details confirmed,trussed rafters made and delivered.

Trussed Rafter FabricatorFabricator designs the required trusses

and provides a Quotation

Roof Designerdesigns TOTAL roof structure required

by Building Designer and returns detailsto give to Trussed Rafter Fabricator

for pricing

either: or:


Section1.2 Information for Specifiers

4

Design ResponsibilitiesThe areas of Design Responsibility for the roofstructure of a building are as follows:-

The Trussed Rafter Designer is responsible for thedesign of trussed rafters as individual components.

He or she must ensure the structural integrity of thetrussed rafter units and inform the Roof Designer (or

Building Designer where there is no specificallyappointed Roof Designer), of any stabilityrequirements needed in the design of the trussedrafters.

If the Building Designer has appointed a RoofDesigner, they are responsible for the design of theroof structure as a whole and must inform theBuilding Designer of all information pertinent to theroof regarding its interaction with the supportingstructure and adjacent elements of the building.

If no person is appointed specifically as the RoofDesigner it falls upon the Building Designer toundertake the responsibilities of the Roof Designer.

On every project it is essential that one personassume overall responsibility as Building Designer.

The Building Designer may be the owner of thebuilding, his appointed Architect, a StructuralEngineer appointed by the owner or Architect, or theContractor or Builder.

The Building Designer is responsible for providingthe information listed in Section: 1.3 (and in section11.1 of BS.5268-3) to the trussed rafter designer andfor ensuring adequate provision is made for thestability of the individual trussed rafters.

The Building Designer is responsible for detailing allelements of bracing required in the roof, includingthat necessary to provide the lateral restraints to trussmembers required by the Trussed Rafter Designer.

The Building Designer is also responsible fordetailing suitable fixings for both the trussed raftersand the wall plates to provide the restraint againstuplift required by the Trussed Rafter Designer.

1. Trussed Rafter Designer

2. Roof Designer

3. Building Designer



5

Section1.3

Exchange of Information

Please refer also to BS.5268-3 Section 11

Information to be provided to the Trussed Rafter Designer by the Building Designer

Information to be provided by the Trussed Rafter Designer to the Building Designer

The Trussed Rafter Designer should provide the Building Designer with the following information, on suitablydetailed drawings, to enable a check to be made that trussed rafters supplied are suitable for their intended use:-

1. The position of roof hatches, chimneys, walkwaysand other openings.2. The service use of the building in respect of anyunusual environmental conditions and type ofpreservative treatment if required.3. The spacing of the trussed rafter and any specialtimber sizes in particular if matching with an existingconstruction.4. The site snow load or basic snow load and sitealtitude, or OS grid reference for the site.5. The position, dimensions and shape of anyadjacent structures higher than the new roof andcloser than 1.5m.6. Any special requirement for minimum memberthickness (eg. For the purposes of fixing ceilingboards or sarking).7. The height and location of the building withreference to any unusual wind conditions.

8. The profile of the trussed rafter (including anyrequired camber).9. The span of the trussed rafter (overall wall plates oroverall length of ceiling tie or both as appropriate).10. The pitch or pitches of the roof.11. The method and position of all supports.12. The type and weight of roof tiles or covering,including sarking, insulation and ceiling finishes. 13. The size and position of water tanks, or otherequipment and plant to be supported by the trussedrafters.14. The overhangs of the rafters at the eaves or apexif appropriate and details of any special eaves details.

1.The methods of support for tanks and otherequipment, together with the capacity or magnitudeof the additional load assumed.2. The range of reactions to be accommodated at thesupport positions including those required to resistwind uplift forces.3. The basis of the design.4. Details of changes in spacing required toaccommodate any opening eg. At a chimney.5. Any special precautions for handling, storage anderection of the roof trusses, in addition to thosecovered by BS.5268-3.

6. Finished sizes, species, strength classes ofmembers.7. The type, sizes and positions of all jointing deviceswith tolerances or the number of effective teeth ornails (or plate areas) required in each member at eachjoint.8. The position and size of all bearings.9. Loadings and other conditions for which thetrusses are designed.10. The spacing of the trussed rafters.11. The position, fixings and sizes of any lateralsupports necessary to prevent buckling ofcompression members.


Section1.4 Information for Specifers

6

Limits of use for Trussed Rafters

Physical Dimensions

Timber Sections

Profile

Trussed rafters provide a flexible method of framingmany required roof profiles. However, due to thecommercial limits of available timber sections,transport limitations for length and height and

manufacturing limitations of the pressing machinery,the following section provides some ideas as to thetypes of truss available in the UK and Eire at present.

Trussed rafters can be manufactured in spans up toapproximately 20 metres and heights up toapproximately 5 metres, although the more normalrange is 15 metres span and 3.5 metres high.

Trusses outside the above ranges may bemanufactured in two or more sections and site-assembled to the required profile (see section 3.4 ontwo-tier construction).

Trussed rafters up to 11 metres in span will generallybe fabricated from minimum 35 mm thick timbers.For trusses over 11 metres and up to 16 metres inspan, thicker timber sections up to 47mm wide will

be used. Above 16 metres in span trusses will consistof multiple trussed rafters permanently fastenedtogether by the manufacturer in the factory, or agreater width than 47mm may be used.

Within the above physical limits, many profiles ofroof truss are possible, depending on therequirements of the roofscape. The creation ofcantilevers over supports, the cutting back of aprofile to form a recessed 'bobtail' area, theintroduction of a pitched ceiling to form a 'scissor'truss, the creation of hip end and corner framing andmany more common and not so common roof shapesare easily achieved by specification of trussedrafters.

It should also be remembered that, to avoid problemswith both manufacturing and deflection of the roofstructure, the trussed rafter profile should be ofsufficient depth overall.

The recommended minimum depth formanufacturing purposes is approximately 600mm.The recommendation for structural depth is that thespan of the trussed rafter divided by its overall depthshould not be greater than 6.67. (This is known as thespan to depth ratio).

Cantilevered hip ends and corners can createproblems due to a pivoting effect if the cantileverdistance is very large and will also require specialpropping arrangements to be made for loose timberhipboards and jack rafters.

Careful geometry checks should be made if acantilevered area and an area with standard bearingabut each other to avoid any problems with roofalignment.



7

Section1.5

Basic Design Principles

Principles of design

Table 3: Maximum Bay Lengths of Rafters and Ceiling Ties

Depth of member Maximum length (measured on plan between node points)

35mm thick 47mm thick

Rafter Ceiling Tie Rafter Ceiling Tie

Mm m m m m

72 1.9 2.5 3.3 3.3

97 2.3 3.0 3.6 4.3

120 2.6 3.4 3.9 5.0

145 2.8 3.7 4.1 5.3

A trussed rafter is an engineered frameworkconsisting of structural members forming triangles.The framework derives its inherent strength from thistriangulation.

The members around the perimeter of the trussedrafter are known as Chords (top and bottom, alsocalled rafters and ceiling ties), and the internal

members providing the internal triangles are knownas Webs (sometimes also called struts and ties).

A true trussed rafter is formed only when the websform triangles between the top and bottom chords.Attic frames and Raised-Tie trusses (see section 1.7and 3.13), do not provide this triangulation and aretherefore technically not trussed rafters.

When loading is applied to a trussed rafter (fromtiles, ceiling construction, snow etc), forces aregenerated in the members forming the truss.

The magnitude of the bending moment in a particularchord is largely due to the Panel Length (the distancebetween the joints at each end of the member, usuallymeasured horizontally, also known as the Bay

Length). The general rule is, the longer the panellength the greater the bending moment and hence thelarger the section of timber required to safely resistthese forces.

Further, BS.5268-3 defines the maximum baylengths permitted in Table 3 (page 5) a copy of whichis given below:

These lengths are to ensure robustness of the trussduring manufacture and handling.The choice of a different truss type with a smaller

panel length (and hence more webs) will usuallyyield a smaller section of timber required.

Figure 1

Panel Length

Pitch

BRG BRG

Top Chord

Web

s

Top Chord

J3

Span

Bottom Chord

J4J2

J6 J7

J1 J5



8

Basic Design PrinciplesDeflectionAnother important criterion in the design of trussedrafters, which must be considered, is the amount ofdeflection, or movement of the truss when loading isapplied to it.BS.5268-3 section 6.5.7 defines the amount of

movement permitted under the differing loadconditions (also see section 2.4).

Additionally. The Trussed Rafter Designer should beaware of the problems which may arise due toDIFFERENTIAL DEFLECTION.

Differential deflection may occur between twoadjacent trusses within a roof when either the supportconditions or the loading conditions change. Forexample, in a hip end or corner condition (seesections 2.9 & 3.5), the heavily loaded girder trussmay deflect more than the truss immediately behindit in the hip sequence. Or, where a bobtail, (stub)truss is used adjacent to a full span truss, thedeflection of the standard truss may be substantiallygreater than that for the bobtail.

In this situation, the Designer should ensure that thedifference in anticipated deflection between the twotrusses is kept within limits, to avoid problemsin producing a smooth line for theceiling construction underneath.

This problem of differential deflection betweenadjacent units is one of the most common causes ofsite problems and, once the roof is erected, one of themost difficult to rectify. The remedy is for theDesigner to be full ware of the potential problem atthe design stage.

Figure 2

Figure 3

Loads Applied

Deflection at ceiling level



9

Section1.6

Guide to Setting Out & DimensioningAs outlined in BS.5268-3, in order to ensure thattrussed rafters are correctly designed and fabricatedand that they are suitable for their intended purposeit is necessary for them to be accurately specified andfor adequate information to be available whenrequired.

At MiTek we have developed a number of standardtrussed rafter configurations, as shown in figure 4, towhich dimensions can be related, this simplifies thespecification for design purposes.

KeyO Overhang SC Span over ceiling tie RW Room widthSOP Span over setting out points C Cantilever RH Room heightSW Span over wallplates N Nib SL Slope length

MiTek MiTek Outside Shape Shape Truss Outside Shape Shape Truss

Number Description Number Description

Triangulated Trusses Triangulated Trusses

Standard Truss Sloping Flat00-02 or 45 with

Duopitch Apex/Double(25-27) (Asymmetric Bobtail

version

also possible)

Single 40, 46 Flat05, 10 Cantilever 52, 58 *Additional shapes

Duopitch 87 ,* with modified

support positions

are available

Double11 Cantilever 59, 63 Half Hip

Duopitch

14-19 Bobtail 64, 68 HipDuopitch

Bobtail14-19 Duopitch Attic Truss

with Nib

20-21 Monopitch Various

20-21 Monopitch Various Attic Trusswith Nib (Centre Supper)

35 Scissors Various ExtendedRafter

(Raised tie)Truss

Non-Triangulated Trusses

O

O

O

O

O

O

O

O

O

O

OO

O O

O

O

SW

SW

SW

SW

SW

SW

SC

SOP

SOP

SW

RW

RW

RH

RH

SOP

SOP

SL

SL

SOP

SOP

O

O

O

C

C C

OSW

SOP

O OSW

SOP

O OSW

SOP

O O

O

SW

SOP

O NSC

SOP

O OSW

SOP

ONSC

SOP

O OSW

SOP

N.B. Not all of the MiTek Range Of Trusses Are Indicated above

Figure 4



10

Guide to Setting Out & DimensioningSetting out and Eaves detailsAlthough often employed as the principle truss typein association with appropriate architectural featuresof a building, the bobtail is most often needed toaccommodate re-entrant areas in perimeter walls asshown in figure 5. The horizontal 'A' dimensionindicated in figure 6 therefore, is conveniently usedto specify the shape for duo-pitch trusses, whiledouble bobtails and bobtailed mono-pitched trusseswhich more often are principle trusses are moreconveniently specified by a vertical 'A' dimension.

Figure 7a shows typical end details when the outerleaf is of masonry, arrangement (b) is best confinedto timber frame construction as separate columns ofmasonry between trusses could be rather unstable. Ifthe end verticals are to be tile clad one of the

arrangements figure 7c or d is suitable. In (c) aspecially wide timber is used as the end vertical ofthe truss so that the tile battens clear the outer leaf ofthe wall; the inside of the end vertical must not belocated to the right of the centre-line of the wallplate. In some cases the arrangement is impracticalowing to the large width required for the end vertical.In many cases the diagonal in the cantilevered part(figure d) can be omitted if there is little load fromthe cladding.

A special bobtail can be designed to suit practicallyany requirement.

Bobtailed trusses must never be formed through do-it-yourself site modifications of standard truss typeswith which they align.

Figure 5

Figure 6

Figure 6a

Figure 6b

Figure 7a

Figure 7b

Figure d

Figure 7c

Span

Masonry inner leaf withtimber nib

Tile clad end vertical

Tile clad end vertical

Timber frame

Span

Span

'A'

'A''A'

'A'



11

Section1.6

Guide to Setting Out & DimensioningSupport details - CantileversThe reaction from the bearing is the greatest load(although upwards) to which a truss is subjected andin order to control excessive bending in thesupported chord it is important, except in thesmallest trusses, to locate a joint at each bearing. Thenormal eaves joint illustrated in figure 8aaccomplishes this if the “Shift” dimension is lessthan 50mm, or one-third of the scarf length,whichever is the greater.

If the Shift is greater than the allowed a stress checkis required on the short cantilever.

Unfortunately there is usually insufficient space foran additional web so should the check fail, as it oftendoes, it is necessary to increase the size of the bottomchord or alternatively incorporate a relief rafter, (as

shown in figure 10b) or a heel wedge. Both of theseoptions can add to the final cost of the truss andtherefore it is best to avoid cantilevers in this range.

If the “Shift” is greater than two scarf lengths, then astandard cantilever truss as show in figure 10a isemployed. The chord sizes are usually no greaterthan the corresponding non-cantilevered standardtruss and the cost is little more. Many variations arepossible by adjusting the position of a joint of a non-cantilevered standard truss type so that it is over abearing. Finally, if required, a non-standardcantilever truss of almost any triangulatedconfiguration can be designed and fabricated. Notethat a brace may sometimes be required on thebottom chord which is untypically in compression.

Figure 8a Figure 8b

Figure 10a

Figure 10b

Figure 10

Figure 9

Scarf length Scarf length

Shift Shift

Shift = max x 50mm or 1/3 scarflength whichever is the greatest

Increase bottom chord

523 823 710 1110

623 923 810 1210

723 1023 910 1310

Alternative configurations

Bottom chord may need brace

Incorporate relief rafter or slider

Standard truss Check bottom chord

Standard cantilever



12

Practical Roof SolutionsHipped EndsThe good performance of MiTek designed hippedends does not depend on tension in battens, amassive wallplate and horizontal thrust on walls.Indeed, with suitable bracing, walls are providedwith the stability called for by the BuildingRegulations. The most simple and lowest cost formof MiTek hipped end, (shown in figure 11a) consistsof a multi-ply girder of standard trusses securelyfixed together and supporting loose rafters andceiling joists. Such constructions are limited to spansgenerally not exceeding 5m. Sizes of rafters and tiescan be found in approved document 'A' of theBuilding Regulations. Hip boards should besupported off the girder by means of a ledger and theceiling joists by means of proprietory joist hangers.

The 'step-down' system incorporates flat-top hiptrusses of progressively diminished height from theridge to the girder. The number of step-down trussesis determined by the necessity of maintainingreasonable sizes for the loose ceiling joists and hipboard on the hipped corner infill areas, as shown infigure 11b. For these reasons the span of the mono-pitch trusses is not usually greater than 3m in thecase of regular hips (where the end pitch is the sameas the pitched of the main roof).

Noggings have to be fitted between the flat chords ofthe step-down hip trusses to support the tilingbattens. The web configurations of the various trusstypes shown (including the mono-pitch) are typicalbut will be chosen to provide the best structuralsolutions.

This step-down hip system is no longer very popularas it requires many different truss profiles to bemade.

The 'flying rafter' hip system show in figure 11c hasthe manufacturing advantage of there being only onebasic hip truss profile. All of the hip trusses,including those forming the girder are similar, andthe mono-pitch trusses supported off the girderusually have the same profile as the sloped part of thehip trusses which speeds up fabrication.

The rafters of the mono-pitched trusses are site cut tosit against the upper hip board and the off-cuts arenailed in position to the rafters of the hip trusses. Theflat parts of the top chords of the hip trusses andgirder are well braced together to prevent instability.

While the hipped corner infill is shown asprefabricated rafter-joist components (open jacks), itis usually cheaper to site fabricate in these areas. Thelower hip board is typically notched and supportedoff a 50 x 50mm post nailed to the girder truss. Theupper hip board can be supported off ledgers and insome cases is propped off the hip trusses underneath.

The system offers the advantage of continuousrafters and consequently easily constructed smoothroof slopes. On long spans it may be necessary to usea second hip girder between the apex and monos.

Figure 11a

Figure 11c

Figure 11b

Rafter noggings not shown for clarity



13

Section1.7

Practical Roof SolutionsT Intersection & Valley InfillThe 'T' is probably the most common kind of roofintersection (as demonstrated in figure 12). The rooftruss arrangement at this feature includes a speciallydesigned girder truss (shown in figure 13), usuallyconsisting of two to four individual trusses connectedtogether with nails or bolts, which support theincoming trusses. Support of the incoming trusses isoff the bottom chord of the girder through girdertruss shoes.

The design of the valley frame infill continues therafter profiles of the opposing roof slopes to form anintersection, and transfers the tile loading uniformlyto the top chords of the underlying trusses.

Figure 13

Figure 12

Typical girder truss

Howe

Double Howe

Standard truss to main roof

Standard trusseson return roof

If load bearing wall or beam is available atposition 'A' to support the standard trusseson the main roof then the compound orgirder trusses can be substituted for astandard truss on the return roof

A

Layboards (if required) are to comply withthe manufacturers details

Diminishing valley frames to benailed direct onto the maintrussed rafters



14

Practical Roof SolutionsCornersThere are two basic methods of forming a corner:1. Hipped CornerA hipped corner is formed by the perpendicularintersection of two roofs which may or may not be ofthe same span.

The principle for the hipped corner construction isthe same as for full hips except that the truss profilesare generally sloped on one side only. The support

across the junction is again provided by either agirder truss or a wall/beam. When a girder truss isspecified provision has to be made for a specialhanger to carry the girder truss supporting the hippedend. Mono valley frames are required to completethe framing of the corner.

