Repair & Maintenance · FAQ

Are the black streaks on my roof actually algae?

Yes — Gloeocapsa magma, a bacteria that feeds on the limestone filler in asphalt shingles. The black color comes from a dark pigment the bacteria produces as UV protection. It shows up most heavily on north-facing slopes (less direct sun) and in shaded areas (more moisture retention).

The streaks are cosmetic, not structural. Algae doesn’t degrade the shingle measurably even after 5 to 10 years of growth. The main reason to address it is curb appeal and resale value. A streaked roof can knock 2 to 4% off home value at sale. Treatment costs $400 to $900 depending on roof size and access; the visual improvement is dramatic and lasts 4 to 7 years with the right cleaning method.