Is the blue green algae on my roof dangerous?
Short answer. The dark streaks on most Southern roofs are a type of cyanobacteria, often called blue green algae, but they are not the toxic water blooms you hear about on lakes. On a roof the concern is appearance and a damp surface, not a health hazard, and it can be removed with a gentle wash and slowed with zinc or copper and algae resistant shingles.
Why it is called blue green algae
Cyanobacteria are an ancient group of organisms that photosynthesize like plants. The species most often blamed for roof streaks forms a dark protective sheath against sunlight, which is why the streaks look black or dark gray instead of green.
Health questions
The cyanobacteria blooms that close lakes produce toxins in water. Roof streaks are a different situation: a thin dry film on shingles, not a bloom in drinking or swimming water. Normal precautions apply when cleaning: gloves, eye protection, and not breathing cleaning chemical mist.
What it does to the roof
- Mostly cosmetic darkening that some HOAs and buyers notice.
- A sign of slopes that stay damp, which may later grow moss or lichen.
- A slight increase in how much heat the roof absorbs, since dark surfaces warm more.
How to deal with it
- Gentle wash with a diluted bleach based or oxygen bleach cleaner, applied at low pressure and rinsed.
- Never pressure wash; it removes the protective granules.
- Zinc or copper strips near the ridge to slow regrowth.
- Algae resistant shingles, which have copper treated granules, at the next replacement.
The cleaning steps and prevention options are covered in black streaks and algae on roofs, and Georgia's climate role in heat and humidity and roof life.