Purlins used to support roof loads must be at least what size?

Purlins must be at least the size of the rafter they are supporting because they serve as a critical structural element in transferring roof loads to the rafters. When purlins are sized appropriately, they ensure the overall stability and strength of the roof system. The load carried by the purlins can be significant, especially in regions with heavy snowfall or other load considerations, making it essential that they match or exceed the capacity of the rafters they support.

This sizing practice helps in maintaining the integrity of the roof structure, preventing potential failures that could arise if the purlins are undersized. Proper sizing also complies with building codes and standards that dictate the necessary dimensions for various structural components to ensure safety and reliability in construction.