What is roof algae?
Why the streaks are black
The organism builds a dark protective coating against ultraviolet light. As colonies spread, rain carries them down the slope, so the stains form vertical streaks rather than spots. Growth usually starts on north and west slopes, which stay damp longest after dew or rain.
Why Georgia roofs are prime territory
Algae thrives on heat plus humidity, which describes most Georgia summers. Shade from trees helps it along, and spores travel on wind and wildlife from roof to roof, which is why whole neighborhoods streak at the same time. Older shingles made without algae-resistant granules are hit hardest.
Is it harmful?
- It does not usually eat through shingles or cause leaks on its own.
- Dark streaks absorb more sun, adding heat to the roof surface.
- It hurts curb appeal and can draw attention from an HOA or a buyer's inspector.
- Moss and lichen, which do more harm, often move in where algae is established.
Getting rid of it
A low-pressure application of a cleaner made for roofs, often a diluted bleach and water mix of the kind shingle makers suggest, followed by a thorough rinse, will lighten the streaks. Protect plants and never pressure wash: it strips granules and can void a warranty. Zinc or copper strips near the ridge release metal ions that slow regrowth below them. For prevention on your next roof, see black streaks and algae and look for shingles with copper-containing algae-resistant granules.