2. Skew Corners or Dog-LegsA skew corner is formed by the intersection of tworoofs at an angle greater than 90 degrees. The corneris generally framed by positioning a girder truss atthe extremity of the two straights with an additionalgirder positioned across the corner as in figure 14b.

The girder units will typically support loose infill on

purlins and binders to maintain the roof plane. Thefeasibility of framing in this manner is dependantsolely upon the span of the longest purlin.

It is not recommended to incorporate hipped endsand tee intersections into skew corners unless afeasibility study has been undertaken beforeplanning has become too far advanced.

Figure 14a

Figure 14b

Standard trusses - Main span

Ridge truss

Seco

ndar

y gi

rder

tru

ss

Pri

mar

y gi

rder

tru

ss

Stan

dard

tru

sses

- S

econ

dary

spa

nHip board

Stand

ard t

russe

s

Standard trusses

Lay board

Mono pitch trusses

Hip corner infill

Girder

Girder across splay(span and pitch differ tomatch standard trusses)

Girder

Looseinfill area

Looseinfill area

Special connection detail required

Noggings

Valleyframes



15

Section1.7

Extended Rafters and Extended JoistsExtended rafters and extended joists, as shown infigure 15 require special consideration because thetrusses are not fully triangulated to the bearings. As aresult of the lack of triangulation, the extendedmember is subject to exceptionally large bendingmoments. In the example shown in figure 16 therafter, or the top chord, is subject to a bendingmoment no less than ten times that which occurs in aconventionally supported truss.

Standard trusses can be adapted and strengthened towithstand the large bending moments and shear forceoccurring in the extended member at the rafter-tiejunction. This may be accomplished by fixing astrengthening piece to each side of the extendedmember, using bolts or a special nailingarrangement. Another way to strengthen the

extended rafter is by using a factory fitted stackchord as shown on the right-hand side of figure 16.

Large rafter extensions will produce outward thrustand movement at the bearings. This is often a criticalfactor in design and is rigorously controlled byBS.5268-3.

Hatch and Chimney OpeningWhere possible, hatches and chimneys should beaccommodated in the standard spacing between trusses.Each member and joint in a truss performs an importantrole essential to the effective functioning of all otherparts and the component as a whole. Trusses must neverbe cut and trimmed except according to details suppliedby the truss designer. The full detailing of theconstruction of these features is given in section 3.3.

Practical Roof Solutions

Extended rafters

Extended joist

Stack chord

0.6m 0.6m7m

Figure 15

Figure 16

Figure 16a

Bearing Bearing

Bearing Bearing



16

Practical Roof Solutions

Room-in-the Roof: Attic FramesThe special advantage of attic frames is that theyenable the upper floor of a building to be totallycontained within the roof, increasing the habitablearea by 40-50% at little extra cost. The bottomchords become the floor joist of the room, their sizehaving been calculated to cater for increased loads.

Attic Frames can be designed to allow 'clear span'supported at eaves only, (as shown in figure 17a),however for longer spans it may be necessary toincorporate an intermediate support (shown in figure17b). This will allow either larger internal roomdimensions or reduce the timber sections required.Since attic frames are non-triangulated, the timbercontent will be considerably greater than thatrequired for a comparable trussed rafter.

Where a more complex attic roof layout is beingplanned, for example where hipped ends, corners orintersections may occur, it is recommended that atruss designer is contacted to prepare a feasibilitystudy at an early stage of the project.

Dormer Window and Stairwell LocationsThe same principles that apply to ordinary rooftrusses also apply to attic frames. If a truss is severedor weakened at any point the structural integrity ofthe whole truss is effected. Therefore, if an openingis planned, the roof must be strengthened byadditional frames at smaller than standard spacingsor girders at each side of the opening. Guidelines tothese details are given in section 3.3.

Having acknowledged these principles, there isrelative freedom in the methods of framing out theactual openings, however there are sensibleeconomic factors to be considered. Obviously it is ofmost advantage to locate window openings ondifferent sides of the ridge and directly opposite eachother in order that they will lie between the same twotrimming trusses. If not, the extent of additionalloose infill timber may completely negate theadvantages of using prefabricated attic frames.Where possible stairwells should be located parallelto the trusses otherwise, once again, the increase insite infill timber may nullify the benefits of usingattic frames.

The following diagram (figure 18) demonstrates themost economic method of incorporating openings tothe roof space, whilst figure 19 requires increasedloose infill timbers and site work if practicalrecommendations are not followed.

Figure 17a

Figure 17b

Figure 18

Figure 19

Room space

Room space

Floor joists

Floor joists

Support points

Support points

Most economical

Least economical



17

Section1.8

Glossary of Terms used in Trussed Rafter ConstructionApex/PeakThe uppermost point of a truss.

Attic Truss/room-in-the-RoofA truss which forms the top storey of a dwelling butallows the area to be habitable by leaving it free ofinternal WEB members. This will be compensated bylarger timber sizes elsewhere.

BargeboardBoard fitted to conceal roof timbers at a GABLEEND.

BattensSmall timber members spanning over trusses tosupport tiles, slates etc.

BearerA member designed to distribute loads over a numberof trusses.

BinderA longitudinal member nailed to trusses to restrainand maintain correct spacing.

BirdsmouthA notch in the underside of a RAFTER to allow ahorizontal seating at the point of support (usuallyused with RAISED TIE TRUSSES).

BlockingShort timbers fixed between chords to laterallyrestrain them. They should be at least 70% of thedepth of the chords.

BobtailA truss type formed by truncating a normal triangulartruss.

Bottom ChordSee CEILING TIE.

BracingThis can be Temporary, Stability or Wind Bracingwhich are described under these headings.

Building DesignerThe person responsible for the structural stability andintegrity of the building as a whole.

CamberAn upward vertical displacement built into a truss inorder to compensate for deflection which might becaused by the loadings.

CantileverThe part of a structural member of a TRUSS whichextends beyond its bearing.

Ceiling TieThe lowest member of a truss, usually horizontalwhich carries the ceiling construction, storage loadsand water tank.

Chevron BracingDiagonal web bracing nailed to the truss in the planeof the specified webs to add stability.

Connector Plate/fastenerSee NAILPLATE.

Cripple RafterSee JACK RAFTER.

Dead LoadThe load produced by the fabric of the building,always long term (see DESIGN LOADS).

DeflectionThe deformation caused by the loads.

Design LoadsThe loads for which the unit is designed. Theseconsider the duration of the loads long term, mediumterm, short term and very short term.

Duo/dual Pitch TrussA truss with two rafters meeting at the APEX but notnecessarily having the same PITCH on both sides.

DwangsSee NOGGINGS.

EavesThe line between the rafter and support wall.

Eaves JointThe part of the truss where the rafter and the ceilingtie intersect. This is usually where the truss issupported.

Extended RafterSee RAISED TIE TRUSS

FasciaHorizontal board fitted around the perimeter of thebuilding to the edge of the truss overhangs.

FastenerSee NAILPLATE.

Fink TrussThe most common type of truss used for dwellings.It is duo-pitch, the rafter having the same pitch. Thewebs form a letter W.

Firring PieceA tapered timber member used to give a fall to flatroof areas.

French HeelAn EAVES joint where the rafter sits on the ceilingtie.

Gable EndThe end wall which is parallel to the trusses andwhich extends upwards vertically to the rafters.



18

Hip EndAn alternative to a GABLE END where the end wallfinishes at the same height as the adjacent walls. Theroof inclines from the end wall, usually (but notalways) at the same PITCH as the main trusses.

Hip SetThe trusses, girders and loose timbers required toform a hip end.

Horn/nib An extension of the ceiling tie of a truss (usuallymonos or bobtailed trusses) which is built intomasonry as a bearing.

Imposed LoadThe load produced by occupancy and use includingstorage, inhabitants, moveable partitions and snowbut not wind. Can be long, medium or short term.

Internal MemberSee Webs.

IntersectionThe area where roofs meet.

Jack RafterAn infill rafter completing the roof surface in areassuch as corners of HIP ENDS or around chimneys.

Live LoadTerm sometimes used for IMPOSED LOADS.

Longitudinal BracingComponent of STABILITY BRACING.

Loose TimberTimbers not part of a truss but added to form the roofin areas where trusses cannot be used.

Mono-Pitch TrussA truss in the form of a right-angled triangle with asingle rafter.

NailplateMetal PLATE having integral teeth punched from theplate material. It is used for joining timber in oneplane with no overlap. It will have an accreditationcertificate and will be manufactured, usually, fromgalvanised steel. It is also available in stainless steel.

NibSee HORN

NodePoint on a truss where the members intersect.

NoggingsTimber pieces fitted at right angles between thetrussed rafters to form fixing points.

OverhangThe extension of a rafter or ceiling tie of a trussbeyond its support or bearing.

Part ProfileSee BOBTAIL.

PeakSee APEX.

Permissible StressesDesign stresses for grades of timber published inBS5268: Part2:

PitchThe angle of the chords to the horizontal, measuredin degrees.

PlateSee NAILPLATE.

PurlinsTimber members spanning over trusses to supportcladding or between trusses to support loose timbers.

Quarter PointThe point on a rafter where the web intersects in aFINK TRUSS.

QueenInternal member (WEB) which connects the APEXto a third point on a FINK TRUSS.

RafterThe uppermost member of a truss which normallycarries the roof covering.

Rafter Diagonal BracingComponent of STABILITY BRACING.

Raised Tie TrussA truss which is supported at a point on the rafterwhich is beyond the point where the rafter meets theceiling tie.

Reducing TrussesSee VALLEY FRAMES.

Remedial DetailA modification produced by the TRUSSEDRAFTER DESIGNER to overcome a problem withthe truss after its manufacture.

Return SpanThe span of a truss being supported by a girder.

RidgeThe line formed by the truss apexes.

RidgeboardTimber running along a ridge and sandwichedbetween loose rafters.

Roof DesignerThe person responsible for the roof structure as awhole and who takes into account its stability andcapability of transmitting wind forces on the roof tosuitable load-bearing walls.

Glossary of Terms used in Trussed Rafter Construction



19

Section1.8

Room-in-the-RoofSee attic truss.

ScabAdditional timber fitted to the sides of a truss to effecta local reinforcement, particularly in raised tie trusses.

Setting Our PointThe point on a truss where the undersides of the rafterand ceiling tie meet.

Skew NailingA method of fixing trusses to the wallplate by drivingnails at an angle through the truss into the wallplatewhich is generally not recommended. (See Truss Clip).

SoffitBoard fixed underneath eaves overhang along thelength of the building to conceal timbers.

SpanSpan over wallplates is the distance between theoutside edges of the two supporting wallplates. This isusually the overall length of the ceiling tie.

Spandrel PanelA timber frame, triangular panel forming the gable wallabove the ceiling line.

SpliceA joint between two members in line using a nailplateor glued finger joint.

Spreader BeamSee bearer.

StrapMetal component designed to fix trusses and wallplatesto walls.

StrutInternal compression member connecting the thirdpoint and the quarter point on a Fink truss.

Stub EndSee bobtail.

Temporary BracingAn arrangement of diagonal loose timbers installed forsafety during erection. Often incorporated withpermanent stability and wind bracing structures.

Third PointPoint on the ceiling tie where the internal webs meet ina fink truss.

Timber Stress GradingThe classification of timber into different structuralqualities based on strength (see BS4978: 1996).

Top ChordSee rafter.

TRADA Quality Assurance SchemeQuality control method in truss manufactureadministered by the BM TRADA Certification.

TrimmerA piece of timber used to frame around openings.

Truss/Trussed RafterA lightweight framework, generally but not alwaystriangulated, placed at intervals of 600mm to supportthe roof. It is typically made from timber members ofthe same thickness, fastened together in one planeusing nailplates or plywood gussets.

Trussed Rafter DesignerThe person responsible for the design of the trussedrafter as a component and for specifying the pointswhere bracing is required.

Truss ClipA metal component designed to provide a safestructural connection of trusses to wallplates. Also toresist wind uplift and to prevent the damage caused byskew nailing.

Truss ShoeA metal component designed to provide a structuralconnection and support for a truss to a girder or beam.

Uniformly Distributed LoadA load that is uniformly spread over the full length ofthe member.

Valley BoardA member raking from incoming ridge to corner in avalley construction.

Valley Frames/SetInfill frames used to continue the roofline when roofsintersect.

VergeThe line where the trussed rafters meet the gable wall.

WallplateA timber member laid along the length of the loadbearing walls to provide a level bearing and fixing forthe trusses.

WebsTimber members that connect the rafters and theceiling tie together forming triangular patterns whichtransmit the forces between them.

Wind BracingAn arrangement of additional timbers or otherstructural elements in the roof space, speciallydesigned to transmit wind forces to suitable load-bearing walls.

Glossary of Terms used in Trussed Rafter Construction


Section2.1

20

Design Compliance

Design loadings accord with the following:-The Building Regulations England and Wales,The Building Regulations Scotland,Irish Standard 193: Timber trussed rafters,BS 6399: Part 1: Code of practice for dead and imposed loads,BS 6399: Part 3: & amendments: Code of practice for imposed roof loads,BS 6399: Part 2: Code of practice for wind loads.

Timber designs accord with the following:-BS5268:-2: Structural use of timber, code of practice for permissible stress design, materials andworkmanship.BS 5268-3: Code of practice for trussed rafter roofs.

Connector plate design accord with the following:-British Board of Agrement Certificate No: 90/2386,WIMLAS Certificate 038/96 - MiTek M20 punched metal plate timber fasteners.

Technical Information

Codes and Standards



21

Section2.2

Timber

The timber used in the manufacture of trussed raftersin the UK and Eire is strength graded softwood.

The common sources of supply for the timber areScandinavia, Baltic States, Canada and the USA. Thelast two countries provide only a minor proportion ofthe timber used in trussed rafters.

Timber is classified by either strength grade orstrength class and this classification defines theworking stresses which may be used to design withthe particular timber involved.

Grading may be either manual, by trained graders, ormechanical, by use of a strength grading machine.

Machine strength graded timbers form the majorityof timbers used in trussed rafters.

As each particular length of timber is classified, agrading mark or stamp is applied to show itsclassification.

When the timber is re-cut for use in trusses, the

Trussed Rafter Fabricator will mark the finishedtruss with the grades or strength classes of timberused, often by means of a label attached near theapex of the truss or by means of a stamp on thetimber near the apex.

Maximum timber lengths of up to 6 metres are used,although lengths of 4.8 metres are commerciallymore common. This means that splice joints arefrequently required in truss chord members, toachieve long spans. Please refer to section 2.4concerning timber splicing.

The Designer will use the strength grade or strengthclass values when designing the members formingthe truss. (See section 2.4, Design Method).

British Standard BS4978: - Specification for visualstrength grading of softwood. BSEN 1313 - 1:Permitted deviations and preferred sizes of softwoodsawn timber, together with BS 5268 - 3: Code ofPractice for trussed Rafter Roofs govern the grading,sizing and use of the softwoods used in TrussedRafter construction


Section2.3 Technical Information

22

Connector PlatesMiTek connector plates are manufactured fromstructural grade galvanised mild steel.

Many common types of nailplate are currently usedin the UK and Eire: The 1.0mm M20, the 1.2mmB90, the 2.0mm M200 and several special platesincluding field splice plates.

For full details of the use of each type of nailplate,please refer to Agrement Board Certificate 90/2386,and Wimlas Certificate 038/96.

The difference in the formation of the nails (teeth)produced by the stamping-out process for each typeof plate, together with the difference in steel

thickness and width used, produces a varying set ofdesign parameters for each type of plate. Further, alarge range of available sizes for each type of plateprovides the designer with a very flexible system forthe design of each particular joint.

To cater for the aggressive roof environments foundin industrial or agricultural buildings, or fordecorative purposes in exposed trussed situations, areduced range of sizes with the M20 nailconfiguration is available in 20 gauge Stainless steel.Please note, however, that these are likely to addconsiderably to the cost of the finished roof trusses.

Figure 20

TYPICAL JOINTS

Apex joint

Heel joint Node joint

Splice joint



23

Section2.4

Design MethodA trussed rafter is an engineered frameworkconsisting of structural members forming triangles.The framework derives its inherent strength from thistriangulation.

The members around the perimeter of the trussedrafter are known as chords (top and bottom, alsocalled rafters and ceiling ties), and the internalmembers providing the internal triangles are knownas webs (sometimes also called struts and ties).

A true trussed rafter is formed only when the websform triangles between the top and bottom chords.Attic frames and Raised-Tie trusses (see section 1.7and 3.16) do not provide this triangulation and aretherefore technically not trussed rafters.

When designing non-standard trussed rafters, it isbeneficial to ensure the full triangulation as above,please refer to MiTek's System Design Office if indoubt.

Principles of DesignWhen loading is applied to a trussed rafter (fromtiles, ceiling construction snow etc), two main kindsof force are generated in the members:1.Bending Moment2.Axial Force

Bending moment causes neighbouring sections oftimber to tend to rotate relative to each other (seefigure 21a).

Axial force may be either tensile, i.e. pullingadjoining sections of timber away from each other, orcompressive, i.e. crushing adjoining sections oftimber into each other (see figure 21b and 21c).

A compressive force may cause the member tobuckle (bending sideways out of the plane of thetrussed rafter) and this may need to be counteractedby bracing (see sections 2.5 and 3.7) or by increasingthe section of timber required for the affectedmember.

Within a trussed rafter, members will be subject toeither axial force alone or a combination of axialforce and bending moment. The design of a trussedrafter must allow for these effects, together with thediffering forces produced by different types of load(see section 2.7 on Loading and Load cases.)

Figure 21a

Figure 21b

Figure 21c

Bending Moment

Tension

Compression



24

Design MethodBending MomentsBending moments are generally induced in the Chordmembers due to the loadings (tiles, ceiling, snow etc)placed directly onto them. It is unusual for Webmembers to be subject to bending moments.

The magnitude of the bending moment in a particularchord is largely due to the Panel Length (the distancebetween the joints at each end of the member, usuallymeasured horizontally, also known as the Bay

Length). The general rule is, the longer panel lengththe greater the bending moment and hence the largerthe section of timber required to safely resist thebending moment.

Further, BS. 5268-3 defines the maximum baylengths permitted in Table 3, a copy of which is givenbelow:

The choice of a different truss type, with a smallerpanel length (and hence more webs), will usuallyyield a smaller section of timber required.

