A rafter is a sloping structural timber in a pitched roof that runs from the wall plate at the eaves to the ridge board or a purlin at the top. Rafters, together with purlins, the ridge and any trusses, form the main load-bearing structure of a rafter or purlin roof, transferring the weight of the roof sheathing, tiles, snow and wind loads down to the supporting walls.
Function and position
The rafter carries:
- Roof sheathing or vapour-permeable membrane with counter battens and tile battens
- Roof covering (tiles, slates, thatch or bitumen)
- Snow and wind loads per the relevant Eurocode (EN 1991)
- Self-weight and any insulation between or above the rafters
Rafters are spaced at regular centres, usually 400-600 mm depending on span and roof covering. At the top they meet a ridge board or ridge beam; at the bottom they rest on the wall plate.
Couple roof vs. purlin roof
The two main framing systems differ in how the rafters are supported:
- Couple roof / rafter roof — rafters meet at the apex and form a self-supporting triangle; no ridge beam is required
- Purlin roof — rafters rest on horizontal purlins (ridge, mid and wall-plate purlins) which are themselves supported by masonry or columns
In modern construction, combinations and engineered timber solutions are common.
Materials and sizes
Rafters are usually made of:
- C16 or C24 softwood — the most common grade for new build
- Larch or Douglas fir — for visible work or higher durability
- Glulam / LVL — for longer spans
Common sizes are 38×175 mm, 50×175 mm up to 75×220 mm, depending on span, spacing and load.
Application
In the UK and Europe almost every new pitched roof is built with timber rafters supported by one or more purlins and trusses. In timber-frame construction rafters form part of prefabricated roof cassettes.
Related terms
- Truss
- Purlin
- Wall plate
- Ridge
- Roof structure
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