Purlins

A purlin is a horizontal structural member in a roof that supports the roof covering and distributes loads to the primary roof framing. It is typically placed perpendicular to the rafters and runs along the length of the roof, helping to stiffen the roof plane and reduce the span of individual rafters.

In timber roof construction, purlins are laid across the rafters at regular intervals and are supported by the rafters or wall plates. They carry the weight of the roofing material—tiles, slate, or metal sheets—between the rafters and transfer loads to the rafter system. Purlins can be continuous across several bays or formed from shorter members joined together; multiple tiers may be used on tall roofs or where snow loads require additional support.

In modern steel or light-gauge construction, purlins are typically metal members (often C- or Z-section) that run along the length of the building and support the roof deck or sheeting. They are positioned between primary framing members and fixed with clips, bolts, or screws. Steel purlins can span larger distances and enable rapid installation of roof coverings.

Functions include supporting roofing material, distributing loads, providing lateral bracing to roof framing, and helping resist wind uplift. Variants and terminology vary by region, but the basic concept remains a horizontal roof member bearing the roof covering and transferring load to the main framing.