what are purlins on a roof

Purlins are horizontal structural members in a roof that provide framing and attach sheathing material. They are typically supported by either rafters or the walls of the building. Purlins must be designed and constructed to resist gravity loads, wind uplift loads, and loads imposed by laterally bracing the tops of the trusses. They create a horizontal frame that supports the weight of the roof deck, no matter which materials are used, and improve the rigidity and breadth of buildings by acting as a mid-span support. Purlins also provide a nailing surface for any end panels and drip edges. The spacing of purlins usually follows the nodal pattern of the supporting trusses and should be placed at the nodes of trusses and not on the members themselves so as not to induce secondary bending and shear.