How do you remove algae from roof shingles?
Short answer. Remove roof algae by spraying a solution of about one part household chlorine bleach to one part water onto the streaks with a low pressure sprayer, letting it work briefly, then rinsing gently with plain water. Never pressure wash or scrub, because both strip the protective granules off asphalt shingles.
The black streaks on Georgia roofs are mostly a type of blue green algae. The method below follows guidance from the Asphalt Roofing Manufacturers Association; check your shingle maker's instructions too, since some specify their own mix.
Step by step
- Pick a cool, cloudy, calm day so the solution does not dry instantly or drift.
- Protect plants and surfaces. Soak shrubs and grass with water first and cover them with plastic or tarps. Rinse anything the solution touches.
- Mix the solution in a garden pump sprayer: roughly equal parts household bleach and water.
- Apply from the bottom up with low pressure, working from a ladder or roof edge where possible rather than walking on wet shingles.
- Let it sit briefly, following the manufacturer's dwell time, then rinse thoroughly with a garden hose at normal pressure.
- Flush gutters and downspouts with clean water.
What not to do
- No pressure washer, even on a low setting.
- No brushes, brooms or scrubbing.
- No mixing bleach with other cleaners, especially ammonia.
Safety
Wear eye protection and gloves. Bleach makes shingles very slippery; manufacturers recommend leaving roof work to trained professionals.
Keeping it from coming back
Cleaning is temporary; streaks usually return within a few years. Zinc or copper strips near the ridge slow regrowth, and algae resistant shingles with copper granules are the long term fix at replacement time. See black streaks and algae on roofs and heat and humidity and roof life.