What causes algae on a roof?
Short answer. Roof algae is caused mainly by Gloeocapsa magma, a blue green alga (cyanobacterium) carried by wind that settles on shingles and thrives in warm, humid, shaded conditions. The dark streaks are its protective outer coating, which darkens as it builds up.
The organism
Gloeocapsa magma forms a dark sheath to protect itself from ultraviolet light. That sheath is what you see as black or dark green streaks, usually starting high on a slope and running down where water flows.
Conditions that feed it
- Humidity and warmth: Georgia summers are close to ideal.
- Shade: north facing slopes and roofs under trees dry slowly after rain and dew.
- Moisture paths: streaks follow drip lines below valleys, gutters from upper roofs and AC condensate outlets.
- Shingle composition: many asphalt shingles contain limestone filler, which is commonly linked to algae growth.
Why it spreads
Spores and cells travel on wind and by animals. Once one roof in a neighborhood is streaked, neighboring roofs are seeded. That is why entire subdivisions often show streaks at the same time.
What does not cause it
- It is not soot, dirt or mildew from the attic.
- It is not a sign the shingles were defective, though older shingles lacked copper granules.
- It is not caused by poor ventilation, which causes different problems underneath.
How to stop the causes you can control
Trim branches to let in sun and air, keep valleys and gutters clear, and redirect condensate lines off the roof. At replacement, choose algae resistant shingles. See black streaks and algae on roofs, trees and your roof and heat and humidity and roof life.