Home

Rafters

Two diagrams - one showing a birdsmouthed rafter, the other showing a non-birdsmouthed rafter, which has maximum overhang 30% single span value of rafter except where overhang for a birdsmouthed rafter permits a greater overhang.

In a conventionally framed roof, the rafters are fixed on a slope and run in the same direction as the roof slope. Rafters are generally erected in opposing pairs or couples. They meet each other directly at their apex on the ridge of the roof. The rafters give the roof its required slope and provide the structure onto which the roof covering is fixed.

Where a rafter meets its supporting top plate it is cut so that it rests directly on the top plate. The cut is a 'birdsmouth' cut. Rafters may be birdsmouthed to a depth not exceeding one third of the rafter depth. Rafters must be single length members or joined over supports.

Table 29 of AS 1684.2 provides details for rafters and purlins.


Two diagrams show a section of a roof. The first shows single span. The distance between rafters labelled as 'rafter spacings', and the distance along a rafter from the ridge board to the beginning of the overhang as 'rafter span'. The second diagram shows a continuous span. In this case an underpurlin running parallel to the ridge board is positioned halfway along the rafter.