Purlins are roof support beams that run across the top of the trusses. Unlike joists which span from the ridge to the wall, purlins run the length of the building, parallel to the ridge, and bear on each truss. They are usually spaced at 12″ to 60″ centers depending on the type of roof panels to be installed. The roof panels are often directly supported by these purlins. If a solid plywood sub-deck is installed atop the trusses, then purlins may or may not be used.