A purlin is a horizontal structural member often made from wood or steel that directly supports the roofing material. Purlins transfer the roof load to the main roof structure such as trusses, load bearing walls, or large rafters.
Purlins run perpendicular to the slope of the roof as opposed to rafters that run up and down the slope of the roof. Purlins vary in size depending on the type of structure, loading requirements, and local codes.
The distance between purlins is depended on the type of roofing material, loading requirements, and local codes. Be sure to check with your local building inspector or check local code requirements.