Purlin - Horizontal Roof Support and Framing Guide

A purlin is a horizontal structural member that spans between rafters or frames to support roof decking or roofing panels.

What It Is

Purlins create intermediate support across a roof so the roof covering does not have to span the full distance between major framing members. In metal buildings they often run perpendicular to the rafters and directly support metal roof panels.

Depending on the roof system, a purlin may be wood or cold-formed steel. It is a structural member, not just nailer lumber, and its size, spacing, and bracing affect the roof load path.

Types

Common types include wood purlins in conventional framing, steel Z-purlins, steel C-purlins, and purlin systems paired with standing-seam or corrugated metal roofs. Some roofs also use purlin-over-rafter arrangements to create ventilation space below the roofing.

Where It Is Used

Purlins are used in metal buildings, barns, pole buildings, sheds, carports, and some conventional roof assemblies. They are typically hidden beneath the roofing but may be visible from inside unfinished agricultural or utility structures.

How to Identify One

From below, purlins appear as repeated horizontal members spanning across roof framing beneath the roof panels or sheathing. In exposed structures they are usually smaller and more closely spaced than the main rafters or trusses.

Replacement

Replace a purlin when it is rotted, bent, split, rusted through, or no longer adequately supports the roofing above. Because purlins are structural, replacement usually involves temporary shoring and matching the original load requirements.