Does bleach remove roof algae, and how should you use it?
Short answer. Yes. Household chlorine bleach diluted about one to one with water is the method the Asphalt Roofing Manufacturers Association describes for removing algae streaks from asphalt shingles. Apply it gently with a low pressure sprayer, rinse with plain water, and protect plants and metal from runoff.
Why bleach works
Sodium hypochlorite kills the algae and bleaches the dark pigment it leaves. Streaks often lighten within minutes and keep fading for days as rain rinses the dead growth.
Mixing and applying
- Use plain, unscented household bleach, not splash free or gel types.
- Mix roughly equal parts bleach and water in a pump sprayer. Some shingle manufacturers specify their own ratio, so check yours.
- Spray at low pressure on a cool, overcast day so it does not dry too fast.
- Rinse gently with a garden hose after the recommended dwell time.
Protecting everything else
- Plants: water them before and after, and cover them during application.
- Gutters and metal: bleach is corrosive to aluminum and fasteners if left on; flush gutters with clean water.
- Siding and decks: rinse drips immediately to avoid spotting.
- Streams and storm drains: do not let concentrated runoff flow into them.
Never mix bleach with
Ammonia or acids, including some cleaners and vinegar, because the combination releases toxic gas.
Limits
Bleach does not prevent regrowth; streaks usually return within a few years. It also kills only what it touches, so thick moss needs more time and gentle removal. For longer term control, see black streaks and algae on roofs and moss and lichen on roofs.