A rafter is a sloping beam in a roof structure that runs from the ridge purlin to the wall plate and carries the roofing. Rafters run perpendicular to the ridge and form the sloping “ribs” of the roof. Battens and then roof tiles, sheets or other roofing material are fixed to the rafters.
Rafter in the roof structure
The hierarchy from top to bottom:
1. Roofing (tiles, sheets, EPDM)
2. Battens (thin strips on which tiles hang)
3. Rafters — this one
4. Purlins (horizontal beams)
5. Trusses (load-bearing frames)
Each rafter spans the distance from purlin to purlin. On a small roof with only a ridge purlin and wall plate, the rafter spans the entire roof surface.
Dimensions
The required rafter size depends on the span (distance between purlins) and the load:
| Span | Rafter size (indicative) | On-centre spacing |
|---|---|---|
| Up to 1.5 m | 38 x 125 mm | 60 cm |
| 1.5 – 2.5 m | 50 x 150 mm | 60 cm |
| 2.5 – 3.5 m | 50 x 175 mm | 60 cm |
| 3.5 – 4.5 m | 63 x 200 mm | 60 cm |
The on-centre spacing between rafters is usually 60 cm — this matches standard insulation and panel sizes.
Rafter vs. truss
| Rafter | Truss | |
|---|---|---|
| Shape | Single sloping beam | Complete triangular frame |
| Function | Carries the roofing | Carries the entire roof |
| Quantity | Many (every 60 cm) | Fewer (every 80-120 cm) |
In a trussed roof, the rafters rest on purlins supported by the trusses. In a rafter roof (without trusses), the rafters run directly from ridge to wall.
Rafter roof vs. trussed roof
Rafter roof
The rafters are the main load-bearing element. They run from the ridge purlin to the wall plate without intermediate trusses. Suitable for small spans (up to approximately 4-5 metres). Simple to build.
Trussed roof
Trusses are the main load-bearing element. The rafters rest on purlins between the trusses. Suitable for all spans.
For DIY projects (garden room, small shed) a rafter roof is the simplest. For larger sheds and garages, trusses are needed.
Installation
1. Cut rafters to size — Calculate the length based on the roof pitch and the distance from ridge to wall
2. Birdsmouth cut — Cut a notch at the bottom that fits over the wall plate or purlin (known as a “birdsmouth”)
3. Fix in place — With angle brackets or screws to the purlins
4. Check spacing — Measure regularly to check the on-centre distance and prevent drift
Related terms
- Purlins
- Truss
- Batten
- Ridge purlin
- Wall plate
- Roof structure