A sag rod is a type of tension rod used in structural applications to provide support to purlins. Purlins are horizontal beams that support the roof deck and distribute loads to the building’s main frame. Sag rods help to maintain the alignment and stability of these purlins, especially in large-span roofing systems. By preventing sagging and ensuring even load distribution, sag rods enhance both the durability and performance of the roofing structure.