A purlin is a horizontal structural member running perpendicular to rafters or roof trusses, providing intermediate support for roof sheathing or decking, particularly common in post-and-beam, timber-frame, or metal building construction where rafters are more widely spaced than conventional stick framing.
Because widely spaced rafters or trusses can’t adequately support roof sheathing directly without intermediate support, purlins bridge these gaps, allowing the sheathing to span a shorter, more manageable distance between supports.
This element is less commonly seen in conventional residential stick-framed roofs, where closely spaced rafters typically eliminate the need for this intermediate support layer.
What Does a Purlin Involve?
- Horizontal lumber or steel running perpendicular to rafters
- Spacing determined by the roof sheathing’s span capability
- Support for roof decking or metal roofing panels
- Common in timber-frame, post-and-beam, and metal building construction
Why Are Purlins Important?
In construction styles using widely spaced structural members, like timber-frame or post-and-beam roofs, purlins provide the necessary intermediate support for roof sheathing to span safely, filling a structural gap that closely spaced conventional rafters wouldn’t otherwise require. This makes purlins an essential rather than optional component in these particular construction styles.
Metal building and pole barn construction relies heavily on purlins running between widely spaced structural frames, supporting the metal roofing panels directly and demonstrating this element’s continued practical importance well beyond traditional timber-frame applications. This continued relevance across construction styles highlights the enduring usefulness of this simple structural concept. This adaptability across building types reflects the enduring value of this straightforward structural solution. This value holds true across many building traditions. This closes the gap comfortably.
Purlin vs. Strut
A purlin provides intermediate horizontal support for roof sheathing between widely spaced rafters. A strut serves a different structural function, typically providing diagonal or compression support within a truss or framing assembly rather than supporting sheathing directly.