A purlin is used to support the rafters between the plate and the ridge of a roof. A purlin is a horizontal or longitudinal member in a roof frame.
There are several different types of purlins used in roof construction. An under purlin sits on the principal rafter of a truss or is propped off a wall to support the rafters. This type of purlin allows a longer span than the rafters alone are capable of spanning. An over purlin sits on the rafters or on a truss or a steel member to which the roofing material is directly attached. There are also several categories of purlins used in wood construction, including through purlins that pass over the top of the principal rafters and butt purlins that tenon into the sides of the principal rafters.