If your roof looks streaked with black or dark green stains running down from the peak, you're looking at Gloeocapsa magma — a hardy algae that feeds on the limestone filler in asphalt shingles.
Why it shows up
Spores travel by wind and land on roofs across the neighborhood. North-facing slopes, shaded areas and humid climates like ours give it the perfect environment to take hold and spread.
Why it's a real problem
- It eats the granules off your shingles, shortening roof life.
- It traps heat and moisture, accelerating wear.
- It can void some shingle manufacturer warranties if left untreated.
Why pressure washing a roof is a bad idea
High-pressure water blasts the protective granules right off your shingles — sometimes voiding your warranty in a single afternoon. The ARMA (Asphalt Roofing Manufacturers Association) explicitly recommends soft washing instead.
The right way: soft washing
A professional soft-wash treatment uses low pressure plus a specialized cleaning solution that kills the algae at the root. Streaks rinse away within minutes, and proper application can keep your roof clean for several years.
— The Louisville Housewash Crew
