Roof bracing is one element in a continuum that provides resistance to horizontal loads. It works with the wall and foundation bracing to supply a total bracing package for a building.
Light timber-framed roofs, whether formed by proprietary trusses or framed-up close-coupled roofs, all require bracing as specified in NZS 3604: 1999 Timber framed buildings. The bracing of trussed or framed roofs is identical. Bracing is also required for lean-to roofs, skillion roofs and exposed rafters.
Bracing the roof framing allows the frame to resist horizontal loads and is a part of the total bracing required within a building to resist wind and earthquake loads. The roof-framing-bracing/wall-framingbracing/ foundation-bracing continuum is similar to the vertical or uplift load paths that must be continuous through the structure. (See Figure 1.)
Assuming that the foundation bracing is taken care of in the design, and adequate connections are made to the walls with wall bracing in place, then the roof bracing must also connect to the wall bracing to enable it to transfer loads back to the ground. Heavy hip roofs will also require the top plate to be secured against bending under load where there is no continuous bottom chord (where jack rafters form the hipped area of roof) by tying it back to a bracing panel with ceiling braces or a ceiling diaphragm.
The number of roof braces required is determined by the weight of the roofing and the area of the roof to be braced. Hip roofs have the advantage that continuous hip or valley rafters may be used for bracing, whereas a gable roof will always require additional gable, roof space or roof plane braces to be installed (see Figure 2). In the roof plane (i.e. the plane of the roofing material) all heavy roofs require braces. These can be formed using a continuous length of timber fixed to the underside of the rafters, or a diagonally opposed pair of continuous steel strips fixed over the rafters (see Figure 3). These braces must be continuous from the ridge to the top plate using fixings specified in Table 10.14 of NZS 3604: 1999.
The following are bracing requirements for all pitched roofs.
Light hip roof
A minimum of three hip or valley rafters per ridge are required. A roofplane brace or sarking may be used as an alternative. Roof plane diagonal braces and roof space or gable braces are not needed.
Heavy hip roof
A roof-plane diagonal brace is required per 35 m2 (or part thereof) of roof-plane area (for each roof plane), plus a minimum of three hip or valley rafters per ridge, plus ceiling-plane braces at 2.5 m centres to the top plate supporting the jack rafters. A ceiling diaphragm may be used as an alternative.
Light gable roof
A roof-plane diagonal brace is required per 25 m2 (or part thereof) of roof-plane area, with a minimum of two for each roof plane. Or a roof space or gable brace can be used at each end and at 7.5 m centres between.
Heavy gable roof
One roof-plane diagonal brace is required per 25 m2 (or part thereof) of roof-plane area, with a minimum of two for each roof plane, plus a roof-space brace per 12 m2 (or part thereof) of roof-plane area, parallel to ridge but not less than 2 m from a parallel external wall.
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