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Roof Is Turning Black

Q: What's causing the black streaks on our roof, and how do I get rid of it?

cleaning algae off of a roof
Photo by Kolin Smith

On the north side of our roof the shingles are covered with long black streaks. Can you tell me what causes this and how to get rid of it?
Geri Chapman, Wooster, Ohio


Kevin O'Connor replies: According to Jim Baker of the Asphalt Roofing Manufacturers Association, those black streaks are caused by Gloeocapsa magma, a type of blue-green algae that thrives on warm temperatures and moisture. Shaded or north-facing roofs are most susceptible because they dry off more slowly after a rain. But even without rain, summertime humidity is often high enough in the eastern United States to foster the growth of this algae, which is a type of photosynthesizing bacteria.

Baker says 1 gallon of bleach and 5 gallons of water with 1 cup of TSP substitute will get rid of the streaks. But before you begin, put on old clothes, protect your eyes with goggles, and spray your foundation plants with clean water or cover them with a tarp. Spray the solution on the roof using a pump sprayer, let it soak in for 15 minutes or so, then rinse it off. The mixture is slippery, so don't stand on the roof as you apply it; work from a ladder instead.

Unless you want to repeat the process every year, hire a contractor to install strips of zinc or copper under the shingles near the ridge. Every time it rains, the strips release minute quantities of metal ions that flow down the roof, killing the algae before it has a chance to spread.


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