A localized swelling in a built-up or modified bitumen roof caused by trapped air, moisture, or solvent vapor expanding under solar heat. Blisters form between membrane plies or between the membrane and the substrate, gradually growing in size and eventually compromising the waterproofing if they rupture.
Heard on the job
Don't step on that blister near the drain, the membrane is stretched thin and might pop.
Insider tip
Stepping on a blister to flatten it is a rookie mistake; it breaks the internal plies and guarantees a leak.