How do you clean algae from roof shingles?

What the black streaks are

Those dark stripes running down north and shaded slopes are colonies of a blue-green algae, often identified as Gloeocapsa magma. Spores travel on the wind, and Georgia's heat and humidity give them ideal conditions. The streaks are mostly cosmetic, but they hold moisture and can make a roof look years older than it is.

Cleaning, step by step

1. Soak nearby plants and grass with water and cover delicate ones with plastic.
2. Mix the solution following your shingle manufacturer's guidance. Published ratios range from equal parts bleach and water to one part bleach in three parts water.
3. Apply with a low-pressure pump sprayer, working from a ladder or roof edge if possible.
4. Let it dwell 15 to 20 minutes, without letting it dry.
5. Rinse gently with a garden hose, then rinse the plants again.

A wet, bleached roof is extremely slippery, so this is a job many homeowners hand off to a soft wash company or roofer.

What not to do

• No pressure washers, stiff brushes or brooms.
• No chemicals the shingle maker has not approved.
• No walking on the roof while it is wet with cleaner.

Keeping it from coming back

Cleaning is temporary, and streaks usually return within a few years. Zinc or copper strips near the ridge release metal ions in rain that slow regrowth below them, trimming trees adds sun and airflow, and algae resistant shingles with copper granules are the long-term fix at replacement time.

See black streaks and algae on roofs and moss and lichen on roofs.