How do you get rid of algae on your roof?
Short answer. Roof algae, the black streaks common on Georgia roofs, can be lightened with a diluted chlorine bleach solution applied with a low-pressure sprayer and rinsed gently, following your shingle manufacturer's guidance. Never pressure wash or scrub, and expect the streaks to return over time unless you add zinc or copper strips or eventually install algae-resistant shingles.
What the streaks are
The dark streaks running down many Georgia roofs are colonies of airborne algae that feed on the limestone filler in older shingles and thrive in warm, humid weather. They are mostly cosmetic, but they make a roof look older and can hold a little moisture on shaded slopes.
How to clean them
- Check the manufacturer's guidance. The asphalt roofing industry's technical guidance suggests about one part household chlorine bleach to one part water, and some manufacturers recommend a weaker mix.
- Protect plants and metal. Wet down landscaping before and after, cover delicate plants, and rinse gutters and metal flashing.
- Apply with low pressure. Use a garden pump sprayer, working from a ladder or a safe position rather than walking on wet, slick shingles.
- Let it work, then rinse gently. Follow the product or manufacturer timing, then rinse with a garden hose at normal pressure.
What not to do
- No pressure washers, which strip granules and can void the shingle warranty.
- No scrubbing with stiff brushes.
- Never mix bleach with ammonia or acidic cleaners.
- Avoid cleaning on a hot afternoon; the mix dries before it works.
Keeping algae away
Zinc or copper strips near the ridge discourage regrowth below them. When it is time to replace the roof, algae-resistant shingles with copper-containing granules carry a separate algae warranty. See black streaks and algae on roofs and the Georgia roof maintenance checklist.