What is a purlin?
A purlin is a horizontal beam in a roof structure that runs parallel to the ridge and supports the rafters or roof panels. Purlins rest on the trusses or on the load-bearing walls and form the intermediate layer between the primary structure (trusses) and the roofing material.
> See also: Purlins (plural) — the article about the role of purlins in the complete roof structure.
Purlin vs. rafter
This distinction is essential:
| Purlin | Rafter | |
|---|---|---|
| Direction | Parallel to the ridge | Perpendicular to the ridge (from ridge to gutter) |
| Rests on | Trusses or walls | Purlins |
| Supports | Rafters or roof panels | Battens and roofing material |
A purlin therefore runs along the length of the building, while a rafter runs from top to bottom.
Types of purlins
- Ridge purlin — At the highest point (the ridge)
- Wall plate — On the wall, the lowest point of the roof surface
- Intermediate purlin — Halfway between ridge and wall plate, on larger roofs
Dimensions
| Span (between trusses) | Purlin size (indication) |
|---|---|
| Up to 2.5 m | 50 x 150 mm |
| 2.5 – 3.5 m | 63 x 175 mm |
| 3.5 – 5.0 m | 75 x 200 mm |
| Over 5.0 m | 75 x 225 mm or steel |
The dimensions depend on the load (roofing material, snow, wind) and the spacing between the rafters.
Application
On a pergola, carport, or veranda, the purlins are often the most visible structural elements. They run from post to post and support the roof sheathing or panels.
In an enclosed building (shed, garage), the purlins are usually hidden within the roof structure.
Related terms
- Purlins
- Ridge purlin
- Wall plate
- Truss
- Rafter