In steel construction, the term purlin typically refers to roof framing members that span parallel to the building eave, and support the roof decking or sheeting. The purlins are in turn supported by rafters or walls. Purlins are most commonly used in Metal Building Systems, where Z-shapes are utilized in a manner that allows flexural continuity between spans.
It is the practice in the steel industry that structural shapes are assigned representative designations for convenient shorthand description on drawings and documentation: Channel sections, with or without flange stiffeners, are usually referenced as C shapes; Channel sections without flange stiffeners are also referenced as U shapes; Point symmetric sections that are shaped similar to the letter Z are referenced as Z shapes. Section designations can be regional and even specific to a manufacturer. In steel building construction, secondary members such as purlins (roof) and girts (wall) are frequently cold-formed steel C, Z or U sections, (or mill rolled) C sections.
Cold formed members can be efficient on a weight basis relative to mill rolled sections for secondary member applications. Additionally, Z sections can be nested for transportation bundling and, on the building, lapped at the supports to develop a structurally efficient continuous beam across multiple supports.