How do you remove algae from a roof safely?
Short answer. Use a gentle wash: apply a diluted bleach based cleaner or an oxygen bleach product at low pressure, let it work, and rinse lightly with a garden hose. Never pressure wash or scrub asphalt shingles, and protect plants, gutters and yourself, or hire a soft wash company that follows the same method.
Before you start
- Stay off the roof if you can. Wet shingles and cleaner are slippery. Many homeowners can treat the lower roof from a ladder at the eave, but steep or two story roofs are a job for a pro.
- Pick a cool, cloudy day so the solution does not dry before it works.
- Soak plants and grass below the roofline with plain water, and cover delicate beds with plastic.
The method
- Mix the cleaner per the label. Industry guidance for asphalt shingles commonly points to a diluted household bleach and water solution; oxygen bleach products are a gentler alternative.
- Apply with a pump sprayer at low pressure, working from the bottom up so streaks do not form.
- Let it dwell for the time on the label, usually several minutes. Do not let it dry.
- Rinse with a garden hose at normal pressure, working from the top down.
- Rinse plants again, and flush gutters and downspouts.
What not to do
- No pressure washer. It strips granules, shortens shingle life and can affect the warranty.
- No scrub brushes or stiff brooms.
- No mixing bleach with other cleaners, especially anything containing ammonia.
Keep it from coming back
Trim shade trees, keep debris off the roof, and consider zinc or copper strips near the ridge. See black streaks and algae on roofs for prevention and the Georgia roof maintenance checklist for the rest of the yearly routine.