Purlin

A purlin is a horizontal beam that runs lengthwise along a roof, parallel to the ridge and eaves. It spans between principal rafters, trusses, gable walls or steel frames and supports either the common rafters above it or, in metal buildings, the roof sheeting directly. Purlins reduce the unsupported length of rafters and allow wider buildings to be roofed with smaller members.

In traditional timber roofs

In a historic "double roof", heavy principal rafters carry principal (side) purlins, and these purlins in turn carry lighter common rafters. Carpentry historians distinguish how the purlin meets the principal rafter:

• Through purlins pass over the backs of the principals.
• Butt purlins are tenoned into the sides of the principals.
• Clasped purlins are held between a collar and the principal rafter.

Large old barns in North America often use purlin plates carried on posts, which support rafters at mid-span. A distinctive "common purlin" system, in which purlins are notched into principal rafters and carry vertical roof boards, is mainly associated with early New England.

In British vernacular roofs, wind braces were often fitted between principal rafters and purlins to stiffen the roof lengthwise.

In steel and metal buildings

In portal-frame and pre-engineered metal buildings, purlins are usually cold-formed steel sections in C, Z or U profiles, running parallel to the eaves between the main rafters. Z purlins can nest together for shipping and are often lapped over supports so they act as a continuous beam. Profiled metal roof sheets are fastened directly to them.

Terminology

Historic spellings include purline and perling. The word should not be confused with battens, which are smaller strips fixed across rafters to carry tiles or slates.