Purlins are horizontal beams in a roof structure that support the roof covering or roof panels. They run parallel to the ridge and rest on the trusses or walls of the building. Without purlins, there would be no structure to fix the roof covering to.
The role of purlins in a roof structure
A roof is built up in multiple layers. Purlins form a crucial intermediate layer:
1. Trusses or walls — The main supporting structure
2. Purlins — Horizontal beams resting on the trusses
3. Rafters or roof panels — Rest on the purlins
4. Tile battens + roof covering — The outermost layer
Purlins carry the weight of the roof covering, wind and snow loads, and transfer these to the trusses or load-bearing walls.
Types of purlins
There are three important purlins in a roof structure:
Ridge purlin
The purlin at the top of the roof, on the ridge. This is the highest point of the roof structure. The ridge purlin connects the trusses at the top.
Wall plate
The purlin that rests on the wall at the bottom. The wall plate distributes the weight of the roof across the wall and forms the connection between roof and wall.
Intermediate purlins
On larger roofs, additional purlins are placed between the ridge and wall plate to reduce the span of the rafters.
Purlins for covered areas, pergolas, and carports
When building a covered area, pergola, or carport, purlins are an essential component:
- For a covered area, purlins carry the roof panels (polycarbonate, glass, or timber)
- For a pergola, purlins are often visible and form a decorative element
- For a carport, purlins span between the posts
The purlin spacing (centre-to-centre) depends on the type of roof covering and the load. For polycarbonate sheets, 60-80 cm is common; for timber roof decking, this can be up to 100-120 cm.
Material and dimensions
Purlins are usually made from:
- Spruce — Affordable, suitable for covered structures
- Douglas fir — More durable, attractive for exposed work
- Pressure-treated wood — Necessary for unprotected outdoor structures
- Steel — For large spans or heavy loads
Common dimensions for timber purlins:
| Application | Dimension (mm) |
|---|---|
| Small covered area (up to 3 m) | 50 x 150 |
| Medium covered area (3-5 m) | 63 x 175 |
| Shed or garage | 75 x 200 |
| Large span | 75 x 225 or steel |
Related terms
- Ridge purlin
- Wall plate
- Truss
- Rafter
- Centre-to-centre (c/c)
- Overhang