In a built-up roof, what is the most likely cause of blisters?

In the context of a built-up roof, blisters are typically formed due to the presence of moisture trapped beneath the roof membrane. When moisture accumulates under the surface of the roofing material, it can create pressure that results in bubbles or blisters forming. The most common scenario for this to occur is through condensation, where warm, moist air rises into the cooler areas of the roof structure, leading to water vapor condensing on the underside of the roofing materials.

While moisture accumulation is related, it's essential to make the distinction that blisters specifically indicate that moisture has become entrapped in a way that leads to pressure build-up, often due to condensation processes rather than long-standing water issues or other factors. Therefore, condensate build-up under the membranes is a crucial element in the development of blisters in built-up roofing systems.