What causes algae on roof shingles?
Short answer. The black streaks on most Georgia roofs are a blue-green algae, usually Gloeocapsa magma, that spreads by airborne spores and thrives where shingles stay damp and shaded. Humidity, north-facing slopes, overhanging trees and the limestone filler in many shingles help it grow, while copper and zinc slow it down.
The conditions algae needs
- Moisture. Morning dew that lingers on a north or shaded slope is enough. Georgia's long, humid summers give algae a long growing season.
- Shade. Trees and taller neighboring roofs keep a slope wet longer, so streaks usually start on the north side and under branches.
- Something to feed on. Many shingles use crushed limestone as filler, which is widely believed to help the algae along.
- Spores. Wind and birds carry spores from roof to roof, which is why whole neighborhoods streak at about the same time.
Why it looks black
The algae builds a dark outer coating to protect itself from ultraviolet light. That is what you see, starting as dots and running down the slope in streaks as rain carries it. Early on it is mostly cosmetic, but a darker roof absorbs more heat, and heavy growth holds moisture that invites moss and lichen later.
What actually helps
- Shingles labeled algae resistant use copper-containing granules that release a little copper with each rain and keep the surface clean for years.
- Zinc or copper strips installed near the ridge work on the same principle for an existing roof.
- Trimming branches lets the sun dry the roof faster.
- For cleaning, industry guidance is a gentle, low pressure application of a diluted bleach mix by a cleaning specialist. Pressure washing strips granules and shortens roof life.
For more, see black streaks and algae on your roof and trees and your roof.