What causes blisters in built-up roofing?

Blisters in built-up roofing occur primarily due to water or air expanding below the roofing material. When moisture seeps into the roofing layers or air becomes trapped, it can form pockets. As temperatures rise, this trapped water or air expands, creating pressure that leads to the blistering effect. This can cause the roofing material to bulge, which may eventually lead to structural integrity issues if not addressed.

Moisture from rain can contribute to the underlying conditions that cause such blisters but it is the expansion of that water or air that directly causes the blisters to form. Direct sunlight exposure can heat the roofing material and dry out moisture but it is not the direct cause of the blister formation. While air pressure changes can affect various roofing systems, it is the specific situation of expanding water or air that leads directly to blisters in built-up roofing. Thus, option C correctly identifies the cause of blisters in this context.