What is a rafter?
A rafter is a sloping beam in a roof structure that runs from the ridge purlin to the wall plate and supports the roofing material. Rafters run perpendicular to the ridge and form the sloping “ribs” of the roof. Battens are fixed to the rafters, and then the roof tiles, sheets, or other roofing material is attached.
Rafter in the roof structure
The hierarchy from top to bottom:
- Roofing material (tiles, sheets, EPDM)
- Battens (thin laths on which tiles hang)
- Rafters <-- this one
- Purlins (horizontal beams)
- Trusses (load-bearing frames)
Each rafter spans 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 (indication) | Centre-to-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 centre-to-centre distance 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 | Supports roofing material | Supports the entire roof |
| Quantity | Many (every 60 cm) | Fewer (every 80-120 cm) |
In a trussed roof, the rafters rest on purlins that are carried 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 primary 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 form the primary load-bearing element. The rafters rest on purlins between the trusses. Suitable for all spans.
For DIY projects (garden house, small shed), a rafter roof is the simplest. For larger sheds and garages, trusses are needed.
Installation
- Cut rafters to size — Calculate the length based on the roof pitch and the ridge-to-wall distance
- Birdsmouth joint — Cut a notch at the bottom that fits over the wall plate or purlin (the so-called “birdsmouth”)
- Secure — With angle brackets or screws to the purlins
- Check spacing — Regularly measure the centre-to-centre distance to prevent shifting
Related terms
- Purlins
- Truss
- Batten
- Ridge purlin
- Wall plate
- Roof frame