Question
I have black streaks on the north side of my roof. What causes them and what would you suggest I use to remove them?
Answer
Those black streaks on your roof shingles are most likely an algae called gloeocapsa magma, or as it is more commonly referred to, a blue-green or black algae. Algae is a solid deposit of spores and dirt, another form of mold.
Mold and algae thrive in moist conditions, which is why you see it mostly on north-facing or deeply shaded parts of your home, including siding. These algae won’t damage your shingles, it just looks bad.
Wash roof shingles with a soft bristle brush or broom (no metal brushes), scrubbing on a mixture of two parts water with one part of a detergent like TSP, Spic ‘n Span, Top Job or Simple Green. This removes most of the solids. Rinse off with straight water, and be sure to rinse any plants that may have been rained on by this mixture.
Do NOT pressure wash your roof; one wrong angle and you can damage the shingles. And please do not stand on your roof while cleaning – use extension poles to safely clean from a ladder or the ground.
If black streaks become a regular maintenance issue for you, consider switching to an algae- and mold-resistant roof shingles. They cost a bit more than regular asphalt shingles, but when you consider they will eliminate the cost of roof cleaning, it may be worth it to you.
Another treatment for avoiding the black streaks is to install zinc metal strips at the top of your roof near the ridge caps, above the algae prone areas. Zinc naturally inhibits algae growth, and when rain hits the metal strip, it carries the zinc down and over the shingles.