Roof Moss & Algae Treatment: What Works and What Wrecks Your Shingles
The chemistry behind roof algae and moss — plus the treatments that actually kill it without stripping granules or voiding your warranty.
Moss and algae are different problems needing different chemistry. Treating one with the other's product either fails or destroys the roof.
Algae — the black streaks
Killed by sodium hypochlorite (bleach) at 50/50 dilution. Dwell 20 minutes, rinse. Streaks fade over 2–4 rain cycles — do NOT scrub.
Moss — the green mounds
Killed by potassium salts of fatty acids (Wet & Forget, Bayer 2-in-1). Apply dry, leave alone. Dead moss releases in 1–3 months and washes off in rain.
Lichen — the crusty circles
Hardest to treat. Same bleach mix as algae but requires 2–3 applications over 6 months. Prevention (zinc strip) is cheaper than cure.
The single best prevention
Trim overhanging branches so the roof gets 4+ hours of direct sun daily. Sun is a natural fungicide — shaded north slopes always fail first.
Frequently Asked Questions
- Are algae-resistant shingles worth it?
- Yes — AR shingles cost $2–$5 more per bundle and carry a 10-year streak warranty. On a full replacement, the upgrade is $150–$400 total.
- Can moss cause leaks?
- Yes. Moss holds moisture against the shingle mat and lifts tabs, letting wind-driven rain under. Treat it before the next storm season.
Add years to your roof — before you spend a dollar on replacement
Estimate how much life you have left, price small repairs, and see whether prevention beats replacement in your situation.