The method of calculation relating to bendingmoment is as follows:

The applied bending stress (calculated from thebending moment divided by the section modules ofthe timber being considered) is compared with thepermitted bending stress for the particular timber

grade or strength class.

The resulting ratio:

This ensures that the actual bending stress in thetimber cannot exceed the permitted stress, causingthe timber to fail.

Axial ForceAxial forces within the trussed rafter are calculatedby analysing the whole frame. The greater thenumber of panels (webs) the greater the axial forcescan be. Also, the lower the pitch of the top chord thegreater can be the axial force.

As mentioned previously, axial force can be eithertensile or compressive and, if compressive, can leadto problems with out-of-plane buckling.

In a similar way to bending moment, the actual axialstress in the timber (calculated from the axial forcedivided by the area of the timber section), iscompared with the permitted axial stress of thetimber grade or strength class being used.

This ensures that the timber never exceeds itspermitted axial stress limit.

Generally, web members will be subjected only toaxial force, whereas chord members will be subjectto a combination of bending and axial stresses.

For chord members therefore, the calculationbecomes:

To ensure that the timber section is within its definedlimits for both bending and axial stress.

This ratio is known as the combined stress index(CSI) or stress summation.

BS 5268 Table 3: Maximum Bay Lengths of Rafters and Ceiling Ties

Depth of member Maximum length (measure on plan between node points)

35mm thick 47mm thick

Rafter Ceiling Tie Rafter Ceiling Tie

Mm m m m m

72 1.9 2.5 3.3 3.3

97 2.3 3.0 3.6 4.3

120 2.6 3.4 3.9 5.0

145 2.8 3.7 4.1 5.3

< 1Actual bending stress

Permitted bending stress

< 1Actual axial stressPermitted axial stress

< 1Actual Actual

bending stress axial stress

Permitted Permittedbending stress axial stress

+



25

Section2.4

Design MethodDeflectionAnother important criterion in the design of trussedrafters, which must be considered, is the amount ofdeflection, or movement of the truss when loading isapplied to it. (See figure 22).

BS.5268-3 section 6.5.7 clearly defines how tocalculate deflection and the permissible limits onrafters, ceiling ties and on overhangs and cantilevers.

This therefore defines the amount of movementunder the differing load conditions permitted.

Additionally, the Trussed Rafter Designer should beaware of the problems which may arise due toDIFFERENTIAL DEFLECTION.

Differential deflection may occur between twoadjacent trusses within a roof when either the supportconditions or the loading conditions change. Forexample, in a hip end or corner condition (seesections 2.8 and 3.5) the heavily loaded girder trussmay show more anticipated deflection than the trussimmediately behind it in the hip sequence. Or, wherea bobtail (stub) truss is used adjacent to a full spantruss, the deflection of the standard truss may beanticipated to be greater than that shown for thebobtail.

In this situation, the Designer should ensure that thedifference in anticipated deflection between the twotrusses is kept within limits, to avoid problems inproducing a smooth line for the ceiling constructionsunderneath.

This problem of differential deflection betweenadjacent units is one of the most common causes ofsite problems and, once the roof is erected, one of themost difficult to rectify. The remedy is for theDesigner to be fully aware of the potential problemat the design stage.

Figure 22

Figure 23

Loads Applied

Deflection at ceiling level



26

Design MethodThe design of joints using Mitek nailplate connectorsis governed by the British Board of AgrementCertificate 90/2386 and WIMLAS Certificate038/96.

Within the approval certificates the conditions of use,assessment of fitness for purpose, sizes of availablenailplates, methods of joint assembly, relevantloadings etc are specified. It is not intended in thisdocument to reproduce in part or in whole thecontents of the Certificates; copies of these areavailable on request from MiTek.

However, to give an insight into the method of jointdesign using the nailplates, the Designer should notethat each nailplate joint must be assessed for shear

strength and lateral resistance to the forces placedupon its integral teeth.

The values for shear and tensile strength are given inthe relevant Certificate, as are the values for the nailanchorage loads. It should be noted that the lateralresistance of a nailplate joint depends upon:1.The number of effective nails in the joint.2.The species of timber used and its condition(moisture content).3.The duration of the loading applied.4.The direction of bearing of the nails in relation tothe grain of the timber (load to grain).5.The direction of the loading in relation to theconnector plate (load to nail).

It should be noted that, when designing a nailplatejoint, the approval Certificates define certainineffective areas at the ends and edges of the timberin which the nails are to be ignored for the design.

Further, the species of timber used and the durationof loading causing the forces must be taken intoaccount.

Finally, the actual position of the nailplate on thejoint will affect the permitted values for each nail.

It can be seen that this leads to a highly complexinteraction, as several different load durations,combined with a number of possible nailplateorientations and a large number of available sizes ofnailplate makes the most economical choice of anyparticular nailplate a difficult decision.

By its nature, the solution of this interaction is nowlargely handled by MiTek's sophisticated computerprograms although manual design is still necessaryfor very special applications.

Figure 24

Angle of load to nails A 90º

Angles of load to nails B (90º-�)Angle of load to nails C 0º

Angle of load to nails A Ø

Angles of load to nails B (ø-�)Angle of load to nails C 90 -Øº

Angle of load to nails A 0º

Angles of load to nails B (�)Angle of load to nails C 90º

(iii)Angle of load to grain 90º

(i)Angle of load to grain 0º

(ii)Angle of load to grain Ø

Indicates direction offastener length

Indicates direction ofgrain

Load

Load

ø



27

Section2.4

Design Method

Splice JointsDue to the need to make long span trusses fromshorter lengths of timber, butt joints called SPLICESoften need to be introduced in the top and bottomchord members.

These joints, like all other nailplate joints, need to beproperly designed in accordance with the abovefactors. Splice joints will normally occur in positions

between 10% and 25% of the panel length in whichthe splice occurs for triangulated trussed rafters. Inother frames and when splices are outside of the code'zone' the software will design the splice to resistshear, axial and moment forces.

Some typical joint details are given in figure 25.

Figure 25

(1) (2) (3) (4) (5)

(6) (7) (8) (9) (10)

(11) (12) (13) (14) (15)

(16) (17) (18) (19) (20)

(21) (22) (23) (24) (25)

(26) (27) (28) (29) (30)



28

Roof and Trussed Rafter BracingBracing in trussed rafter roofs is essential andperforms specific and separate functions:

1. TEMPORARY BRACINGTemporary bracing is required during erection of thetrussed rafters to ensure that the trusses are erectedvertically plumb, at the correct centres and in a stablecondition for the continuation of construction.

This bracing is the responsibility of the roof erector,(see later for recommendations).

2. TRUSS INTEGRITY BRACINGBracing may be required by the trussed rafter designto prevent out-of-plane buckling of a member ormembers within the truss. This bracing must beprovided to ensure the structural integrity of thetrussed rafter. It is the responsibility of the TrussedRafter Designer to inform the building designer ifthis is required.See figure 26a, 26b and 26c.

LATERAL WEB BRACINGOne shown (with splice) at mid point of webs. For two braces,locate at third-point of webs. Diagonal anchor braces asshown at 6m intervals. All braces 25 x 100 free of majordefects and fixed with two 3.35 x 65mm galvanised nails at allcross-overs.

3. ROOF STABILITY BRACINGIn addition to the above bracing, extra bracing willoften be required to withstand external and internalwind forces on the walls and roof. This area of bracingdesign is the responsibility of the Building Designer (orRoof Designer if one has been appointed) and includessuch areas as diagonal wind bracing, chevron bracingto internal members, longitudinal bracing at truss notepoints, etc.

Figure 26a

Figure 26b

Figure 26c

TRUSS INTEGRITY BRACING(Specified by Trussed rafter Designer)

25 x 100mm member nailed toedge of web fix using 3.35 dia x65mm long R/W galvanisednails, at 150mm centres.

ALTERNATIVE WEB STABILITY BRACE(use when less than three

trusses in line)

Section A-A

Lateral braceto truss chordor web member(web shown)



29

Section2.5

Roof and Trussed Rafter Bracing

Design responsibility Specifiers and designers should understand thatTruss integrity bracing is the responsibility of theTrussed Rafter Designer who must inform theBuilding Designer if such bracing is required.Whereas Roof Stability bracing (and any additionalspecialist bracing) is the responsibility of theBuilding Designer (or Roof Designer if one has beenappointed). The Building Designer is responsible fordetailing ALL bracing.

BS.5268-3 gives some recommendations for certainspecific cases of roofs; for other types of roof thebracing pattern for roof stability should be designedby a competent person. See figure 27a, 27b, 27c and27d.

The Building Designer has access to informationpertinent to the structure i.e. walls, and the forcesbeing transferred from them, which the TrussedRafter Designer cannot determine. (See also section1.2 on Design Responsibilities).

Please refer to BS 5268-3 for further guidance onbracing of roofs for domestic situations.

Figure 27a

Figure 27b

Figure 27d

Figure 27c

(Specified by the building or roofdesigner) for guidance onlyplease refer to BS5268-3.

RAFTERDIAGONALBRACING

Longitudinal binder atnode points

Ceiling diagonalbraces

Note web chevron and rafter diagonal bracingomitted for clarity, see following details.

Should be inclined at approximately 45º and each nailed to at least threetrusses. All 25 x 100mm free of major defects and fixed with 3.35 x 65mm

galvanised nails at all cross-overs.

(One only shown and spliced) webs and all other bracingomitted for clarity. Braces to be 25 x 100mm free of allmajor defects and fixed with two 3.35 x 65mm galvanizednails at all cross-overs including wall plate. Braces to beincluded at approximately 45( to the tiling battens andrepeat continuously along the roof.

WEB CHEVRON BRACING

INTERVAL VIEW

Binder at nodepoint

Binder at node pointCeiling diagonal bracing

Rafter diagonal brace

Web chevronbracing



30

Loading and Load CasesIt is important that all truss loadings are specifiedbefore quotation to ensure correct design. Unless

otherwise advised, trusses will normally be assumedto be for normal domestic use.

TOP CHORD (Rafter)TilesWeight to be as laid. Nearly all commonly usedinterlocking concrete tiles are within 0.575kN/m2,which is regarded as the standard loading. It isimportant that the actual tile weight to be used isnotified to the Trussed Rafter Designer. This loadingis specified as a long term loading on slope; i.e.applied along the length of the sloping rafter.

Felt, Battens, Self WeightThe allowance usually made for felt, battens and selfweight of trusses is 0.11kN/m2.

As are the tiles, this is regarded as a long termloading slope.

WindExcept in the case of vertical and near verticalchords, wind loading is not often a critical criterionin the design of fully triangulated trusses.

All trusses should be designed for wind loading inaccordance with BS 6399: Part 2 code of practice forwind loads. Wind load data should be provided bythe Building Designer to the Trussed RafterDesigner.

Wind loading is treated as a very short term loading,applied at right angles to the relevant members.

SnowDesigns for snow loadings are in accordance with BS6399: Part 3: Actual design loads are dependant uponseveral factors, such as building location, altitudeand roof plane geometry. The loadings imposed bysnow are regarded as medium term loadings, onslope. Where appropriate, snow drifting should beconsidered.

Man Load on RafterThis is specified as 0.75 x 0.9kN in any position. Testhave shown that, in normal circumstances, tiles andbattens provide sufficient transverse load distributionfor this loading not to be a critical criterion in design.However it can dictate the design of a long overhang.This loading is treated as short term loading.

BOTTOM CHORD (Ceiling Tie)Plasterboard, Self Weight etcThe standard ceiling construction of one layer of12.5mm plasterboard and skim coat is taken asgiving a load of 0.25kN/m2 (including truss selfweight).

This load is treated as a long term loading on slope(although generally bottom chords will have noslope).

Light StorageFor normal domestic applications, the specifiedallowance for storage over the length of the bottomchord (ceiling tie) is given as 0.25kN/m2 (on slope).For anything other than this condition, the BuildingDesigner should inform the Trussed Rafter Designerof the required storage loads to be used.

This load, as for the ceiling construction load, istreated as a long term loading on slope.

Man Load on Ceiling TieTo allow for loadings imposed by a person workingin the roof void, an allowance of 0.75 x 0.9kN at anylocation on the bottom chord, either in the bays or atthe node points (joints) should be made. Thisloading is treated as short term loading.

Loadings for Domestic UseThe great majority of trusses fall into this category. Therelevant document, BS 5268-3 describes the minimumloadings which should be taken into account.

The following data provides a useful guide to typicalloading factors in roof design:



31

Section2.6

Loading and Load CasesLoadings - Water TankWater tanks in trussed rafter roofs should besupported by a system of bearers and cross-bearers insuch a fashion that the loadings imposed on thetrusses are transferred to a position as close aspossible to the node points (joints) of the trusses. Thestandard 230 litre water tank is usually supportedover three individual trusses, or 300 litre tank overfour trusses. The long term loading from thisarrangement is taken as 0.9kN/truss (0.45kN pernode).

Loadings - Agricultural BuildingsLoadings for agricultural buildings are described inBS 5502 and are based on weight of the actualmaterials in the fabric of the building. Snow andwind loading criteria depend on occupancyclassification determining the acceptability ofcollapse and expected life of the building.

Compliance with BS 5502 has been a condition ofobtaining certain capital grants and an up-to-datebriefing on the matter should be obtained beforespecification.

PurlinsTrussed rafters are generally used in conjunctionwith tiling battens fixed to the upper edge of the topchords and this provides an excellent method of out-of-plane restraint to the top chords. If tiling battensare not to be used, it is vital to specify the maximumpurlin spacing to be used for two reasons:1.To allow the Trussed Rafter Designer to apply theloads in the correct way.2.To allow the Trussed Rafter Designer to applycorrect top chord restraints.The Trussed Rafter Designer will require thisinformation in order to obtain a correct design.

Load Duration (load cases)

The load-carrying characteristics of timber are suchthat it can sustain heavier loading for a short timethan it can for a long time.

This effect is used in establishing the allowablestructural properties of a particular timber grade (orStrength Class).

Trussed rafters and other structural timbercomponents are then designed taking into accountthe differing durations of the various loadings whichthey are required to carry.

The main loadings encountered in dealing withtrussed rafters (see earlier in this section) are:

1.Roof Coverings:Tiles, slates etc. are considered as long terms loads,as they will be present throughout the life of thebuilding.

2.Ceiling Construction:Plasterboard etc at ceiling level is, like the roofcovering, considered as long term.

3.Ceiling Storage: The allowance for storage in the roof void at ceilinglevel is also treated as an ever-present, long termload.

4.Water Tanks:As these will also be present throughout thebuilding's life loads applied by water tanks aretreated as long term loads.

5.Snow Loadings:The design allowances for loadings due to snow onthe roof are treated as medium term loads, i.e. theseloads will not be present at all times, but will affectthe roof structure only for a period of weeks ormonths at a time.

6.Man Load on Rafter and Ceiling:Where this is applicable, this load is treated as a shortterm load, ie this load will be present within thestructure for a period of minutes or hours only.

7.Wind Loadings:Always considered for design, the loadings due towind are treated as very short term loads. These loadswill be present on the structure for a period ofminutes or seconds only.

The above loadings cover the most usual types ofload carried by trussed rafters.

Other loads may be present within the roof in specialcircumstances These may include air conditioningequipment, patient hoists, climbing ropes etc andmust be allowed for in the design, in the appropriateload case.



32

Loading and Load Cases

The load cases which normally dictate the resultsare:-1.Long TermDesigning for the effect of all long term loads (allloads which will be present throughout the life of thebuilding) i.e.:-Roof coverings-Ceiling constructions-Ceiling storage-Water tanks-any special long term loads.

2.Medium TermTaking into account loads which will be present for aperiod of weeks or months on the building i.e: - Allthe long term loads Plus - snow loads

3.Short TermFor loads which may occur for minutes or hoursduring the buildings life i.e:-All the long term loadsPlus - Snow loadsPlus - The man load on the bottom chord

It will be seen that this load case will, in fact, be amultiple load case as the man load must be checkedat every bottom chord bay and node position. Notethe man load on top and bottom chords are inseparate load cases.

Full details of load cases see BS 5268-3

Figure 28a

Figure 28b

Figure 28c

Water tank

Water tank

Water tank

Snow

Snow

Snow

Man

Snow

Ceiling construction



Storage in ceiling

Storage in ceiling

Storage in ceiling

Tiles/roof covering

Tiles/roof covering

Tiles/roof covering



33

Section2.7

Selecting Trussed Rafter ProfilesDetermination of the required truss rafter profileassumes the ability to read a two dimensionaldrawing and visualise the required structure in threedimensions.

This skill is acquired by experience, although certainpowerful graphical aids in the form of computerprograms are now available to assist the designer inthis area.

The designer's task is to take the roof plans, sectionsand elevations presented to him by his client andform the required roof shape in his mind. Thetechnique of 'sectioning' the roof will then give the

required profile of the truss required at any point.

'Sectioning' requires the designer to consider theheight dimension of a particular point on the roofline, as well as its plan position. The profile changepoints for a truss are often seen on the roof plan aship or valley intersection lines, or at points where theapex or eaves line changes in some respect.

The following figures illustrate the transition fromtwo-dimensional roof plan to three-dimensionalimpression of the roofscape to the required trussedrafter profiles.

Figure 29a

Figure 29b

Figure 29c

ROOF PLAN

FRAMING PLAN

13.0

00

8.000

8.000

16.000

16.000

8.00

0

8.00

0

13.0

00

THREE DIMENSIONALREPRESENTATIONOF ROOF



34

Selecting Trussed Rafter Profiles

As mentioned earlier, computer graphic programs areincreasingly assisting the designer to visualise theroofscape.

The figures in this section have been produced fromMiTek's MI2000 layout, take off and design software.

Figure 30a

Figure 30b

T1:TRIANGULAR T2:TRIANGULAR T3:HALF HIP T4:HALF HIP30.0 30.0 30.0 30.0

8.000 8.000 8.000 8.000

Quantity = 30. Quantity = 7. Quantity = 1. Quantity = 1.T5:HALF HIP T6:HALF HIP GIRDER T7:MONOPITCH T8:VALLEY30.0 30.0 30.0 30.0

8.000 8.000 2.200 0.600

Quantity = 1 Quantity =1 Quantity = 10. Quantity = 1.

