Roof bracing to NZS 3604
How a roof's bracing is worked out to NZS 3604, step by step, with why each step is there and the terms it uses.
- NZS 3604:2011
Brace the roof in the calculator
The calculator counts and places the braces on your own roof. This page explains how.
The steps
See whether the roof needs braces
It needs none where its pitch is under 5°, where a roof plane is smaller than 6 m², such as a dormer or porch, where sarking covers the whole roof or a structural ceiling diaphragm is fixed to the rafters, or where a mono-pitch roof's high wall is braced full height.
Why Rafters and trusses stand side by side like books on a shelf: pushed along the ridge they lean over together. A flat or tiny roof barely leans, and sarking, a diaphragm or a braced high wall already holds it square.
NZS 3604:2011 10.3.1, 10.3.4, 10.3.5
Unbraced, the rafters lean over together when pushed along the ridge; crossed braces hold them upright. Count the braces
One brace for every 50 m² of plan on a light roof, or every 25 m² on a heavy one, overhangs included, and never fewer than 2 for each ridge line. A mono-pitch roof's highest support counts as its ridge.
Why A heavier roof has more to resist when it is pushed, so each brace serves less of it, and each ridge needs restraining at both ends whatever its area.
NZS 3604:2011 10.3.2 to 10.3.4, Table 10.16
Count the hip and valley rafters
On a roof framed with rafters, hip or valley rafters running whole from the ridge to the top plates count as braces where there are at least 3 of them. A trussed roof's hips are trusses and count as none.
Why A hip rafter already runs diagonally from the ridge to the wall, which is what a brace does.
NZS 3604:2011 Table 10.16 notes
Choose roof plane braces
In the roof from the ridge down to the wall: a 90 × 19 mm length of timber at 45° to the ridge, a crossed pair of steel straps each taking 4 kN in tension, or a hip or valley rafter
Why A brace in the plane of the roof turns the rafters into a stiff triangle, so they cannot lean as one.
NZS 3604:2011 10.4.2, Table 10.17
Or roof space braces
Diagonally inside the roof, no steeper than 45°, from the ridge or top chords down to ceiling level: one 90 × 45 mm up to 1.85 m long, or two spaced apart up to 4.8 m; its foot on a runner over a ceiling diaphragm or within 300 mm of a braced wall
Why Where the roof plane is busy, a diagonal across the roof space does the same job down to ceiling level, and its foot needs something that will not move.
NZS 3604:2011 10.4.3, Table 10.17
Place them
Put a brace at each end of every ridge and spread the rest along it, alternating in direction; the two kinds may be mixed.
Why Braces at both ends hold the whole ridge; alternating them resists a push from either direction.
NZS 3604:2011 10.4.2.1
Crossed braces from the ridge to the wall at each end of the ridge, and the rest spread along it. Or sark the whole roof
Hit and miss boards 140 × 19 mm at 40° to 50° to the ridge, no more than a board's width apart; or a sheet over the whole roof fixed to the rafters: three-ply plywood of 6 mm or more, or a wood-based sheet of 4.5 mm and 880 kg/m³, or 6 mm and 600 kg/m³
Why Boards on the diagonal or sheets fixed to every rafter make the roof plane one stiff surface, so no separate braces are needed.
NZS 3604:2011 10.3.1, 10.4.4
Or fix a ceiling diaphragm to the rafters
Rectangular, no longer than 2 times its width between bracing lines, lined with a gypsum-based sheet of 10 mm and 600 kg/m³ or more for a ceiling up to 15° and 7.5 m long, or plywood or a wood or fibre-cement board up to 25° and 12 m, or 45° and 7.5 m
Why A stiff ceiling fixed to the rafters holds them square from below, so the braces it stands in for are not needed.
NZS 3604:2011 10.3.1, 13.5
Show it on the drawings
A roof plan with each ridge line, every brace's kind and position, the hip or valley rafters counted, and the extent of any sarking or ceiling diaphragm, with its lining and fixing.
Why It is what the building consent authority checks the roof against, and what the builder builds from.
NZS 3604:2011 10.3, 10.4
The terms
- Plan area
- The roof's area seen from above, overhangs included. NZS 3604:2011 10.3.2
- Ridge line
- A ridge, or a mono-pitch roof's highest support, which sets the fewest braces. NZS 3604:2011 10.3.4
- Roof plane brace
- A brace in the slope of the roof, from the ridge down to the wall. NZS 3604:2011 10.4.2
- Roof space brace
- A diagonal inside the roof, from the ridge or top chords down to ceiling level. NZS 3604:2011 10.4.3
- Hip or valley rafter
- A rafter on the line where two roof slopes meet, running from the ridge to the wall. NZS 3604:2011 Table 10.16
- Sarking
- Boards or sheets fixed over the rafters across the whole roof. NZS 3604:2011 10.4.4
- Ceiling diaphragm
- A ceiling lined and fixed to act as one stiff plate. NZS 3604:2011 13.5
- Runner
- A member a roof space brace's foot bears on, over a ceiling diaphragm or near a braced wall. NZS 3604:2011 10.4.3.4