Pitched roofs

7.2

Also see:

BS EN 1995-1-1 and PD 6693-1

7.2.9Bracing for trussed rafter roofs

Trussed rafters shall be suitably braced to support applied loads and self-weight without undue movement.

For the purposes of this chapter, the guidance and use of standard trussed rafter bracing does not apply to homes on or near exposed sites, e.g. flat coastal fringes, fens, airfields and moorland. In such cases, bracing should be designed by an engineer in accordance with Technical Requirement R5.

Standard trussed rafter bracing, in accordance with Table 4, 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, i.e. garages, additional diagonal ceiling bracing and longitudinal binder bracing at each ceiling node point is required).

Table 4: Location, height and span for standard bracing conditions

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 (where the site is in an exposed location, the design should be checked for additional requirements, and the bracing completed as specified suitably fixed to the wall plate)

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

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 rigid sarking boards are used, e.g. 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 and mono-pitched roofs, where the braces cross, the intersection detail (below) should be used.

Diagonal rafter bracing

Applicable to all trussed rafter roofs unless rigid sarking, such as timber boarding or plywood, is used.

Diagonal rafter bracing should be approximately 45° to the rafters on plan.

Bracing for roofs that are approximately square

Bracing for larger roofs

Bracing for roofs less than 6.6m wide on detached or staggered/stepped buildings

Bracing for mono-pitch trusses

Intersection details should be formed by:

22mm x 97mm x 600mm timber splice plate

nailing, using a minimum of four 35mm x 65mm galvanised round wire 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 rigid sarking, such as OSB, timber boarding or plywood sheeting, is 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

rigid sarking, such as OSB, timber boarding or plywood sheeting, is 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

Where the span exceeds 8m. For mono-pitch roofs of any span and duo-pitch roofs over 11m span, bracing should be designed by an engineer in accordance with Technical Requirement R5.
It should be approximately 45° to the web members.

Diagonal bracing to end vertical of mono-pitch trusses

Applicable where the truss is not restrained by:

a masonry wall, or

cladding, i.e. plywood.