T9:VALLEY T10:VALLEY T11:VALLEY T12:VALLEY30.0 30.0 30.0 30.0

1.200 1.800 2.400 3.000

Quantity = 1. Quantity = 1. Quantity = 1. Quantity = 1.T13:VALLEY T14:HIP GIRDER T15:MONOPITCH T16:HIP30.0 30.0 30.0 30.0

3.600 8.000 2.800 8.000Quantity = 1. Quantity = 1. Quantity = 5. Quantity = 1.

ROOF PLAN

T1:TRIANGULAR

35.0

Quantity = 18

T2:CANTILEVER

35.0

Quantity = 7

T3:BOBTAIL

35.0

Quantity = 6

FRAMING PLAN

Figure 30c

10.0

00

4.000 4.0004.000 4.000 3.0003.0003.0003.000 0.80

0

0.80

0

0.80

0

0.80

0

10.0

00

Another illustration of 'sectioning' to find the requiredprofile would be the choice of either a cantilevered or

bobtail truss, as shown in the following figures.



35

Section2.8

Framing Common RoofscapesBobtail (Stub Ends)Bobtail or stub-end trusses are used where thesupporting wall position on one or both sides is set infrom the normal heel position. (see section 3.9,bearing details). This commonly occurs whererecesses occur in the outside wall line of a buildingor where walls are built up to tile level to provide afirebreak compartment within the building.

The horizontal 'A' dimension (figure 32), known asthe cut-back, is therefore conveniently used tospecify the shape for duo-pitch trusses, while doublebobtailed trusses and bobtailed mono-pitched trussesare often more conveniently specified by a vertical'A' dimension, known as the End Height.

Figure 33 shows typical end details when the outerleaf is of masonry. Arrangement 33b is best confinedto timber frame construction as separate columns ofmasonry between trusses could be unstable. Thereshould be sufficient depth of masonry on figure 33ato anchor the roof down against wind uplift.If the end verticals are to be tile clad, one of thearrangements in figures 33c or d is suitable. In figure33c a specially wide timber is used as the end vertical

of the trusses so that the tile battens clear the outerleaf of the wall; the inside of the end vertical memberof the truss must not be located to the right of thecentre-line of the wallplate. In some cases thisarrangement is impractical owing to the large widthrequired for the end vertical. In many cases thediagonal in the cantilevered part (figure 33d) can beomitted if there is little load from the cladding.

Bobtailed trusses must never be formed through on-site modifications of standard truss types with whichthey align, following the basic rule that a trussed

rafter should never be cut, notched, drilled orotherwise modified without first checking with theTrussed Rafter Designer.

Figure 31

Figure 32

Figure 33a Figure 33b Figure 33c Figure 33d

EFFECT OF BOBTAILON ROOF LINE

Span

Timbernib

MASONRY OUTER LEAF

Masonry inner leaf Timber frame

TILE CLAD END VERTICAL

‘A’

‘A’ ‘A’ ‘A’

Span Span



36

Framing Common RoofscapesCantilevered TrussesThe reaction from the bearing is the greatest load(although upwards) to which a truss is subjected, andin order to control excessive bending in thesupported chord, it is important, except in thesmallest trusses, to locate a joint at each bearing. Thenormal eaves joint (figure 34a) accomplishes this ifthe bearing shift is less than one-third of the scarflength or is less than 50mm.

If the shift is greater than the allowed a stress checkis required on the short cantilever. Unfortunately,there is often insufficient space for an additional webso it is usually necessary to increase the bottomchord (figure 34c) or alternatively, to incorporate arelief rafter (figure 34d) or a heel wedge. Both ofthese options can add to the final costs of the trussesand therefore it is best to avoid cantilevering thetrusses in this range.

If the shift is greater than two scarf lengths, then anordinary standard cantilever truss is employed. (fig 35)

Note that the chord sizes are not normally greaterthan the corresponding non-cantilevered standardtruss and the cost is very little more. The MiTeksystem offers many standard cantilevered truss types.Many other variations are possible by adjusting the

position of the joint on a non-cantilevered standardtruss type, so that it is over a bearing.

Finally, if required a non-standard cantilever truss ofalmost any triangulated configuration can bedesigned and fabricated. Note that a brace maysometimes be needed on the bottom chord, which isuntypically in compression.

Figure 34bFigure 34a

Figure 34c

Figure 34d

Figure 35

Standard truss

Increase bottom chord

Standard cantilever

Incorporate relief rafter or slider

Bottom chord may need brace

Scarf Length

ShiftShift

Scarf Length

Check bottom chord



37

Section2.8

Framing Common RoofscapesTypical Roof Features - Hipped EndsThe most common end shapes are the Gable End,which allows the simplest roof framing and usesmost support wall surface; the Hipped End whichoffers a simple wall solution at the expense of a more

complex roof structure, and the Dutch Hip and GableHip, which are compromises between a gable andhip, easily formed using trussed rafters.

Most traditional hipped ends behave like an invertedconical basket and, under load, the tendency for itsrim (the wall plate) to spread is resisted by friction(lateral force on the wall), tension in the rim (tensionand bending in the wall plate) and tension in the weft(the tiling battens). In the long term the results aresagging hip boards and rafters, bulging walls andcharacteristic horizontal cracks in the masonry at theinside corners of the dwellings roughly 300-600mmbelow ceiling level.

However, hipped end systems develop by MiTek donot depend on tension in battens, or a massivewallplate and horizontal resistance from the walls.With suitable bracing, the trussed rafter hip roofprovides the walls with the stability required byBuilding Regulations.

Figure 36a Figure 36b

Figure 36dFigure 36c

Figure 37a

Gable end

Dutch Hip

Action of hip end

Traditional hipped end

Gable Hip

Hipped end

Figure 37b


Framing Common Roofscapes

Hipped EndsThe simplest form of hipped end consists of a multi-ply girder of standard trusses securely nailed orbolted together, which support loose rafters andceiling joists, as in figure 38.

This is the most inexpensive form of hip because nospecial trusses are needed other than the girder, buttheir use is limited to spans up to 5m.

Loose rafter and ceiling joist sizes should be takenfrom Approved Document A to the BuildingRegulations. Hip boards should be supported off thegirder by means of a ledger. The ceiling joists shouldbe supported by proprietory joist hangers.

If the end pitch is different to the pitch of the mainroof, the eaves details should be discussed with yourtrussed rafter supplier. It is advisable to ensure thatthe top extremities of rafter overhangs are at thesame level to provide for continuous guttering. Notethat whilst adjustments can be dealt with on site inloose timber construction, the mono-pitched trussesused in other hip types must be made correctly in thefactory.It should also be noted that all forms of hipconstruction employing a hip board exerts ahorizontal thrust at the wallplate corner junction.Having taken up any horizontal movement, ofcourse, the structure becomes stable. Movement ofthe wallplate can be controlled by fixing a 1200mmlength of galvanised steel restraint strap around theoutside. See figure 41.

MiTek trussed rafter suppliers can provide detailedadvice on hipped end roof details.


38

Figure 38

Figure 40

Figure 41

Figure 39

Simple hip end

LEDGER DETAILS Multi-ply girder

Minimum 38 x 100 ledger

Multi-ply girderHip boards notched

over ledger


EAVES MATCHING

WALLPLATE CORNER REINFORCEMENT



39

Section2.8

Framing Common Roofscapes

Hipped Ends - ‘Stepdown’The step-down hip system uses flat top hip trusses ofprogressively diminishing height from the ridge tothe girder truss position. This system is rarely used aseach truss is different to make. The number of step-down hip trusses is determined by the need tomaintain reasonable sizes for the loose ceiling joistsand hip board in the hipped corner infill areas. Forthese reasons, the span of mono-pitch trusses is notusually greater than 3 metres in the case of regularhips, where the hip end pitch is the same as the pitchof the main roof.

Noggings must be fitted between the flat chords of the step-down

hip truss to support tiling battens. The webconfiguration of the various truss types shown(including the mono-pitch) are typical, but will bechosen to provide the best structural solution.Fortunately, this system is flexible inaccommodating large spans and irregular hips withunequal roof pitches and employs standard, designedtruss types throughout.

Figure 42

Figure 43a

Figure 43b

Figure 43c

Common trusses

Nogging

NoggingNogging

Hipped cornerinfill

Hip board

Proprietory truss shoe

Mono-pitched trusses

Step-down trusses

Girder truss

PLAN VIEW

SECTION

TRUSS COMPONENTS

Step-down 1

Step-down 2

Noggings omitted for clarity

Girder

mono - pitch



40

Framing Common RoofscapesHipped Ends - 'Flying Rafter'Of the many types of hip systems this one has anobvious manufacturing advantage: There is only onebasic hip truss profile. All the hip trusses, includingthose forming the girder truss are identical; and themono-pitch trusses supported off the girder have thesame profile as the sloped part of the hip trusses,which speeds up fabrication and reduces the overallcost of the hip system.

The rafters of the mono-pitched trusses and/or hiptrusses are extended and are site cut to fit against theupper hip board. Off-cuts may be required to benailed in position to the rafters of the hip trusses. Forthe longer rafters props may be required to run downto the trusses underneath.

The flat parts of the top chords of the hip trusses andgirder must be securely braced together to ensurestability.

The hip corner may be constructed from pre-fabricated rafter/joist components commonly calledOpen Jacks or all the corner can be framed with looserafters, joists and hipboards on site. The hip board isnotched over the girder truss and supported offledgers at the apex of the hip.

This system offers the advantage of continuousrafters and thus easily constructed smoothroof slopes.

Typical spans usingthis construction withone primary multi-ply hip girderis 5 - 9.6 metres.

Larger spans, up to 13.2 metres,may be accommodated by the use ofintermediate girders between the main girdercarrying the mono-pitch trusses and the hip apex.

It is possible to construct several types of hip endusing the 'Flying Rafter' concept, or indeed, tocombine the 'Step-down' concept within the hiptrusses with the 'Flying Rafters' on the hip endmono-pitch trusses.

Please contact your truss supplier if you have apreference for a particular method of construction, asthe MiTek design system can encompass any method.

1 Flat top chords require bracing

2 Ledger under to support hip boards

3 'Flying Rafters' on hip trusses(may require props to trusses below)

4 Girder

5 Infill rafters

6 Hipboard

7 Infill ceiling joists

8 Mono-pitch trusses

Figure 44

Rafters omitted for clarity



41

Section2.8

Framing Common RoofscapesHipped CornersA hipped corner is formed by the intersection, at 90degrees, of two roofs which may, or may not be thesame span or pitch.

Hipped corners for mono-pitched and other roofshapes are based on the same principles describedbelow for duo-pitched roofs.

The common framing consist of a SECONDARYhalf-hip girder truss supported by a PRIMARY duo-pitch girder truss. An internal load-bearing wall orbeam support can often be used to perform thefunction of the primary girder truss.

The duo-pitch girder truss is specially designed forthe exceptional loads it carries and includes a widerthan normal vertical web to which a proprietorygirder hanger can be fixed to carry the half-hipgirder.

The roof is built up in the valley area using a mono-pitched valley set so that the half-hip girder carriesthe mono-pitch trusses and hipped corner infill, inthe same way as at a hipped end. The span of themono-pitch trusses is not generally greater than 3metres and more than one half-hip truss may beneeded between the ridge truss and the half-hipgirder.

The details shown correspond to the method ofconstruction used in the Step-down hipped end, inwhich noggins have to be sited between trusses tosupport the tiling battens.

Hipped corners with a Flying Rafter can also beprovided.

Figure 45

Figure 46b

Figure 46a

COMPONENTS OF HIPPED CORNER

PLAN VIEW

Standard truss

Ridge truss

Secondary half-hip girder truss

Primary duo-pitch girder truss

Standard trusses - main span

Seco

ndar

y ha

lf-h

ip g

irde

r tr

uss

Pri

mar

y du

o-pi

tch

gird

er t

russ

Stan

dard

tru

sses

- s

econ

dary

spa

n

Ridge truss

Lay board

Hip board

Noggings

Mono-pitch trusses

Valleyinfill

Hipped corner infill

Mono-pitch truss

Wide vertical web toprovide fixing for girdertruss shoe



42

Framing Common Roofscapes'Tee' Intersections and Valley InfillThe basic junction of two roofs is known as a 'Tee'intersection, where a valley line will be formed at thepoint of intersection of the two sloping planes. Theconstruction around the valley area is commonlyformed by the use of either timber rafters,valleyboards and ridgeboards (not recommended) orby the use of pre-fabricated valley frames (Fig 47b).

It is strongly recommended that valley frames beused in junction areas, as these provide the quickest,cheapest and most structurally effective solution tothe roof framing in these areas.

The use and function of the valley frames are moreimportant than they appear. The individualcomponents transfer the roof loadings to the topchords of the underlying standard trusses in auniform manner. Acting with the tiling battenbetween each neighbouring pair of components, theyprovide lateral stability to the same chords.

Some variations on the basic system are shown infigure 49. Others occur from time to time andsuitable layouts can be easily devised by MiTektrussed rafter suppliers.

The layboards shown in figure 48 are in short lengthsand supported off battens nailed to the sides of therafters, to lie flush with the tops of the rafters. Thisenables the felt and tiling battens to be carriedthrough into the valley. The tile manufacturers adviceshould be sought to ensure correct tile and pitchsuitability.

In many cases, the support for the main roof trussesmay be provided by a multi-ply girder truss as shownin figure 48, with the incoming trusses supported inproprietory Girder Truss Shoes at each intersection.

It is common practice on site to erect the girder trussfirst and position the incoming trusses afterwards.

All MiTek girders are designed to resist stressesinduced in the bottom chords by the supportedtrusses. The connector plates on girders will typicallybe considerably larger than those on the standardtrussed rafters.

As described above, the valley construction shouldinclude intermediate tiling battens betweenneighbouring valley infill trusses, to provide thecorrect restraint for the rafters of the underlyingtrusses.

Figure 47a

Figure 48

Figure 49

Figure 47b

Figure 50

TEE INTERSECTIONSTiling batten betweenneighbouring valleyinfill trusses

Girdertruss

Valleyinfilltrusses

Layboards

TYPE 800

TYPICAL GIRDER TRUSSES

TYPE 1200

Return spanstandard trusses



43

Section2.8

Framing Common RoofscapesDog Leg or Skew CornersA dog leg or skew corner is formed by theintersection of two roofs at an angle other than 90degrees.

It is usual for the incoming and outgoing roofs tohave the same span and pitch, although it is possibleto frame the roof using trussed rafters, if these differ.

The cross section may be of any of the usual shapesbut is generally mono-pitched or duo-pitched.

The feasibility of this framing method depends onthe design of the longest purlin. Although installationof loose ceiling ties from girder to girder may besimpler in carpentry requirements, it is generallypreferable to adopt the layout shown (incorporatingloose timber binders at ceiling level), in order tosimplify plasterboarding.

The ceiling binders should be supported on the bottomchords of the girders and located against the verticalwebs. A robust structural connection for example, withtwo proprietary angle plates should be made betweenbinders and loose ceiling joints. The ends of thebinders may need to be notched or blocked-up off thegirders, to ensure that the undersides of the loose joistsare level with the girders.

The typical framing plan shown for a duo-pitch roofis characterised by the minimum number of differenttruss types and provides a practical solution to theproblems raised in these situations but using looseinfill.

The multi-ply girders used have a number of verticalwebs to allow the fixing of the loose timber purlins,which support loose timber rafters.

The example shown needs a lot of site loose work.However now that there are proprietory metalhangers available to join trusses to girders at anglesother than 90 degrees the whole of the corner couldbe framed with trussed rafters.

Figure 52a

Figure 51

Figure 52c

Figure 52b

DOG LEG

Girder truss

Girder truss

Looserafters

Looserafters

Purlin

Purlin

Purlin

Purlin

Looseceiling ties

Angleplates

Purlin bearing postnailed to web of girder

Binder

Binder

Binder(shown notched)

Binder

Looseceilingjoists

Plan onrafters

Plan onceiling

Type 800

Type 920

Type 1200

Loose rafters notched onto purlin

CONNECTIONS

Girder truss



44

Connection DetailsCareful erection, fixing and strapping is essential if atrussed rafter roof is to provide a sound platform forroof coverings and contribute effectively to thestability of the roof and gable ends.

Strapping gables to ceiling tiesCeiling ti straps may be excluded from thespecification for roof pitches below 20 degrees.Check with the building designer. If they are needed,fix as shown for rafter straps, but attach to the upperedge of the ceiling tie. Use a twisted strap to engagea vertical joint if horizontal courses do not coincide.

Strapping at the separating wallIn addition to the normal strapping to walls,additional straps may have been specified to providelongitudinal bracing between roofs, these should berun over the top of the separating wall and fixed tothe specified number of trusses on each side. Includenoggings and packing to transmit loads properly.

How to fix rafter strapsEngage at least three trusses with each strap. Usegalvanised steel straps 30 x 5mm or approved profilegalvanised steels straps.

Holding down roofs to wallsRoof to wall (vertical) strapping is not requiredunless the location of building construction is knownto be wind stressed, then it is essential to carry outthe roof designer's specifications. Lighter roofcoverings in areas of higher wind load requireholding down straps as may be specified forbrick/block construction. In extreme cases the designmay call for direct strapping of rafters to the walls(see figure 54).

Straps are normally 30 x 2.5mm section galvanisedsteel but any higher specification should e followed.The tops of the straps should be nailed (three 30 x3.75mm nails or more) to the wall plate, or the rafterin the case of a rafter to wall strap. When fixing tothe wall, it is critical that the straps are long enoughto run over the specified number of blocks, and thatat least two of the fixings engage the last full blockat the base of the strap.

Figure 54

Figure 53

Strap fixed to solid noggin with a minimumof four fixings of which at least one is to be

in the third rafter or in a noggin beyondthe third rafter

Use only corrosion resistant nails (65 x3.35mm)

Noggings to be provided and set horizontal unlessthe strap has a twist to line it up with the roof slope.

Strap beddedunder a cut block

Packing piece betweeninner leaf and first rafter.

100mmmin



45

Section2.9

Connection DetailsHeavy-duty joist hanger to BS6178 Part 1These are used to carry trusses or joists at masonryload bearing or fire break walls. Carefulconsideration must always be given to the method ofsupport. We would recommend that advice isobtained from the responsible Building Designer orStructural Engineer since in a number of casesspecial hangers may have to be manufactured. TheBuilding Designer may also specify high densitybrick courses above and below the hangers to avoidcrushing of blocks. The bearing length for these joisthangers is approximately 90mm. (See figures 55 and56).

