Purlins used to support roof loads must be at least the size of which component?

The correct choice focuses on the relationship between purlins and rafters in roof framing systems. Purlins are horizontal members that provide support to the roof covering and are typically installed perpendicular to the rafters. They are crucial for transferring the roof loads to the rafters, which in turn support the overall roof structure.

When determining the minimum size for purlins, it is essential to ensure they can adequately support the loads transferred from the rafters. Consequently, purlins must be at least the same size as the rafters, or even larger in some instances, depending on the specific load requirements and construction methods being utilized. This ensures structural integrity and safety for the roof system.

In contrast, trusses, beams, and joists have different roles and load-bearing capacities within a structure. Trusses are pre-engineered components designed to handle specific loads, while beams play a significant role in carrying loads across larger spans. Joists work primarily in floor systems, supporting floor loads rather than roof loads. While all these components are integral to a building's structure, the specific sizing requirements for purlins relate directly to the rafters, making this answer the most applicable in the context of roof construction.