C-Purlin is a horizontal beam or bar used for structural support in buildings, most commonly on a roof. Purlins are supported either by rafters or the walls of the building. Steel Purlins are lightweight, dimensionally stable, accurate, and straight. They expand and contract reasonably in extreme temperature changes.

A purlin (historically purline, purling) is a structural member in a roof. In traditional Timber framing, there are three basic types of purlin: purlin plate, principal purlin, and common purlin. It also appears in Steel frame construction. Steel purlins may be painted or greased for protection from the environment.