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7.2.9 Bracing for trussed rafter roofs
Trussed rafters shall be suitably braced to support applied loads and self-weight without undue movement. Issues to be taken into account include:
- general requirements
- duo-pitched roofs
- mono-pitched roofs
- attic roofs.
For the purposes of this chapter, the guidance and use of standard trussed rafter bracing applies to all homes designed within the parameters set out below. For large houses with truss spans of over 12m, homes with complex roofs or roofs near exposed sites, the bracing should be designed by an engineer in accordance with Technical Requirement R5.
Standard trussed rafter bracing, in accordance with Table 6, is generally acceptable, where the home:
- has a rectangular roof (including hip ends) and is either a duo-pitched or a mono-pitch structure
- is not taller than 8.4m (to the underside of the ceiling tie)
- is braced in accordance with this chapter
- is braced according to the conditions of the site and in accordance with the design
- does not have trusses which span more than 12m
- has trusses which are only supported at each end
- does not have unsupported masonry spanning more than 9m (between buttressing walls, piers or chimneys)
- has a ceiling of plasterboard directly under each truss (where there is no plasterboard, ie, garages, additional diagonal ceiling bracing and longitudinal binder bracing at each ceiling node point is required).
Table 6: Location, height and span for standard bracing conditions for buildings at site altitudes ≤150m
Notes
1. For details of area specific wind zoning please refer to PD6693-1.
2. Wind zones in accordance with PD6693-1.
Where trussed rafter designs are outside the parameters above, the guidance in PD6693-1 should be followed.
General bracing requirements#
Roof bracing should be:
- in accordance with this chapter or PD6693-1
- in accordance with the design and not altered without prior approval from the designer
- appropriate for the site (for large houses with truss spans of over 12m, homes with complex roofs or roofs near exposed sites, the bracing should be designed by an engineer in accordance with Technical Requirement R5)
- completed before the roof covering is laid
- provided using a minimum timber size of 100mm x 25mm (3mm tolerance)
- nailed twice to each rafter it crosses; fixings should be 3.35mm x 65mm (10 gauge) galvanized round wire nails or minimum 3.1mm x 75mm mechanically driven gun nails
- where braces and binders are not continuous, they should be lap jointed and nailed to a minimum of two trusses.
When bracing pitched roofs:
- diagonal and longitudinal bracing should be provided at rafter level (this may be omitted where sarking sheets or boards are used, eg chipboard, plywood or OSB, which are fixed to each trussed rafter with 3mm x 50mm galvanised round wire nails at 200mm spacing)
- diagonal and chevron bracing should pass across each rafter in the roof, however, small gaps, such as two trussed rafters between sets of bracing, or one trussed rafter adjacent to gable or separating walls, is permitted in the middle of an otherwise fully braced roof
- longitudinal bracing members should extend the full length of the roof, tightly abut gable and party walls and permit diagonal bracing to pass (they may be lap-jointed providing the overlap is nailed to a minimum of two trussed rafters)
- there should be a minimum of four diagonal rafter braces in each roof; in narrow fronted roofs (Figure 22) and mono-pitched roofs, where the braces cross, the intersection detail (Figure 23) should be used
- the diagonal bracing should extend over and be directly fixed to the wall plate, fixings should be 3.35mm x 65mm (10 gauge) galvanized round wire nails or minimum 3.1mm x 75mm mechanically driven gun nails, see Figure 19. Where the bracing cannot be directly fixed to the wall plate the bracing should be terminated as detailed in PD6693-1:2019, Figure E.9.
Duo-pitched roofs#
Diagonal bracing for duo-pitch trusses
Applicable to all trussed rafter roofs unless sarking sheets or boards, are used.
Diagonal bracing should also be provided in-between hipped ends, where the length of roof between the hip ends exceeds 1.8 m.
Diagonal rafter bracing should be approximately 45° to the rafters on plan.
Figure 20: Diagonal rafter bracing – square roofs
Figure 21: Diagonal rafter bracing – larger roofs
Figure 22: Diagonal rafter bracing – narrow fronted roofs
Figure 23: Intersection detail
Intersection details should be formed by:
- 22mm x 97mm x 600mm timber splice plate
- nailing, using a minimum of four 3.35mm x 65mm galvanised round wire nails or minimum 3.1mm x 75mm mechanically driven gun nails to each side of the intersection, with nails driven through bracing and clenched over.
Longitudinal bracing member at ridge node point
Applicable to all trussed rafter roofs. Not necessary where sarking sheets or boards are used.
Longitudinal bracing member at rafter node point
Applicable to all rafter node points. Not necessary where:
- spacing between braced nodes is less than 4.2m, or
- sarking sheets or boards are used.
Longitudinal binders at ceiling node points
Applicable to all ceiling node points. Not necessary where the spacing between braced nodes is less than 3.7m.
Chevron bracing between webs
Chevron bracing should be:
- provided where the span exceeds 8m
- at approximately 45°
- nailed to at least three trusses.
For duo-pitch roofs over 11m span, chevron bracing should be designed by an engineer in accordance with Technical Requirement R5.
Mono-pitched roofs#
Diagonal bracing for mono-pitch trusses
Applicable to all mono-pitched trussed rafter roofs unless sarking sheets or boards are used.
Diagonal rafter bracing should be approximately 45° to the rafters on plan.
Diagonal bracing to end vertical member of mono-pitch trusses
Applicable where the truss is not restrained by:
- a masonry wall, or
- cladding, ie, plywood.
Longitudinal bracing to mono-pitch trusses
Longitudinal bracings should be located at the apex and either:
- all other node points (excluding support points); or
- where intermediate longitudinal bracing members are omitted, the resultant spacing between longitudinal braced node points does not exceed 4.2 m measured along each rafter and 3.7 m measured along each ceiling tie.
Chevron bracing between webs
Chevron bracing should be:
- provided where the span exceeds 5m
- at approximately 45°
- nailed to at least three trusses.
For mono-pitch roofs over 8m span, chevron bracing should be designed by an engineer in accordance with Technical Requirement R5.
Attic trusses#
Attic trusses should be braced in accordance with the design.
Diagonal bracing
Applicable to all attic trussed rafter roofs unless sarking sheets or boards are used.
Diagonal bracing is required:
- at each gable end at approximately 45° on plan
- at intervals along the roof to ensure each truss is braced
- to be fixed to the underside of the rafters (where diagonal bracing passes through the room space, ceiling boards can be fixed over the diagonal bracing onto battens fixed to the rafters on each side of the bracing).
Diaphragm bracing
Where the diagonal bracing passes through the room space and needs to be interrupted to allow boarding to be fixed without battening out the rafters the following options may be used:
- continuous internal 9mm plywood or OSB sheathing (see Table 9) minimum 900mm width, as shown in Figure 33, face fixed to underside of rafters for the full length of the roof, or
- diaphragm bracing closely fitted between the rafters as shown in Figure 34 and Figure 35. The bracing should be 9mm plywood or OSB sheathing (see Table 9), at least 1200mm long fixed to a 50mm x 50mm timber framework.
Figure 33: Attic truss – diaphragm bracing
Figure 34: Attic truss – diaphragm bracing
Figure 35: Attic truss – diaphragm bracing
Longitudinal bracing
Longitudinal bracing is required:
- at node points (excluding support points).
Last updated: 2nd January 2024