Heavy-duty girder to girder truss showsThese are designed to support a secondary girder offthe main girder ensuring that the loads are transferredefficiently. The shoe is usually fixed to the maingirder (A) by means of bolts as specified by themanufacturer with washers under the bolt heads andnuts. The bearing length for these shoes isapproximately 120mm. (See figure 57).NB. Refer to manufacturers instructions for thecorrect application and procedure.

Girder truss shoe and long legged hangersGirder truss shows are used to fix single trusses tocompound girders or for other truss to trussconnections. The bearing length is approximately65mm.

The shoe or hanger must have side flanges of a sizewhich suits the depth of the girder chord to which itis fixed. Some joist hangers are suitable only fortimber or timber to truss connections not for truss totruss connections, always use the appropriate hanger.(See figure 58).

Metal fixings used in timber roof structures shouldhave safe working loads which can be substantiatedby freely available reports in accordance withBS6178 and TRADA recommendations. Theyshould always have a manufacturer's mark and showthe certified safe working load.

It is strongly recommended that timber to timberfixings and timber to brick fixings should be suppliedby the Roof Truss Fabricator, and delivered to sitewith the trusses.

NB. For all the hangers and shoes described above,every fixing hole requires a 30 x 3.75mm squaretwisted sheradised nail unless otherwise specified bythe manufacturer.

Figure 55

Figure 56

Figure 57

Figure 58

Hanger for building intobrick or block walls

Incoming trussessupported by bolted

heavy duty shoes andhangers, should be

notched to provide asmooth ceiling line

Straddle hanger forsupporting joists eitherside of a wall or beam



46

Connection DetailsRaised Tie Support Clip (Glide Shoe)This is a special application fixing that has beenspecifically designed to allow horizontal movementat a truss bearing without affecting the overallstability of the truss whilst continuing to provideresistance to lateral and uplift forces.

Used in trussed rafter roof construction the (mediumterm/long term) horizontal deflection should berestricted to a maximum of 6mm per side (trussbearing). A minimum 100mm horizontal seat cutmust be made to fix the upper bearing plate. Thelower bearing plate is fixed to the inner (or inner andouter) edge of the wallplate using 3.75 x 30mmsquare twisted sheradised nails.

The truss is temporarily secured by single nailinginto the centre slots to allow lateral spread betweenthe bearing plates after the roof structure iscompleted. The longer the period of constructionlasts, together with the absolute stiffness of the trussconfiguration, the greater the lateral movement willbe (up to the design limit). Finally additional nailsshould be inserted (3.75 c 30mm long square twistedsherardised) for stability or uplift resistance in theremaining fixing holes.

Truss to be nailed to shoe only after all dead weightshave been imposed

Steel plate fastened to truss(upper part)

Show secured to wallplate(lower part)

Figure 59



47

Section2.9

Connection Details

Figure 60

Figure 61

Figure 62

Horizontal anchor strap forpitched roof and gable ends

Horizontal anchor strap forpitched roof and gable ends

Twisted and benthorizontal restraint strap

Wallplate anchor strap

Truss clip

Truss clips are for fixing timber trusses to wallplates.They avoid the damage often caused by skew nailing.Follow the manufacturers recommendations for safeapplication of truss slip.



48

Problems to be aware ofSome of the more common problems which mayoccur when designing roofs containing trussedrafters are listed below.

1.Trussed rafter or frame?Trussed rafters are fully triangulated frameworks andthere is often confusion when raised tie or Atticframes are required. Such frames are not fullytriangulated trusses rafters, although they are ofsimilar appearance, and their design calls for acompletely difference approach than that for truetrussed rafters.

2.Trusses to match existing constructionWhere trusses are required to align with existing roofconstructions, great care must be taken to obtain thecritical dimensions to which the new trusses shouldbe made. The most common problems in this area arethe misalignment of the outside rafter line and themis-match of overall roof height.

3.Changes in wall thicknessTrussed rafters are commonly supported on the innerleaf of a cavity wall construction. Where both innerand outer leaves of the masonry are required tosupport trusses (common when a garage abuts directlyonto a bungalow for example), great careshould be taken to ensure continuityof the outside rafter line.

4.Changes in adjoining roof pitchIf two adjoining areas of roof are required to be atdifferent pitches, care should be taken to ensure thatthe outside top edges of the rafter overhangs are ofcommon height to provide continuity of fixing forfascia boards.

5.Variations in support conditionsIt is increasingly common for the support conditions fortrusses to be varied from the standard type of heelsupport, to create cantilevered and bobtailed (stub)trusses, where the cantilever or bobtail distance is short(in the order of 100 to 400mm). Such support conditionsmust be specially designed for, (See section 2.8).

6.Depth limits for trussed raftersThe general limit for the manufacture of roof trussesis in the order of 600mm overall depth minimum.Further, it is recommended that the span-to-depthratio (span of truss divided by its overall depth) is notgreater than 10. (See section 1.4).

7.Check that everything fitsEnsure that all water tanks, air-handling plant, servicesetc will fit within the outside profile and will clear theinternal webs. A further point here, is to check that anydeep hangers used to carry special loads will fit withinthe depth available to them, as such items tend to berelatively deep.

8.Fixings to support trussesTimber to timber connections are best made at 90degrees wherever possible, as angled connectionsincrease costs. (See section 2.9).

9.Fixing of hangersWhere hangers are used on the bottom chords ofgirder trusses to support trusses and/or infill members,it is often more practical to provide a deeper bottomchord, usually 125mm or greater, in the girder truss toavoid the need for blocking-out on site.

Where hangers are used in masonry walls to carrytrusses, a sufficient depth of masonry above thehanger should be provided to ensure a secure fixing.(See section 3.9).

10.Alignment of websIn some cases, it is important to align webs on adjacenttrusses within the roof to enable bracing members tocontinue in a straight line or for connections to be madefrom purlins. (See section 2.7).

Figure 63

Figure 64

Figure 65

Figure 66

Figure 67

Truss

Truss

Frame

Frame

Frame

Outside rafter edgedoes not align

If trusses are supported on outer leaf (or single skin wall)some trusses may require short cantilever details

Traditional rafter:Large depth and birdsmouthedover wallplate

Trussed rafter small rafterdepth no birdsmouth

Tank does not fit

Cantilever produced by change of pitch


49

Section3.0

Modifying Trussed Rafters on Site

The basis rule here is DO NOT MODIFY TRUSSED RAFTERS on site unless the prior permission for themodification is obtained from the Trussed Rafter Designer.

Trussed rafters are designed for a purpose and should never be cut, notched, drilled or otherwise modifiedwithout full consideration of the resultant effect.

If for some reason an adjustment in the geometry or internal structure of the truss is required by the site,REFER BACK to the Trussed Rafter Fabricator. He will have the engineering design for the units suppliedand is in the ideal position to co-ordinate any action which may be needed.

Informationfor Site Use


Section3.1 Information for Site Use

50

Do's and Don'ts on SiteSite storageBearers should be placed on a level, hard and drysurface. A waterproof covering must be used toprotect components against rain and sun and to allowgood air circulation. In vertical storage, bearers mustbe high enough to keep rafter overhangs clear of theground. In horizontal storage, bearers must bearranged at close centres to give level support.

Fit it rightFixing problems are eliminated in roof constructionby the use of proprietory Builder Products.Proprietory joist hangers, restraint straps, connectorplates, framing anchors, truss clips and shoes savetime, money and produce a quality job. Your MiTekfabricator will advise you as to the right products forthe job.

Think don't cutTrussed rafters are designed and fabricated for aparticular purpose - and to save work. Trusses mustnot be cut under any circumstance. Truss spacingscan usually be adjusted to take hatch openings andchimney breasts. For large chimney breasts, trussesare specially designed and supported. Don't cut or guess, consult your trussed raftersupplier if you are in doubt.

HandlingCare in the handling trusses must be taken at alltimes. 'See-sawing' trusses on walls or scaffoldingmust be avoided and where necessary an extra manshould be used to prevent the truss being distorted.Large trusses handled mechanically should beadequately supported.

BracingThe Building Designer will have detailed thepermanent roof bracing. Permanent roof bracing isrequired to ensure the stability of the roof. Each roofrequires longitudinal and diagonal bracing. Inspecific cases bracing is also required to stabiliselong web members. Temporary bracing must be fixedduring erection to ensure that trusses are maintainedin a vertical plane.

Support your tankThe extra load of a water tank requires carefulsupport. Additional trimmers across the joistssupport the bearers.

DO STORE ON DO HANDLE WITH DO FIX ITSITE CAREFULLY CARE RIGHT

TRUSSES DO NEED DON'T CUT DO SUPPORTBRACING YOUR TANK

IF IN DOUBT - ASK!


Information for Site Use

51

Section3.2

Supporting Water TanksWater tank support for standard Fink Trusses

Water tank support platform

Figure 68

Figure 69

Figure 68Tank support to be asDetail A or B (See page 54)

Truss spacings shown are based ona maximum module of 600mm c/c

Tank placed centrally

Ties and bracing omitted for clarity

Node point

Node point

Bearer 'a' placed as close tonode point as possible

Tank support to be asDetail A or B (See page 54)

46 x 97mm minimumsole plate tight to node

point and securelynailed to trusses


Note: Always carefully brace elevated tankplatforms back to main truss

25 x 100mm bracing Studs notched to fullysupport bearer 'a'

Span of trusses Ls

Span of trusses LsBay size

Bay size



52

Supporting Water TanksWater tank support for Mono Pitch Trusses

Water tank support platform for Mono Pitch Trusses

Figure 70

Figure 71

Tank support to be as DetailA or B (see page 54)


Standard mono pitched trussas main roof

xDo no fix to diagonal webunless truss has beendesigned to carry the load

Bay size

Mono pitched trusses withreversed internal members

Node point

Bearer 'a' placed as closeto node point as possible

If mono pitch trusses aresupported on wall as

shown then a pack willbe required under

bearer 'b'

Alternatively

If supported at the wallface on hangers then the

bearer 'b' should bebuilt into wall 100mm

Mono truss configuration haslimited space for a water tank

unless of a reasonably highpitch. Alternatively member 'X'

can be reversed as shown in theabove illustration

Bearer 'a' securely nailed totruss verticals

50mm x truss width bearerssecurely nailed to truss

verticals to support bearers 'a'

A minimum of 25mm should be left betweenbearer 'c' and the truss ceiling member to

allow for long term deflection



53

Section3.2

Supporting Water TanksWater tank support for Small Span Trusses

Water tank support for Open Plan Attic Trusses

Figure 70

Figure 70



Ridgeboard supportledger (Nailed tomultiple girder truss)

Standard truss to main roof

Infill rafters(to be

minimum25mm deeperthan trussed

rafter memberto allow for

birdsmouthingat wallplate

Bay size(2.0m Max)

Compound bearer andbinder 'a' placed as close tonote point as possible

Infill ceiling joist 1.8mm Max

Bearer 'a' placed asclose to node point

as possible

High level water tank support

Alternative support positions

Low level watertank support

Packs

Bay size

Bay size


A minimum of 25mm should be left betweenbearer 'c' and the truss member to allow

for long term deflection



54

Supporting Water TanksDetail 'A' Detail 'C'

Table 1: Sizes for support member

Tank capacity to marked Minimum member size (mm) Max span Ls for fink trussed Max bay size for otherwater line a and c b rafters (m) configurations (m)

Detail 'A' not more than 300 47 x 72 2/35 x 97 or 6.50 2.20litres on 4 trussed rafters 1/47 x 120

47 x 72 2/35 x 120 or 9.00 2.801/47 x 145

47 x 72 2/35 x 145 12.00 3.80

Detail 'B' not more than 230 47 x 72 1/47 x 97 6.50 2.20litres on 3 trussed rafters 47 x 72 2/35 x 97 or 9.00 2.80

1/47 x 120

47 x 72 2/35 x 120 or 12.00 3.801/47 x 145

Detail 'C' not more than 300 1/72 x 145 or 1/72 x 145 or 6.00 2.00litres on 2 multiple trusses 2/35 x 145 2/35 x 145

as shown

Figure 74

Figure 75

Figure 76

Detail 'B'

If water tanks are to be supportedon the trussed rafters in accordancewith the details shown then theprovision for these must be takeninto account at the design stage.

Otherwise, the additional loadsimposed by the water tanks must besupported independent of thetrussed rafters.

Where space is limited this detail may be used between members a & b and b& c inorder to gain head room. However a minimum clearance of 25mm above the ceilinglining or truss member must be allowed for possible long term deflection.

Note: Support members may be of any species with a permissible bending stress not less than that of European redwood/whitewood of C16strength class or better.

Longitudinal ties to be offset to cleartank bearers and placed as close tonode point as possible

Longitudinal ties to be offset to cleartank bearers and placed as close to

node point as possible

Lap joint to be over2 No trussesS=trussed rafterspacing max 600mm c/c

Node point

Node point

Node point

Bay size 2m max

Joist hanger

Min 25mm

Alternative supportbetween membersfor detail ‘A’ & ‘B’



55

Section3.3

Hatch and Chimney openingsWhere possible, hatches and chimneys should beaccommodated in the standard spacing betweentrusses.

Each member and joint in a truss performs animportant role, essential to the effective functioning ofall other parts and the component as a whole. Trussesmust never be cut and trimmed except according todetails supplied by the Trussed Rafter Designer.

The principle behind the methods and details given inthis section is to ensure that no individual trussed rafteris subject to a load significantly greater than thatapplied, were it at standard spacing.

Figure 77a shows a system suitable for openingsgreater than 10% over standard and up to twicestandard spacing. Battens and plasterboard should begiven extra support.

Support of the loose timbers is provided in line witheach truss joint by a purlin, binder or ridge board andby trimmers at the actual opening.

When the two trimming trusses at each side of theopening (figure 77b) are actually nominally fixedtogether with nails, at say 600mm centres along allmembers, an opening of up to three standard spacingsmay be used. Deeper purlins, binders and ridge boardof typically 47 x 175mm and trimmers of 47 x 125mmshould be installed.

Depending on the design of the chimney flue andstack, appropriate clearance should be allowedbetween timber and chimney.

Figure 77a

Figure 77c

Figure 77b

Figure 77b

Although intended primarily for trussedrafters, the above principles can also beused for framing with Attic Frames.Raised Tie or Extended joist trussesrequire careful consideration whenframing around hatch or chimneyopenings, as often reinforcing timbers(Scabs) are already required on the'standard' unit and it is often not possibleto design multiple ply units of this type.

S

S

B

C

B

S

S

S SC up to 3S

B

A A

BC up to 2S

Trimmer Ridgeboard

Loose rafter deeper than trussesbirdsmouthed onto wallplate

and purlin Ridgeboard

CollarLedger supportingridge board of trusses

PurlinTrimmer

Trimmer Trimmer

S = Standard spacingC = Chimney openingB = Reduced spacingA = Effective spacing



56

Two Tier ConstructionIt is necessary to use two tier trussed rafters when thevertical dimension of a single component would betoo large for manufacturing or transportation. This

dimension is generally 3.9 - 4.4m but your MiTekfabricator will advise you when to expect trusses inthis form.

The two tier truss (figure 78) comprises a flat-toppedbase truss and a triangular capping truss, fittingalongside the base truss on longitudinal bearers.Each truss may be one of a large selection of types.The base truss will generally be made as high aspractical but not so high that the span of the cappingtruss is less than 2-3m. Although a duo-pitched shapeis shown in figure 78, all basic configurations can beconstructed by the two tier method.

The bracing of the flat top chord of the base truss isimportant in ensuring its performance incompression.

The base trusses should be erected, full permanentbracing installed and battens fixed, up to the topposition of the capping trusses. The resultingstructure then forms a safe, rigid working platformfor the erection of the capping trusses. Tiling orloading of the base trusses should not proceed untilthe capping trusses are fully installed and braced.

Figure 78

Figure 79a

Figure 79d

Figure 79c

Figure 79b

Capping truss

Capping truss

Capping trusses

Minimum 38 x 75ledger

Splice. N.B. Not in linewith other edge of basetruss

Flat top chord of basetrusses25 x 100

brace

47/50 x 100bearer

25 x 100braceBase trusses

Base trusses

minimum 47/50 x100 bearer

Minimum 63.75 x 65mmgalvanised

round wire nails

Base truss

50 x 100 bearerscontinuous forlength of roof(see fig79b)

25 x 100 bracesat 600mm centres

(see fig79c)

400 to 500mmrafter overlap(see fig79a)

Cap

Often the cap truss sits in the sameplane as the base truss and they areconnected together using a MiTekField Splice plate

Field splice plate pressed intoone part in factory and nailedin other part on site

Base



57

Section3.5

Hip Ends and CornersTypical Roof Features - Hipped EndsThe most common end shapes are the Gable End,which allows the simplest roof framing and usesmost support wall surface; the Hipped End whichoffers a simple wall solution at the expense of a more

complex roof structure, and the Dutch Hip and GableHip, which are compromises between a gable andhip, easily formed using trussed rafters.

Most traditional hipped ends behave like an invertedconical basket and, under load, the tendency for itsrim (the wall plate) to spread is resisted by friction(lateral force on the wall), tension in the rim (tensionand bending in the wall plate) and tension in the weft(the tiling battens). In the long term the results aresagging hip boards and rafters, bulging walls andcharacteristic horizontal cracks in the masonry at theinside corners of the dwellings roughly 300-600mmbelow ceiling level.

However, hipped end systems develop by MiTek donot depend on tension in battens, or a massivewallplate and horizontal resistance from the walls.With suitable bracing, the trussed rafter hip roofprovides the walls with the stability required byBuilding Regulations.


Figure 36dFigure 36c

Figure 37a

Gable end

Dutch Hip

Action of hip end

Traditional hipped end

Gable Hip

Hipped end

Figure 37b



58

Hip Ends and CornersHipped EndsThe simplest form of hipped end consists of a multi-ply girder of standard trusses securely nailed orbolted together, which support loose rafters andceiling joists, as in figure 38.

This is the most inexpensive form of hip because nospecial trusses are needed other than the girder, buttheir use is limited to spans up to 5m.

