A meandering, tunnel-like blister or ridge in a built-up or single-ply roof system. Unlike ridges that follow the straight lines of insulation joints, these random bumps are usually caused by trapped moisture expanding or the membrane detaching from the substrate in irregular patterns. They compromise the roof's wind uplift resistance and are easily damaged by foot traffic.
Heard on the job
Watch your step near the parapet, there's a massive mole run waiting to trip you up.