Purlin function as a form of support for rafters and are horizontal structural members in a building, architecture or structural engineering. They are used to increase roof spans without the need for increasing rafter sizes or compromising wall stability.
Purlins are also used to build wider roofs, as they provide mid-span support. In addition, they increase the rigidity of the roof and support the overall weight of the roof deck. Three types of purlin can be used in wood construction for framing: purlin plate, common purlin and principal purlin. In steel construction, purlins are used as roof framing members that support the roof sheeting or decking.