Loose rafter and ceiling joist sizes should be takenfrom Approved Document A to the BuildingRegulations. Hip boards should be supported off thegirder by means of a ledger. The ceiling joists shouldbe supported by proprietory joist hangers.

If the end pitch is different to the pitch of the mainroof, the eaves details should be discussed with yourtrussed rafter supplier. It is advisable to ensure thatthe top extremities of rafter overhangs are at thesame level to provide for continuous guttering. Notethat whilst adjustments can be dealt with on site inloose timber construction, the mono-pitched trussesused in other hip types must be made correctly in thefactory.It should also be noted that all forms of hipconstruction employing a hip board exerts ahorizontal thrust at the wallplate corner junction.Having taken up any horizontal movement, ofcourse, the structure becomes stable. Movement ofthe wallplate can be controlled by fixing a 1200mmlength of galvanised steel restraint strap around theoutside. See figure 41.

MiTek trussed rafter suppliers can provide detailedadvice on hipped end roof details.

Figure 38

Figure 40

Figure 41

Figure 39

Simple hip end

LEDGER DETAILS Multi-ply girder


Multi-ply girderHip boards notched

over ledger


EAVES MATCHING

WALLPLATE CORNER REINFORCEMENT



59

Section3.5

Hip Ends and CornersHipped Ends - ‘Stepdown’The step-down hip system uses flat top hip trusses ofprogressively diminishing height from the ridge tothe girder truss position. This system is rarely used aseach truss is different to make. The number of step-down hip trusses is determined by the need tomaintain reasonable sizes for the loose ceiling joistsand hip board in the hipped corner infill areas. Forthese reasons, the span of mono-pitch trusses is notusually greater than 3 metres in the case of regularhips, where the hip end pitch is the same as the pitchof the main roof.

Noggings must be fitted between the flat chords of the step-down

hip truss to support tiling battens. The webconfiguration of the various truss types shown(including the mono-pitch) are typical, but will bechosen to provide the best structural solution.Fortunately, this system is flexible inaccommodating large spans and irregular hips withunequal roof pitches and employs standard, designedtruss types throughout.

Figure 42

Figure 43a

Figure 43b

Figure 43c

Common trusses

Nogging

NoggingNogging

Hipped cornerinfill

Hip board

Proprietory truss shoe

Mono-pitched trusses

Step-down trusses

Girder truss

PLAN VIEW

SECTION

TRUSS COMPONENTS

Step-down 1

Step-down 2

Noggings omitted for clarity

Girder

mono - pitch



60

Hip Ends and CornersHipped Ends - 'Flying Rafter'Of the many types of hip systems this one has anobvious manufacturing advantage: There is only onebasic hip truss profile. All the hip trusses, includingthose forming the girder truss are identical; and themono-pitch trusses supported off the girder have thesame profile as the sloped part of the hip trusses,which speeds up fabrication and reduces the overallcost of the hip system.

The rafters of the mono-pitched trusses and/or hiptrusses are extended and are site cut to fit against theupper hip board. Off-cuts may be required to benailed in position to the rafters of the hip trusses. Forthe longer rafters props may be required to run downto the trusses underneath.

The flat parts of the top chords of the hip trusses andgirder must be securely braced together to ensurestability.

The hip corner may be constructed from pre-fabricated rafter/joist components commonly calledOpen Jacks or all the corner can be framed with looserafters, joists and hipboards on site. The hip board isnotched over the girder truss and supported offledgers at the apex of the hip.

This system offers the advantage of continuousrafters and thus easily constructed smoothroof slopes.

Typical spans usingthis construction withone primary multi-ply hip girderwould be 9.6 metres.

Larger spans, up to 13.2 metres,may be accommodated by the use ofintermediate girders between the main girdercarrying the mono-pitch trusses and the hip apex.

It is possible to construct several types of hip endusing the 'Flying Rafter' concept, or indeed, tocombine the 'Step-down' concept within the hiptrusses with the 'Flying Rafters' on the hip endmono-pitch trusses.

Please contact your truss supplier if you have apreference for a particular method of construction, asthe MiTek design system can encompass any method.

1 Flat top chords require bracing

2 Ledger under to support hip boards

3 'Flying Rafters' on hip trusses(may require props to trusses below)

4 Girder

5 Infill rafters

6 Hipboard

7 Infill ceiling joists

8 Mono-pitch trusses

Figure 44

Rafters omitted for clarity



61

Section3.5

Hip Ends and CornersHipped CornersA hipped corner is formed by the intersection, at 90degrees, of two roofs which may, or may not be thesame span or pitch.

Hipped corners for mono-pitched and other roofshapes are based on the same principles describedbelow for duo-pitched roofs.

The common framing consist of a SECONDARYhalf-hip girder truss supported by a PRIMARY duo-pitch girder truss. An internal load-bearing wall orbeam support can often be used to perform thefunction of the primary girder truss.

The duo-pitch girder truss is specially designed forthe exceptional loads it carries and includes a widerthan normal vertical web to which a proprietorygirder hanger can be fixed to carry the half-hipgirder.

The roof is built up in the valley area using a mono-pitched valley set so that the half-hip girder carriesthe mono-pitch trusses and hipped corner infill, inthe same way as at a hipped end. The span of themono-pitch trusses is not generally greater than 3metres and more than one half-hip truss may beneeded between the ridge truss and the half-hipgirder.

The details shown correspond to the method ofconstruction used in the Step-down hipped end, inwhich noggins have to be sited between trusses tosupport the tiling battens.

Hipped corners with a Flying Rafter can also beprovided.

Figure 45

Figure 46b

Figure 46a

COMPONENTS OF HIPPED CORNER

PLAN VIEW

Standard truss

Ridge truss

Secondary half-hip girder truss

Primary duo-pitch girder truss

Standard trusses - main span

Seco

ndar

y ha

lf-h

ip g

irde

r tr

uss

Pri

mar

y du

o-pi

tch

gird

er t

russ

Stan

dard

tru

sses

- s

econ

dary

spa

n

Ridge truss

Lay board

Hip board

Noggings

Mono-pitch trusses

Valleyinfill

Hipped corner infill

Mono-pitch truss

Wide vertical web toprovide fixing for girdertruss shoe


Valley Intersections'Tee' Intersections and Valley InfillThe basic junction of two roofs is known as a 'Tee'intersection, where a valley line will be formed at thepoint of intersection of the two sloping planes. Theconstruction around the valley area is commonlyformed by the use of either timber rafters,valleyboards and ridgeboards (not recommended) orby the use of pre-fabricated valley frames.

It is strongly recommended that valley frames beused in junction areas, as these provide the quickest,cheapest and most structurally effective solution tothe roof framing in these areas.

The use and function of the valley frames are moreimportant than they appear. The individualcomponents transfer the roof loadings to the topchords of the underlying standard trusses in auniform manner. Acting with the tiling battenbetween each neighbouring pair of components, theyprovide lateral stability to the same chords.

Some variations on the basic system are shown infigure 49. Others occur from time to time andsuitable layouts can be easily devised by MiTektrussed rafter suppliers.

The layboards shown in figure 48 are in short lengthsand supported off battens nailed to the sides of therafters, to lie flush with the tops of the rafters. Thisenables the felt and tiling battens to be carriedthrough into the valley. The tile manufacturers adviseshould be sought to ensure correct tile and pitchsuitability.

In many cases, the support for the main roof trussesmay be provided by a multi-ply girder truss as shownin figure 48, with the incoming trusses supported inproprietory Girder Truss Shoes at each intersection.

It is common practice on site to erect the girder trussfirst and position the incoming trusses afterwards.

All MiTek girders are designed to resist stressesinduced in the bottom chords by the supportedtrusses. The connector plates on girders will typicallybe considerably larger than those on the standardtrussed rafters.

As described above, the valley construction shouldinclude intermediate tiling battens betweenneighbouring valley infill trusses, to provide thecorrect restraint for the rafters of the underlyingtrusses.

Figure 47a

Figure 48

Figure 49

Figure 47b

Figure 50

TEE INTERSECTIONSTiling batten betweenneighbouring valleyinfill trusses

Girdertruss

Valleyinfilltrusses

Layboards

TYPE 800

TYPICAL GIRDER TRUSSES

TYPE 1200

Return spanstandard trusses


62


LATERAL WEB BRACINGOne shown (with splice) at mid point of webs. For two braces,locate at third-point of webs. Diagonal anchor braces asshown at 6m intervals. All braces 25 x 100 free of majordefects and fixed with two 3.35 x 65mm galvanised nails at allcross-overs.3.

3. ROOF STABILITY BRACINGIn addition to the above bracing, extra bracing willoften be required to withstand external and internalwind forces on the walls and roof. This area of bracingdesign is the responsibility of the Building Designer (orRoof Designer if one has been appointed) and includessuch areas as diagonal wind bracing, chevron bracingto internal members, longitudinal bracing at truss nodepoints, etc.

Figure 26a

Figure 26b

Figure 26c

TRUSS INTEGRITY BRACING(Specified by Trussed rafter Designer)

25 x 100mm member nailed toedge of web fix using 3.35 dia x65mm long R/W galvanisednails, at 150mm centres.

ALTERNATIVE WEB STABILITY BRACE(use when less than three

trusses in line)

Section A-A

Lateral braceto truss chordor web member(web shown)


63

Section3.7

Bracing Trussed Rafters and RoofsBracing in trussed rafter roofs is essential andperforms specific and separate functions:

1. TEMPORARY BRACINGTemporary bracing is required during erection of thetrussed rafters to ensure that the trusses are erectedvertically plumb, at the correct centres and in a stablecondition for the continuation of construction.

This bracing is the responsibility of the roof erector,(see later for recommendations).

2. TRUSS INTEGRITY BRACINGBracing may be required by the trussed rafter design toprevent out-of-plane buckling of a member or memberswithin the truss. This bracing must be provided toensure the structural integrity of the trussed rafter. It isthe responsibility of the Trussed Rafter Designer toinform the building designer if this is required.See figure 26a, 26b and 26c.



64

Design responsibility Specifiers and designers should understand thatTruss integrity bracing is the responsibility of theTrussed Rafter Designer who must inform theBuilding Designer if such bracing is required.Whereas Roof Stability bracing (and any additionalspecialist bracing) is the responsibility of theBuilding Designer (or Roof Designer if one has beenappointed). The Building Designer is responsible fordetailing ALL bracing.

BS.5268-3 gives some recommendations for certainspecific cases of roofs; for other types of roof thebracing pattern for roof stability should be designed bya competent person. See figure 27a, 27b, 27c and 27d.

The Building Designer has access to informationpertinent to the structure i.e. walls, and the forcesbeing transferred from them, which the TrussedRafter Designer cannot determine. (See also section1.2 on Design Responsibilities).

Please refer to BS 5268-3 for further guidance onbracing of roofs for domestic situations.

Figure 27a

Figure 27b

Figure 27d

Figure 27c

(Specified by the building or roofdesigner) for guidance onlyplease refer to BS5268-3.

RAFTERDIAGONALBRACING

Longitudinal binder atnode points

Ceiling diagonalbraces

Note web chevron and rafter diagonal bracingomitted for clarity, see following details.

Should be inclined at approximately 45( and each nailed to at least threetrusses. All 25 x 100mm free of major defects and fixed with 3.35 x 65mm

galvanised nails at all cross-overs.

(One only shown and spliced) webs and all other bracingomitted for clarity. Braces to be 25 x 100mm free of allmajor defects and fixed with two 3.35 x 65mm galvanisednails at all cross-overs including wall plate. Braces to beinclined at approximately 45( to the tiling battens andrepeat continuously along the roof.

WEB CHEVRON BRACING

INTERVAL VIEW

Binder at nodepoint

Binder at node pointCeiling diagonal bracing

Rafter diagonal brace

Web chevronbracing

Bracing Trussed Rafters and Roofs



65

Section3.8

Loose Timber Connection DetailsThe use of loose infill members and purlins is quitecommon on the more complex trussed rafterroofscapes. The nett result is an increased loadimposed upon the trussed rafters, which has to beaccommodated in the design and the requirement ofa secure fixing of the loose timbers to the trusses.

It is important to position incoming purlins at thenode points of the trusses and details 80 to 83 showpractical fixing methods for variants in webarrangements at a joint.

It is necessary to manufacture trussed rafters eitherside of a loose infill area, with webs that align to easethe fixing.

It is also practical to manufacture trussed rafters withwider than normal webs to allow more tolerance forthe fixing of the infill members, and is essential forthe fixing of special girder hangers where larger sizebolts are required.

Figure 80

Figure 81

Figure 83

Figure 83a

Figure 82

Figure 80a

For skew corner situation read in conjunctionwith figure 82

Use similar detail at apex for hipboard support

Purlin (width = w)

65 x 25 Purlinrestraint fixedwith 6 no 3.35 x75mm long roundwire galvanisednails at 80mmcentres

Purlin restraint

Purlin

W + 25

View A

8080

8080

8080

Load appliedto lower joint

Supporting truss

Purlin fitted tight againstunderside of chord and edgeof web (no notching of trusspermitted)

50 x 75 Purlin support postfitted tight against truss downto bottom chord joint, fixed withone row of 3.35 x 75mm longround wire galvanised nailsOne row of nails at

centreline of edge ofweb/purlin support post

Supporting truss,chain line indicatesvertical web

NB. Centreline of oncoming purlinis set out to intersect with centrepoint of purlin support post/ledger

50 x 125 purlin/hipboardledger

Purlin (hipboards notchedto sit in similar

positions)

Purlin supportpost fixedcentrally toweb asfigure 80

Purlin support post. Size 50 x (W + 25), extending toceiling joist. Fix with one row of 4.0 x 100mm long roundwire galvanised nails at 80mm centres for full length of post

Max allowable load for this detail: 4.0Kn for C245.0Kn for TR26

NB. To support lower edge of upper hipboard use 50 x 150hipboard ledger bolted to vertical/angled web using M12bolts (refer to purlin, hipboard support at apex with non-vertical webs for typical fixing details See figure 81)

X

X

X

Line ofloose rafter

Line ofloose rafter

Lower hipboard

Lowerhipboard

Hip girdertruss

Hipgirdertruss

75 x 50mm hipboard supportpost extending to ceiling joist.Fix with one row of 4.0 x100mm long round wiregalvanised nails at 80mmcentres for full length of post

Angledweb

Loadapplied tolower joint

M12 bolts with50 x 5mm thick

round washers ateach end. (Boltlength = truss

thickness +75mm)

Load applied tolower joint


Ledger support to upperhipboard if required

75 x 50mm hipboard support post

1st mono-pitch truss

1st mono-pitch truss


Section X - X

Web Post

Purlin

Purlin

For ply of truss see calculations



66

Bearing DetailsThe greatest loads to which normal trusses aresubjected are the upward forces/reactions from thesupport through the bearings. Except for very smalltrusses, the line of action of these forces should beclose to the centre of a joint, or a structural penalty,in the form of very large timber sizes, will beincurred owing to large bending moments.

The standard eaves detail (figure 84a) is satisfactoryif the shift is not greater than 50mm, or not greaterthan on third of the scarf length. The 'Alternate' or'French' heel (figure 84b) is considered in the sameway but the key position is where the line of theunderside of the rafter intersects the underside of theceiling tie.

Another point to note is that as a truss ends at avertical chord, (figure 84c) there is little scope fortolerance on length or verticality.

Where trusses are to be supported off the face of awall (figure 84d), placing a nib at the heel of the trussis the most common solution. It is good practice toallow a nominal gap between the vertical chord ofthe truss and the masonry, for constructionaltolerance (figure 84e and 84f). Depending on thereaction, and the grade and size of the timber in thebottom chord, a simple extension of the bottomchord may suffice (figure 84e) to form a 'nib'.

Should the bending or shear stress in the nib beexcessive the whole joint can be reinforced. (figure84f). At greater spans it is possible to use the detailin figure 84g to locate the point of intersection of theprinciple forces vertically over the bearing.

Figure 84a

Figure 84c Figure 84d

Figure 84e Figure 84f

Figure 84b

Standard heel joint

Constructional tolerance Constructional tolerance

Frenchheel joint

(GirderHeel)

Shift Shift



67

Section3.9

Bearing DetailsConsideration should also be given to protecting thetimber and fasteners against dampness andaggressive ingredients in the mortar by using a dpcbetween truss and masonry.

Trusses can also be supported off the face of a wallby use of suitable hangers. The installationinstructions should be noted especially concerning

the minimum height of masonry above the hangerflange and the maximum gap between the end of thetrusses and the back plate of the hangers. The effectof the eccentric loading on walls loaded in this wayshould also be assessed.

Top hung fixings (figure 84j) are not common since,at ceiling level, the wall generally needs lateralrestraint from the roof against wind and the ceilingties need to be stabilised.

Flat topped girders supporting trusses (figure 85) canbe supplied with a shaped fillet (figure 85a); ortrusses may have a wedge or block plated into thejoint to provide a horizontal bearing surface (figure85b).

Figure 84g

Figure 85


Figure 84h Figure 84jWall

Blockplated totruss

Flat toppedgirders

Shaped filleton girder

Wedge platedto truss

Hangerfitted tightto wall

Maximum6mm gapbetweenend oftruss andface ofhanger



68

Ventilation and CondensationGeneralRoofs incorporating trussed rafters should bedesigned to service class 1 & 2 as defined in BS5268: Parts 2 & 3. Guidance on the prevention ofcondensation in roofs is given in BS 5250.

Trussed rafters using metal fasteners should not beused where there is likely to be aggressive chemicalpollution, unless special precautions are taken toensure durability of the roof timbers and fasteners.Consideration should also be given to the possibilityof the corrosion of fasteners in contact with sometype of insulation materials.

Reasonable access to the roof space should beprovided to allow periodic inspection of the timberand fasteners.

Thermal InsulationIn the majority of trussed rafter roofs, the insulationrequired to comply with the statutory regulations forthermal transmittance (U value), is provided byplacing the insulating material between the ceiling tiemembers on top of the ceiling board. Placinginsulation at this level results in a COLD ROOFSPACE.

Alternatively, the insulation may be fixed at rafterlevel, resulting in a WARM ROOF SPACE. A warmroof space is normally constructed where habitablerooms are to be provided within the roof, as in Atticor Room in the Roof construction.

