How shall purlins be sized in relation to the rafters they support?

Purlins, which are horizontal members used to support the roof decking and provide intermediate support between rafters, must be sized appropriately to ensure they can safely carry the loads imposed on them. This includes both dead loads, like the weight of the roofing material, and live loads, such as snow or maintenance activities.

Choosing a size for purlins that is less than the size required for rafters can compromise the structural integrity of the roofing system. Purlins must be capable of supporting the rafters while adhering to design standards and building codes, which often specify minimum sizing based on the spans involved and the loads anticipated. Therefore, it is critical that purlins are sized at least to the minimum requirements for what the rafters need to handle, ensuring the entire structure remains sound and safe.

While purlins could be larger than rafters for specific design benefits, it is essential they are not undersized relative to the rafters they are intended to support. This ensures that all components work together within the framework of load distribution and safety factors in building construction.