What is black streaks (gloeocapsa magma)?
Gloeocapsa magma is a blue-green algae that forms dark streaks and stains on roofs, thriving in shaded, moist environments and commonly removed with soft washing rather than high-pressure equipment.
Gloeocapsa magma is a cyanobacteria (blue-green algae) that colonizes roof surfaces, creating the characteristic black or dark green streaks visible on asphalt shingles throughout the Southeast. The algae feeds on the limestone filler in shingle granules and thrives in shaded, damp areas, making it especially common on north-facing or tree-covered roofs in humid climates like Columbia, SC.
The streaks are not just cosmetic. The algae and associated debris trap moisture against the roof surface, potentially accelerating shingle degradation and shortening roof life. Unlike moss or lichen, gloeocapsa magma does not have root structures, but its waxy protective coating makes it stubborn to remove.
High-pressure washing can strip shingle granules and damage the roof membrane, which is why roof soft washing is the preferred method. Soft washing uses low-pressure water combined with biodegradable algaecide solutions to kill the algae and allow stains to rinse away without mechanical force. This approach protects the roof integrity while effectively eliminating both the visible streaks and the living organism underneath, reducing the likelihood of rapid regrowth.