VentilationIt is essential that cold roof spaces are effectivelyventilated to the outside to prevent condensation,which may form in the roof void.

In addition, to minimise ingress of water vapour intothe roof space from rooms below, all joists andservice entry holes in the ceiling construction shouldbe sealed effectively.

The location and size of ventilation openings shouldbe determined by the Building Designer, takingparticular account of possible blockage by insulatingmaterials, sound transmission, spread of fire andentry of birds, driving rain or snow. Openings shouldbe located near ceiling level in the eaves or externalwalls enclosing the roof space, or both, and should beequally distributed between at least two oppositesides of the roof. Additional ventilators may also beplaced in the ridge.

The size and number of openings may be calculated,taking into account all the relevant factors, butdisregarding any fortuitous ventilation through theroof covering, or they may be specified inaccordance with the recommended minimumopenings given on the following page. These areexpressed as the minimum width of a continuous gapbut, alternatively, a series of discrete openings of anequivalent total area may be specified, provided thatthe least dimension of any opening, gap or mesh isnot less than 4mm.

Figure 86

Bring insulation materialdown into cavity



69

Section3.10

Ventilation and CondensationThe ventilation of mono-pitched roofs at ceiling levelonly may allow air to stagnate at the apex of the roof.To prevent this, high level or ridge ventilation,equivalent in total area to that given in the table,should be provided in addition to the ventilation atceiling level.

Similarly, air stagnation may occur in duo-pitchedroofs of more than 20 degree pitch, or 10.00m spanand consideration should be given to the provision ofadditional high level or ridge ventilation, equivalentto a continuous gap 5mm wide.

When insulation material is close to the roofcovering, as at the eaves, or where it is placed atrafter level to form a warm roof space, (as in attic andraised tie construction),

Minimum ventilation openings

it is essential to provide an air gap or not less than50mm between the top of the insulation and theunderside of the roof covering or sarking. This gap,which should allow uninterrupted air circulationimmediately above the insulation, should beventilated to the outside at the eaves and, wheninsulation is placed at the rafter level, also at theridge. The minimum opening at the eaves to provide

adequate ventilation of the air gap above theinsulation in a warm roof space should not be lessthan that shown in the table for low level ventilationand the ridge ventilation should be not less that thatprovided by a continuous gap 5mm wide. In normalcircumstances, further ventilation of warm roofspaces is not required.

Figure 87

Figure 88

Figure 89

Roof space ventilator

Roof space ventilator

Eaves soffit ventilator

Felt or tile underlayOptional tilt fillet

Pitch of roof (degrees) 0 to 15 Above 15

Low level ventilationat ceiling level.

Minimum width of 25 10continuous gap on at leasetwo opposite sides of roof.

High level ventilationfor mono-pitched roofs

at or near the ridge. 5 5Minimum width

of continuous gap.



70

Site Storage and HandlingIntroduction(General Information relating to Health andSafety issues in Trussed Rafter Construction).

When the Construction (Design and Management)Regulations were published in 1994, a fundamentalchange in approach was initiated with regard to theattitude toward and significance of issues relating toHealth and Safety in the Construction Industry.Since that time, a raft of further supportinglegislation has been drafted and published whichtogether now document in great detail the duties,obligations and responsibilities of those engaged inthe process of Construction, from members of theoriginal design team to trainee operatives working onsite.

In order to fully understand and implement therequirements of these Regulations it is necessary toappreciate and recognise these new philosophies bymaking the necessary changes in working practicesto elevate the profile of Health and Safety issuesacross the full spectrum of Construction Activities.This can be achieved by undertaking RiskAssessments, designing out hazards where evident,providing sufficient resources at all times, propertraining and good levels of communication channelswithin the design team and on site.

The advice that is set out within the Sections of thishandbook which provide assistance relating to issuesof Health and Safety is therefore illustrative only anddoes not form prescriptive advice on any of thematters discussed. It is vital that each project shouldbe approached by the parties involved as a freshchallenge from the point of view of Health andSafety to allow creative and innovative solutions tobe developed. Readers of this handbook are thereforeencouraged to fully acquaint themselves with thevarious Regulations, and particular:-

Health and Safety at Work Act 1974Construction (Design and Management) Regulations1994Management of Health and Safety at workRegulations 1992Provision and Use of Work Equipment Regulations1992

Construction (Health, Safety and Welfare)Regulations 1996 - (CHSW Regulations 1996)Manual Handling Operations 1992Workplace (Health, Safety and Welfare) Regulations1992

Unloading Trussed Rafters

(Information for the safe unloading of trussedrafters).

When the delivery of trussed rafters arrives on sitethe contractor(s) involved should be prepared andalready allocated sufficient and suitable resources toensure that trussed rafters are unloaded safely and ina manner so as not to overstress or damage thetrusses. This operation will have been subject to aContractors General Risk Assessment and thendetailed in a safe working method statement that hasbeen approved by the principal contractor or theperson responsible for Health and Safety on site.Normally trussed rafters will be delivered in tightbundles using steel or plastic bindings. This willoften require mechanical handling equipment, suchas a forklift or crane, to enable the safe manoeuvringof these large units. The safe working methodstatement should accommodate any special handlinginstructions or hazards specified by the designer inhis risk assessment for the truss design.

Site Storage of Trussed Rafters

(Methods for the proper and safe storage of trussedrafters on site).

Trussed Rafters can be safely stored vertically orhorizontally at ground level or on any other properlydesigned temporary storage platform above groundlevel. Whichever method and location is chosen thetemporary support should be set out to ensure that theunits do not make direct contact with the ground orany vegetation and be so arranged as to prevent anydistortion. The delivery of trussed rafters shouldwherever possible be organised to minimise sitestorage time, however where longer periods ofstorage are anticipated then the trusses should beprotected with covers fixed in such a way as to allowproper ventilation around the trusses.

When stored vertically, bearers should be positionedat the locations where support has been assumed tobe provided in the design with stacking carried outagainst a firm and safe support or by using suitableprops.

Figure 90SAFE VERTICALSTORAGE

Trestle prop

Bearer height to allow overhangto clear ground

Trestle prop



71

Section3.11

Site Storage and HandlingWhen trusses are stored horizontally, level bearersshould be positioned beneath each truss node(minimum) to prevent any deformation and distortion.(See figure 91 below).

No other method of storing trussed rafters is consideredto be suitable, except where specific provision has beenmade in the design for an alternative temporary supportload case.

At such time when it is necessary to remove the pre-tensioned bindings from a bundle of trusses, extremecare should be exercised. As a precaution againstdestabilisation of the whole bundle of trusses, it isrecommended that prior to the removal of the bands,timber battens are fixed across the bundle at severallocations with a part driven nail into every truss. Sucha simple precaution will allow the safe removal ofsingle trusses once the steel bands are removed. Asuggested arrangement of batten locations for astandard Fink truss is shown in figure 92 below.

Alternative details relating to this procedure and whichinvolve the unbundling of the trusses whilst on theback of the lorry should be communicated by thecontractor to the truss manufacturer prior to theirdelivery to site.

Manual Handling of Trussed Rafters

(Information relating to manoeuvring trussed raftersaround the site using manual handling techniques).

With careful consideration manual handling methods

can be safely employed to move trussed rafters arounda construction site, although the choice of method willdepend to a large extent on the particular circumstancesof the lifting operation. Such operation will generallybe identified in a contractors safe working methodstatement that takes account of all the assessed risksand which utilises and refers only to the resourceswhich are available to the site. The preparation of thismethod statement should be undertaken sufficiently inadvance to ensure the adequate planning and co-ordination of the task and sourcing of any specialequipment that may be required. For example, asituation where the manual handling of trussed raftersmay be appropriate might be the lifting of singletrusses on to residential units not exceeding two storeysin height.

Whatever technique is adopted to manually manoeuvretrussed rafters it is vital that the technique takes fullaccount of any special instructions issued by thedesigner to ensure that the structural integrity of theunits is maintained and that there is no risk of damageto the trusses.

Mechanical Handling of Trussed Rafters

(Information relating to manoeuvring trussed raftersaround the site using mechanical handlingtechniques).

Where it is not possible for reasons of safety or otherpractical considerations to implement manual handlingtechniques to manoeuvre trussed rafters, other meansthat involve the use of mechanical handling or liftingequipment will be necessary. Using such equipmentgives the option of being able to move larger andheavier loads and consequently, the ability to raisecompletely or partially assembled sections of roof thathave been pre-assembled at another location (forexample, on the ground level superstructure of anadjacent plot). Similar considerations to thoseidentified in the section relating to manual handlingremain relevant, although as the size of the loadsincrease, issues of instability and potentialdistress/damage to the trussed rafters becomes morecritical. For this reason, it is vital that trusses orsections of roof are only lifted at locations approved bythe truss designer, such locations being preferablymarked on the units at the time of their manufacture.Where appropriate, the use of spreader bars andstrongbacks may be required to ensure an evendistribution of lifting points.

Figure 91

Figure 92

SAFE HORIZONTAL STORAGE

DIAGRAM ILLUSTRATING SAFE METHOD OFBREAKING A BUNDLE OF TRUSSES

Bearers vertically in line andat close centres

Ensure that the battens arefixed to each truss prior torelease of the binding



72

Site Storage and HandlingAn example of the use of a spreader bar is shown infigure 93 below.

Where bundles of trusses are raised to roof level,caution should be exercised in the removal of therestraining bands (see section 3.11 figure 92). Shouldthese bundles of trusses be stored either on atemporary working platform or at eaves level, thecontractor should take the necessary steps to ensurethat the supporting structure has sufficient strengthand that a storage system as illustrated in eitherfigure 90 or 91 is constructed.

Designated slewing areas should be cordoned off andthe movement of operatives either restricted orprohibited within this area during all liftingoperations.

At all times, strict adherence with the Contractorsmethod statement should be observed.

Where circumstances and design considerationsdictate that pre-assembled sections of roof, such aships etc., (or indeed, complete roofs) are raised inone single lifting operation, particular attentionshould be given to the method of lifting theassembled sections. Such large and unwieldy loadsrequire that checks should at least be made regardingthe following:-

Prevailing weather conditions, with particular reference to wind speed.

A survey of obstacles in the slewing area, includingscaffolds, towers and overhead services.

A survey of the accuracy of construction and settingout of the pre-assembled roof structure.

Underground services locations to avoid damage bythe use of large cranes etc.

These sorts of techniques have the potential to savesignificant amounts of time and money on site whilstadditionally offering significant Health and Safetybenefits to all employees and personnel, althoughthey generally require early design input andplanning to ensure sufficient strength is inherentduring the lifting procedure. Typical benefits whichmay be associated with improvements in mattersrelating to Health and Safety include:-

The immediate provision of stable sections of roof,away from which infill sections of roof can beconstructed, rather than relying on temporarybracing.

All assembly operations are carried out at groundlevel and therefore the risk of operatives falling is totally eliminated.

The risk of operatives being struck by falling objects during an alternative roof level assembly is significantly reduced.

Clearly, there are many other benefits relating tospeed, efficiency and the overall costs associatedwith the construction process.

Mechanical handling and lifting operations areessential where the scope of the works falls outsideof simple residential scale projects.

Figure 93

MECHANICAL HANDLING

Spreader bar

Rope guide

Nodepoints

•

•

•

•••



73

Section3.12

Erection ProcedureAssembly of trussed rafter roofs

(Information relating to the assembly of trussedrafter components and infill)

Once the trussed rafters have been safely raised toeaves level utilising either the methods or principlesoutlined previously and assuming that all thenecessary information has been forwarded by theRoof Designer to the contractor, then it is possiblefor the assembly of the trussed rafter roofconstruction to commence. In similar fashion to theother work tasks associated with trussed rafter roofconstruction, the assembly of the roof componentsshould be carried out in strict accordance with acontractor prepared safe working method statement(see section 3.13 for a typical example of aContractors General Risk Assessment andsupporting Method Statement).

Whichever method of raising the trusses is utilised,the principal risks associated with assemblingtrussed rafter roofs in their final location are eitherfalling, temporary instability and collapse of thepartially complete structure or being struck by afalling truss/object. All of these issues need to beaddressed to safely proceed with the operation. Themanner in which any other residual site hazardsshould be dealt with should be based on the principleof a hierarchy of risk control. This principle statesthat the most desirable option is to design out thehazard and subsequent risk completely at the designstage and the least desirable option is to providepersonal protection systems such as restraintharnesses (i.e. protection after a fall).

With regard to assembling trussed rafter roofstructures, the most desirable approach for standardstorey height construction (up to 3.0m from floor toceiling) is to provide both a perimeter workingplatform externally and either a full or partialworking platform internally and erecting the trussesusing the standard erection procedure as shown infigure 94a. A useful modification to the basic bracingprocedure is to rigidly brace the first truss back to theexternal scaffold to allow roof assembly to proceedunencumbered in a direction away from that firsttruss.

Alternatives to this approach might involve thecombination use of working platforms and safetynets or, in situations where the potential fall distancesare sufficient to allow their safe use, the installationof larger nets and/or restraint harnesses.

At all times, the Designers and Contractors shouldundertake proper Risk Assessments of the tasks inhand and draft appropriate method statementsaccordingly. Where the trussed rafter

designer/manufacturer is also engaged to erect theroof structure then the method statement would beprepared by him and approved by the principalContractor (who is responsible for the Health andSafety of all personnel, directly employed orotherwise, on the site). Some amendment orreassessment of the proposed working method maybe necessary before the Principal Contractor allowsthe work to commence.



74

Immediately prior to the fixing of the permanentbracing and the tiling battens or sarking, all trussedrafters should be checked for straightness andvertical alignment. Whilst every effort should bemade to erect trusses as near vertical as possible, themaximum deviations from the vertical shown in thefollowing table may be permitted.

Maximum deviation from verticalAfter erection, a maximum bow of 10mm may bepermitted in any trussed rafter provided that it isadequately secured in the complete roof to preventthe bow from increasing. For rafter members, thismaximum bow is measured from a line between theapex and the eaves joint.

Figure 94a

Figure 94b

Erection ProcedureThe builder should consider, in conjunction with theBuilding Designer, the erection procedures to beused and the provision of temporary bracing, riggingand any other specialised equipment required to erectthe trusses safely and without damage, in accordancewith the design requirements and having due regardto possible windy conditions.

Permanent bracing should be of minimum size 22 x97mm free of major defects and fixed with two 3.35x 65mm galvanised round wire nails at each cross-over.

The following procedure is suggested for mostdomestic size roofs.

1. - Mark the position of each truss along bothwallplates.

2. - Erect the first truss (truss A in figure 94a) at thepoint which will coincide with the uppermost pointof the diagonal brace F when it is installed later. Usethe temporary raking braces B fixed to the raftermembers and the wallplates to hold this truss in thecorrect position, straight and vertical. For clarity,only on raking brace is shown in the figure but theyshould be fixed to both rafter members and be of

sufficient length to maintain the truss in position,during the erection of the remaining trusses.

3. - Erect truss C and brace back to A with temporarybattens D at suitable intervals along the rafter andceiling tie members. Repeat this procedure until thelast truss E is erected.

4. - Fix the permanent diagonal braces F ensuringthat each top end is as high up the last trussed rafterA as is possible and that each bottom-end extendsover the wallplate to which it should be fixed. Forclarity, only one permanent brace is shown in thefigure, but they should be installed on both sides ofthe roof.

5. - Fix the longitudinal members G, making surethat the ceiling ties are accurately spaced at thecorrect centres.

6. - Fix all remaining longitudinal, diagonal andchevron bracing required on the internal members ofthe trusses as specified.

7. - Additional trusses may be erected by temporarily'bracing-off' the completed end.

Gable end

A

B

C

D

E F

G

G

GG

G

Alternative direction of bracingon rafters is equally acceptable

Lap jointif required

Gable end

Rise of truss (m) 1 2 3 4 or more

Deviation from 10 15 20 25vertical (mm)



75

Section3.13

(This section is intended to give general guidance toContractors regarding appropriate controls forassessing and documenting the risks associatedwith construction task).

Perhaps it is appropriate under this section to notethat the undertaking of Risk Assessments andcompilation of Method Statements (whereappropriate) is the LEGAL DUTY OF ALLCONTRACTORS as it is for Designers under theConstruction (Design and Management) Regulations1994. Such Assessments are necessary to appraisehazards and their associated risks in order that theserisks may be either minimised or controlled.

The responsibilities and obligations of Contractorsare primarily laid down in the following Regulations:

Health and Safety at Work Act 1974

Construction (Design and Management) Regulations1994

Management of Health and Safety at WorkRegulations 1992

Provision and Use of Work Equipment Regulations1992

Construction (Health, Safety and Welfare)Regulations 1996 - (CHSW Regulations 1996)Manual Handling Operations 1992

Workplace (Health, Safety and Welfare) Regulations1992

Examples of a typical Risk Assessment andsupporting Method Statement are given on pages 76and 77. These are presented to illustrate thedifference between a Contractors Standard Healthand Safety Policy which should include provision forall 'Standard' risks - as documented in theContractors General Risk Assessment (which maysimply be an amended sheet from the CompanyHealth and Safety Policy Manual) and PPE/ManualHandling Risk Assessments and/or detailed MethodStatements which are custom written to deal withspecific, non-standard or particularly risky aspects ofwork.

Risk Assessments and Method Statements



76

Contractors general risk assessment for theerection and assembly of roof trussesUnder the Management of Health and Safety at WorkRegulations 1992 contractors are required toundertake and record risk assessments for sitespecific tasks and locations of work. These RiskAssessments can be used to i) identify provision within tender/contract documents regarding matters

relating to Health and Safety, ii) check Health andSafety conditions on site, iii) developing safe systemof work and Method Statements where required andiv) provide information on hazards tooperatives/personnel at the place of work.

By way of an example which illustrates typicalcriteria for assessing the risks associated with aparticular work task the following exampleassessment has been prepared:-

Harm:Significant injuries or fatalities without controls

Persons in Danger:Roof operatives, other operatives in the vicinity,general public as passers by

Controls:This section should typically include informationrelating to the design and use of the following:-Ladders, Scaffolds, Working Platforms, StorageAreas, Edge Protection and Barriers, LiftingEquipment, Disposal of waste, PPE, WarningNotices, Checking Procedures, Adverse weather,Plant Maintenance etc.

