tar boil
A small bubble or blister in the flood coat of an aggregate-surfaced built-up membrane
tar boil: blister that forms under bitumen membranes
A tar boil is a localized bubble or blister that forms in the flood coat (the top layer of bitumen) of a built-up roofing membrane. It occurs when moisture, air, or other volatile substances become trapped beneath the bitumen surface and expand under heat, pushing the solidified layer upward into a dome shape. The blister typically measures from less than an inch to several inches across and compromises the weathertightness of the roof assembly.
The primary cause is moisture trapped in the substrate, felts, or insulation layers that vaporizes when the roof surface temperature rises in direct sunlight. Built-up roofs built in damp conditions or without proper drying time between plies are especially prone to boiling. Vapors can also be released from the bitumen itself if applied at excessive temperatures or if the material contains residual water. Poor ventilation in the roof cavity and sealed edges that prevent vapor migration increase pressure and frequency of boils.
Tar boils differ from other roofing failures in their characteristic appearance and cause. Unlike ridges or wrinkles formed during installation, boils are true protrusions caused by internal pressure. They are distinct from alligatoring or crazing, which result from material aging and shrinkage. A boil left untreated will eventually rupture, exposing the underlying felts and bitumen to water penetration and accelerating roof deterioration.
The term reflects the violent expansion occurring inside the membrane as trapped material approaches boiling point. Repair requires cutting open the blister, allowing moisture to escape, drying the cavity thoroughly, and re-sealing the roof surface with fresh bitumen and aggregate. Prevention requires controlling substrate moisture content before application and managing application temperatures and timing carefully. In existing roofs, proper ventilation or intentional roof venting can reduce the frequency of new boils by allowing trapped moisture to migrate outward before vapor pressure builds dangerously.