A small blister or bubble that forms in the flood coat of a hot-applied built-up roof. It occurs when moisture or trapped air expands under the heat of the liquid asphalt or coal tar, pushing upward through the surface before the material cools and sets. These boils can eventually pop, exposing the underlying felts to water damage.
We've a cluster of tar boils near the parapet that need to be scraped and patched before we spread the gravel.
Tar boils often indicate that the substrate or the felts were slightly damp during application.
Related terms
Blister
A raised, trapped pocket of air or moisture vapor within a roof membrane.
Hot mop
Waterproofing built from layers of felt mopped down with hot asphalt, or the act of applying it.
Outgassing
Gas escaping into curing sealant or membrane from punctured rod or damp concrete, leaving bubbles and pinholes.