PPE:Safety Helmets, Protective Footwear and Glovesshould be worn

Additional Assessments Required? Manual Handling (where appropriate) activitiesand PPE

Method Statement Required?Yes, see method statement ref. MG/GEN/OJ

Can the Work Task be adequatelycontrolled?YYeess

Specific Legislation and other InformativeGuidance Documents:CHSW Regs 1996: CDM Regs 1994: ManualHandling Regs 1992 etc

Information, Instruction and Training:See Company Training Information - Nooperatives shall carry out any activity withoutproper training as noted therein

Emergency Procedures:Display Procedure in site offices, Ensure personnelknow how to raise alarm, provide Adequate FirstAid Kit

Monitoring Procedures:This shall be the responsibility of the Site Managerto organise and implement according to establishedprocedure

Any other Items:AAss aapppprroopprriiaattee

Signed:

Date:

Project Title: HHoouussiinngg EEssttaattee,, AAnnyywwhheerree

Client: JJ BBllooggggss && CCoo

Description of Works: GGeenneerraall RRooooff AAccttiivviittiieess

Document Reference No: RRAA//GGeenn//OOJJAA

Date: ****//****//****

Author: AAJJFF

Hazards: Risk Ratings

(This list should also refer to those hazards Without Controls With Controlsidentified in the roof Designers Risk Assessmentand also those contained in the siteHealth and Safety Plan), e.g:

Persons falling - HHiigghh LLooww

Falling objects - MMeeddiiuumm LLooww




77

Section3.13

Safe Working Method:

For additional reference regarding this methodstatement refer to Contractors sketch ref. ***/** asillustrated on page (?) of this site installation guide.At all times this method statement assumes that allappropriate design considerations have beenincorporated and allowed for within the design andlayout of the temporary working platforms.Additionally, it should be noted that this methodstatement refers only to those operations which havebeen designated as having a higher level of risk, forall matters associated with this operation referenceshall be made and working practices adopted whichcomply with the Contractors general RiskAssessment for roof work.

Part 1:1. Construct external perimeter scaffold as per detailin a manner to ensure sufficient manoeuvring spacearound loading platform. Locate vertical trussrestraint standards at position to allow unobstructedlifting to eaves level working platform. All edgeprotection to both the eaves level and the loadinglevel platforms must be constructed and fixed beforeany lifting operations take place. Similarly, erectinternal working platforms at a level (typically)300mm below ceiling tie level.

Under no circumstances whatsoever shall any edgeprotection be removed to facilitate these operations.

2. According to the recommendations of the ManualHandling Risk Assessment use x No. Personnel tomanually lift individual trusses via the truss restraintstandards to the eaves level working platform. Movetrusses along the length of the roof to their finalposition (where they shall immediately be fixed bycarpenters using temporary/permanent bracing - seepart 2 of this method statement). NB. Girder trussesshall be raised as single component plies and then theceiling tie members (min) bolted together accordingto the details provided by the truss manufacturer andin locations marked by him on the trusses; rafter andweb members may be nailed according to furtherdetails provided by the truss manufacturer.

NB. Roof Bracing Details which will include sizesand location of Rafter and Chevron Bracing etc,shall be installed in accordance with the roofdesigners layout drawing.

Part 2:3. When the first truss has been raised and located inits final position by the truss handling team, thecarpenters shall immediately provide temporarydiagonal restraints at a minimum of three locations tohold the truss vertical and so as to act as a rigid startpoint for the erection of the remainder of the trusses.This temporary restraint shall preferably be locatedoutside of the roof structure i.e. Fixed to the externalperimeter scaffold. The positioning of the temporarybraces in this way will then allow unobstructedpassage to the truss handling team as further trussesare raised and located in their final position.

NB. Wherever possible, Carpenters should use pre-nailed bracing members (accurately marked out tocoincide with the truss centres) to ensure that trusserection progresses smoothly and quickly.

4. As soon as sufficient trusses have been temporarilypositioned, the carpenters shall commence the fixingof internal permanent bracing to create fully stablesections of roof. Where necessary for carpenters towork at higher levels than the main internal workingplatform then either stepladders or temporary trestlesshall be used between trusses constructed orpositioned on the main platform. Under nocircumstances shall operatives be allowed to climbwithin the temporarily braced roof structure.

5. As soon as permanently braced sections of roofhave been completed, it shall be allowed foroperatives to locate working platforms within theroof structure by positioning suitable boards directlyon top of the ceiling ties. These platforms can then beused for the installation of services etc. Similarly, atthis time it is appropriate to allow the removal of theexternal temporary props in order to allow any gablemasonry construction to be commenced. Gableconstruction should not have been allowed tocommence prior to this stage as it is the stability ofthe roof construction which provides restraint to thegable masonry construction.

NB. The dismantling of the internal workingplatform shall only be allowed to commence belowcompleted areas of roof construction as such timewhen no work is being carried out overhead.

6. Further areas of roof construction (if appropriate)shall be carried out according to the identicalprinciples outlined above.

Task Description: Project Title: Ref:EErreeccttiioonn ooff TTrruusssseedd RRaafftteerr RRooooff HHoouussiinngg DDeevveellooppmmeenntt aatt No:SSttrruuccttuurree uussiinngg MMaannuuaall hhaannddlliinngg MMuuddddyy LLaannee,, Date:MMeetthhoodd RReeff 0011.. NNeewwttoowwnn,, SSmmookkee CCiittyy Author:

Location of Work Task: Project Title:HHoouussee ttyyppee AA ((SSoouutthh FFaacciinngg oonnllyy)) EErreeccttiioonn aanndd IInnssttaallllaattiioonn ooff ttrruusssseedd RRaafftteerr

oonn MMuuddddyy LLaannee RRooooff SSttrruuccttuurree ttoo HHoouussee TTyyppee AA

This Safe Working Method Statement has been prepared for the following work.No other work than that referred to must be carried out.




78

Construction Check List

Job No: Contractor:Site: Block:Inspector: Date:

OK NOT OKYes No

Trussed Rafters

Correct quantity, positions and orientation

Centres not greater than specified

Verticality and bow after erection within code limits

No damage or unauthorised modifications

Girders/Multiple trusses connected together in accordance with specification

Properly seated on wallplates, hangers, etc.

Bracing correct size and in correct position

Bracing connected to each truss as specified

Bracing laps extended over a minimum of 2 trusses

Bracing of truss rafter compression members are installed as specified

Valley set is correctly set out and braced as specified

Valley set is supported on bevelled bottom chord or supported on fillet

Loose Timbers

Correct sizes, position and grade

Centres not greater than specified

Birdsmouth, joints, scarfs etc., accurately and correctly made

Properly seated on wallplates, hangers, etc.

Fixings are to specification

Structural Metalwork

Truss clips, framing anchors and other vertical restraints present and fully nailed

Hangers correct to specification and fixed as specified

Gable restraint straps present and correctly fixed including pack between members

Tank Platform

Correctly positioned and constructed as specified

Loads applied to trusses as allowed for in design

Special Items

Services in position specified and do not clash with webs

Roof ventilated as specified

Trap hatch formed to specification

Sarking if applicable, is to specification

Tiles fixed are correct weight as specified in design

Comments

✔ ✖



79

Section3.15

Nailing and BoltingScab MembersRafter sizes in raised tie trusses often need to beincreased, since the entire weight of the roofstructure is supported on the extended raftersresulting in large bending forces. Even then, timberscabs or reinforcing members are often necessaryand it is essential that they are correctly fittedwhenever specified. Scabs may be required on one orboth faces of the extended rafter and may also berequired on multiple trusses. The truss manufacturermay fix the scabs in the factory prior to delivery ormay provide the scabs loose, with a fixing detail toallow them to be secured on site. Scabs on multipletrusses will invariably require bolting - large platewashers should be used with all bolts.

Typical Scab Nailing PositionsScabs may be fixed by the manufacturer or on siteusing a nailing or bolting details provided by themanufacturer.

Girder TrussesGirder trusses are designed to carry more load thanthat from the standard trussed rafter spacing. Theyconsist of two or more trussed rafters fastenedtogether. Typically, girder trusses carry other trussedrafters or infill timbers on shoes attached to theceiling tie of the girder.

Girders are fastened together by nails or bolts. Whenfastened together on site, bolts must be used for atleast the ceiling tie members, in positions marked bythe truss manufacturer. In all cases, the nails or boltmust be positioned strictly in accordance with themanufacturer's instructions.

See TRA Information Sheet 9804 'Girder Trusses(Principal Trusses) Definitions and ConnectingTogether On Site'.

Washers must be used under the head and nut of eachbolt.

Nails and bolts should either be inherently corrosion resistant or protected by a corrosion resistant coating.

Figure 95

Figure 96

Figure 97

Bolt Diameter Washer SizeDiameter Thickness

M8 24mm 2mmM12 36mm 3mmM16 48mm 4mmM20 60mm 5mmM24 72mm 6mmRafters and webs

nailed

Truss TrussTruss

Nail

group

Nail

group

Nails at

300 ctrs

Nails at

300 ctrs

Ceiling membersbolted togetherBolts spaced to avoid incoming truss fixings

To be effective scabs must be in firmcontact with the wallplate Multiple trusses may also require scabs

These will be invariably bolted into place



80

All the trusses (or frames) are generally of two basictypes depending on how they are supported.

Type 1:(Figure 98a) is characterised by a load-bearingsupport at or near mid-span and as a result generallyhas heavy joists propping relatively light rafters. Thetruss may need to be supplied in kit form forcompletion on site if it is too high for fabrication or

transportation. The kit form, while requiring somesite fabrication, does make for straightforwarderection as the floor joists can be installed first,providing a safe, rigid working platform.

Type 2:(Figure 98b) is free spanning between wallplates andas a result the floor is suspended from the rafterswhich consequently are relatively heavy and often asheavy as the floor joists. The associated kit form isusually different to that for type 1 in order tofacilitate erection and to ensure that the moreimportant joints are made under factory controlled

conditions. However, substantial connections, oftenemploying MiTek field splice plates, fully nailed orbolts, have to be made between the capping and basecomponents, handling and erection of these heavyunits needs to be carefully supervised.

Figure 98a

Figure 98b

Figure 98c

Camplated nailedsite connections

Field splice plate,bolted or nailedsite connections

Cap

Base

Often the cap truss sits in the same planeas the base truss and they are connectedtogether using a MiTek Field Splice plate

Field splice plate pressed into one part infactory and nailed in other part on site

Attic Frames 2 & 3 Part Construction



81

Section3.17

Attic Frames - BracingPermanent bracing is required in all roofs for fourreasons:

a. to maintain rafter stabilityb. to prevent dominoeing c. to form diaphragms to transmit wind loads to

shear wallsd. to maintain the stability of internal compression

members

By far the most serious matter which arises in roofsurgeries is rafter (roof) instability, arising from lackof suitable bracing.

Permanent bracing is the responsibility of theBuilding Designer. The advice and recommendationsgiven here are given in the interests of good buildingpractice and are not to imply responsibility acceptedby MiTek. They should be considered as thenecessary minimum.

Figure 99a

Figure 99b Figure 99c

Figure 99d

Figure 99e

Note: Bracing shown must be installedboth sides of ridge and repeat atintervals (with a minimum of two)along roof. Alternate rising to left andto right. Where the roof is short thesecond line of bracing may cross asshown by the broken line.

Tight fitting 12mm plywoodfixed with 60mm nails at250mm centres

50 x 50 ledgersfixed with 100mmnails at 250mmcentres

The structural action of diagonal bracing is the completionof triangulation in various planes, in order to form rigiddiaphragms. For example, in the plane of the rafters this isprovided by rafters, tiling battens and the bracingmembers.

The effectiveness of the noggined parts of the diagonals infigure 99b might be open to question, as it is verydependent on the quality of installation. Suitablealternatives are plywood diaphragms (figure 99e).

Diagonals repeat continuously along building, they mayrise to left or right or vary

Two diagonals each end. One diagonal alternatively eachside of centre line elsewhere

25 x 100mm diagonal brace

50 x 50 nogging at plaster boarded

area extended into void areas

Alternative to one line of diagonal rafter bracing

View D

View C

View BView A

View A View B

View D

ViewC

Tiling battens

45º

Insulation

At least 4trusses perdiagonal

At least 4trusses perdiagonal

2 No 3.35 x 65mm galvanisednails at all cross overs

25 x 100mm longitudinalbinders shown thus

25 x 100mm diagonal bracingshown thus

Solid blocking or herringbone strutting at least at allbearings and mid span, check NHBC requirements



82

Attic Frames - EnvironmentFireRoom-in-the-roof constructions is in an unusualposition in regard to fire regulations. The floor, ofcourse, must have the usual, minimum modified half-hour endurance. However, additional precautionsshould be taken to prevent spread-of-fire into theroof cavities and to ensure the integrity of theconnectors for the full half hour.

Alternative solutions are:a. Continue the floor boarding into the side trianglessealing it to the wall plate as shown (figure 100a) andprotect the connectors with 12.7mm plasterboardcover plates.

b. Install under the floor joists a ceiling liningcapable of providing full or almost full protection eg:

1.12.7mm 'Fireline' plasterboard2.Normal 12.7mm plasterboard plus a 5mm plasterskim coat3.12.7mm plus 9.5mm plasterboard with staggeredjoints

If compliance with the ventilation requirements ofthe Building Regulations is to be effected througheaves vents, these should be made impassable to fire.

Insulation and VentilationThought should be given to the type and location atan early stage, as this might well determine the depthof rafter to be adopted.

A cool regime (figure 100b) required ventilation tocontrol condensation. An airgap of not less than50mm should be provided between the top of theinsulation and the underside of the roof covering.

With a 100mm mineral wool quilt the smalleststandard finished size of timber to provide thenecessary depth is 147mm.

Warm roof regimes (figure 100c) need the sameventilation with, in addition, ridge vents providing atleast a 5mm minimum continuous gap.

Figure 100a

Figure 100c

Figure 100b

Floor boarding

Seal/blocking

50mm ventilationgap

Vents equivalent to 25mmminimum continuous gap

Vents to equivalent to 5mmminimum continuous gap at

ridge

Vents equivalent to25mm minimumcontinuous gap

Insulation

Insulation

Vapour barrier

Vapour barrier

Plasterboard coverplate

Connector



83

Section3.19

General Construction detailsIn general, it is preferable to use one of theproprietary types of fixings, 'A', between the ends ofthe trussed rafters and the wall plates or bearings asshown in figure 103.

Where proprietary fixings are not used, the minimumfixing at each bearing position should consist of two4.5 x 100mm long galvanised round wire nails,which are skew nailed from each side of the trussedrafter into the wallplate or bearing. Where nailingthrough the punched metal plate cannot be avoided,the nails should be driven through the holes in thefasteners. This method of fixing should not be usedwith stainless steel metal plate fasteners or where theworkmanship on site is not of a sufficiently highstandard to ensure that the fasteners, joints, timbermembers and bearings will not be damaged bycareless positioning or overdriving of nails.

Internal non-loadbearing wallsIt is advisable to erect non-loadbearing walls afterthe tiling has been completed thus allowingdeflection to take place under the dead load, therebyreducing the risk f cracking appearing in the ceilingfinishes. If partitions are of brick or block, then as analternative the final course may be omitted untiltiling has been completed

Figure 102

Figure 105

Figure 101

Figure 103

Figure 104

The Building Designer should ensure that, when required,adequate holding down fixings, 'C', are specified for boththe trussed rafter and the wall plates or bearings.Restraint strap at ceiling level

Restraint strap at rafter level

Noggings to beprovided and sethorizontal unlessthe strap has atwist to line it upwith the roofslope

Strap bedded undera cut block

Truss clip

Partition

Omit final courseuntil tiling is

completedPacking piece betweeninner leaf and first rafter

Strap fixed to solid noggings with aminimum of four fixings of which at leastone is to be in the third joist/rafter or ina noggin beyond the third joist/rafter

A

C



84

General Construction detailsHogging over party wallsParty walls should be stopped 25mm below the topsof rafters. During construction layers of non-combustible compressible fill such as 50mm mineralwool should be pressed onto the locations shown toprovide a fire stop as figure 106.

Continuity across party wallsIf the tiling battens are required to be discontinuedover a party wall, then lateral restraint must beprovided in addition to that required to transferlongitudinal bracing forces.

This should consist of straps adequately protectedagainst corrosion. These straps should be spaced atnot more than 1.5m centres and be fixed to two raftermembers and noggins on each side of the party wallby 3.35mm diameter nails with a minimumpenetration into the timber of 32mm.

HipboardsFixing over flat-top girder

Where hipboards pass over and are supported on flattop girder trusses, the hipboard must be notched inorder to achieve the correct height for the hipboardand to provide horizontal bearing. The flying rafterof the truss may need to be trimmed but in nocircumstances should the flat chord or the rafterbelow the joint be cut. In most cases the hipboard issupplied in two parts which can be joined over theflat top truss. One method of providing the necessaryfixing is illustrated in figure 108.

Hipboard to be notched over girder truss and buttedtogether over centre of girder.

Figure 109

Figure 106

Figure 107

Figure 108

50 x 150 ledger nailedto truss using 3.75 x90mm galvanisedround wire nails

Hipboards

X

X

X

25mm

XFill

Tiles

Felt

Trussed rafter

Fill between battens

Tiling battens

Tiling batten

Compressible fill

Top of wall

Underside of rafter

Hand nail plate2 No per connection

Use 3.75 x 30mmsquare twisted

sheradised nails 2 ply (typical)hip girder truss

Hipboard to benotched over

ledger

Lower hipboard

Y/4(max)

Section X - X

Y

Batten



85

Section3.20

Builders Metal WorkMetalwork for timber-to-timber and truss to masonryconnections are always required at some point in aroof structure.

MiTek Industries Limited are leading suppliers of allfixings necessary for the erection of trussed rafterroofs.

A separate catalogue is available detailing the fullrange of fixings we stock and supply, but detailedbelow are just some of the products available.

Truss Hanger

Angle Bracket

Masonry Hanger

Vertical Restraint StrapLateral Restraint Strap

Truss Clip

Joist Hanger

Truss Hanger

Girder Truss Hanger


MiTek Industries LimitedMiTek House

Grazebrook Industrial ParkPeartree Lane

DudleyWest Midlands DY2 0XW

Tel: 01384 451400Fax: 01384 451411

£10
