Modified Bitumen Roof Repair

Modified bitumen is an asphalt-based roofing membrane reinforced with polymers for added flexibility, commonly installed as cap sheets in either torch-applied, hot-mopped, or self-adhered systems. It’s a common upgrade over older built-up roofing on many Downey homes.

The standard professional repair method is a three-course application: a base layer of flashing cement (a heavy, trowel-grade asphalt compound), a fiberglass reinforcing membrane embedded into that wet layer, and a top coating of flashing cement over the fabric, feathered out onto the surrounding roof. This method works even on roofs with some ponding water, since certain flashing cements are formulated to bond even in wet or underwater conditions.

Modified bitumen repairs hold up well when done correctly because the fiberglass reinforcement prevents the patch from cracking under thermal movement — a bare cement patch without fabric reinforcement is far more likely to crack and fail within a year or two.