While square tubing and purlins are both used for support, purlins play a different role. Typically, steel purlins are used to help support your roof . They’re used to build out the framing of your roof, and offers a support system for your roof decking. After the decking is supported and secure, you either build the roof out with shingles (wooden or asphalt) or with U-Panels or R-Panels.
Purlins, most commonly, come in two different types. Z Purlins are more commonly used for overlapping projects, while C Purlins are used for single span projects.