Roof algae appears as black stains (gloeocapsa magma). It grows on the shingles by feeding on the shingle limestone filler. OxiOut Algae & Mold Remover and JOMAX by ZINSSER Company will remove it. Roof algae facts
About 20-years ago, organic mat was used to absorb the asphalt in the manufacture of shingles. When the mat became cost prohibitive, limestone was used to add body and weight to the shingles. Prior to the use of limestone, algae did not have a food source.
Cleaning the algae off a roof is a dangerous job and usually requires the services of a professional. Falling off a dry roof is easy enough, but adding a cleaner and water is very slippery and twice as dangerous.
Before using either of the following products, thoroughly water all plants exposed to runoff, then rinsing a second time when finished.
We sell a product, OxiOut Algae & Mold Remover, that is for roofs, fencing, decks and more. This is the safer of two products we sell. It is mixed according to directions, applied to a wet surface with a tank sprayer and let to work for 5 to 10-minutes. Lightly scrub with a brush (on a broom handle) while keeping the surface wet, and then rinsed. Stubborn stains may need a third application and possibly with a stronger solution, which is given in the directions. One container will clean about 1,000 square feet of roof.
The other roof cleaner is JOMAX by ZINSSER Company. This is the mean machine of our two products. They make several products for removing mold and mildew off roofs and buildings. It is mixed 3/4 cup JOMAX with 2 1/4 cups bleach and 3 1/4 cups water to make a gallon of cleaner. Apply to a dry roof with a tank sprayer and let sit 5-minutes and rinse. Stubborn areas may require a second application. If possible, apply the second application to a dry roof for best results. One gallon of JOMAX can clean 2,000 square feet of roofing.
Once the roof is clean you can prevent further algae growth by installing zinc or copper strips (with several inches showing) near the peak of the roof. The leaching from the strips will prevent further algae growth. Another barrier is OxiShield, which is a liquid that can be sprayed on a dry roof to prevent new algae and mold growth.
Be safe, work off a ladder. And when in doubt, hire it out!