A purlin roof is a roof structure in which the roof covering is carried by horizontal beams — the purlins — that run between trusses or bents. It is one of the two main types of roof structure, alongside the rafter roof.
How is it built?
In a purlin roof the trusses or bents carry the purlins, which run parallel to the ridge. The rafters or roof sheathing rest on the purlins, and on top of those come the tiling battens and roof tiles. The purlins allow the trusses to be spaced further apart.
Common purlins are:
- Ridge purlin — at the top, near the ridge
- Intermediate purlin — halfway up the roof slope
- Wall plate (foot purlin) — at the bottom, on the wall
Difference from a rafter roof
In a rafter roof the rafters run directly from wall plate to ridge and carry the roof covering themselves; no purlins are needed. In a purlin roof the purlins take the load and pass it on to the trusses. Purlin roofs are suitable for larger spans and are very common.
Application
- House building — many traditional roofs are purlin roofs
- Large spans — barns, sheds and halls
- Prefab roofs — prefabricated trusses with purlins between them
Related Terms
- Purlin
- Truss
- Roof structure
- Roof sheathing
- Wall plate
Want to learn more about construction terms? Visit the full knowledge base at fredsdiyplans.com.