In architecture or structural engineering, a purlin (or purline) is a horizontal structural member in a roof. Purlins support the loads from the roof deck or sheathing and are supported by the principal rafters and/or the building walls. The use of purlins, as opposed to closely spaced rafters, is common in pre-engineered metal building systems and some timber frame construction.
In timber roof construction prior to the introduction of trusses, under purlins were used to support rafters over longer spans than the rafters alone could span. Under purlins were typically propped off internal walls. For example, an 8"x 4" under purlin would support the center of a row of 6"x 2" rafters that in turn would support 3"x 2" roof purlins to which the roof cladding was fixed.
In metal building roof systems, purlin members are frequently constructed from cold-formed steel 'Z' sections. These sections can be lapped and nested at the supports which creates a continuous beam configuration between the bays. Some people call any horizontal member even wall members purlins.
An equivalent element in wall construction is a girt.