My Asphalt Shingle Roof Is Turning Black

Cause

• What looks like black dirt on rooftops with asphalt shingles is actually gloeocapsa magma, blue-green algae also known as cyanobacteria. It is spread by airborne spores and thrives in humid areas, where it commonly causes roof discoloration. Damage from gloeocapsa magma algae is minimal but it can affect the resale value of a house. Homeowners have several options for cleaning algae off asphalt shingles.

Cleaning

• Algae can be removed from asphalt shingles with professionally manufactured products created to get rid of black stains and streaks. Follow directions on the container. Homeowners can also mix their own algae-removal solution of 1 quart bleach, 1 gallon of water and 1/4 cup trisodium phosphate. In addition to the homemade cleaner, homeowners will need a ladder, rubber gloves, safety glasses, water hose with a spray nozzle and a pump sprayer. Pour the cleaning mixture into the pump sprayer, shake well and begin spraying black spots and streaks. Leave the solution on the roof for 15 minutes and rinse with a water hose. Homeowners can also hire a professional to remove the algae.

Prevention

• While cleaning will get rid of black stains and streaks, algae will come back. Preventive measures to keep algae away from asphalt shingles include keeping the roof clean and installing copper or zinc strips, which are toxic to algae. Use roofing nails or screws to attach metal strips on all sides of the roof, just below the ridge. When the time comes to replace the roof, consider using algae-resistant shingles with copper granules added during production. These preventive methods also keep moss off rooftops.

Considerations

• Roof maintenance can be dangerous, especially when the roof is wet. Wear non-slip shoes to avoid falling and eye and skin protection when working with chemicals. Use a safety rope and if the roof is steep, consider letting a professional do the job. Bleach or chemicals may be harmful to grass, plants and shrubs, so wet all of them before cleaning the roof. Covering bushes in plastic is also a good idea. Cleaning algae from a rooftop should not be done too often because chemicals can damage shingles and shorten their lifespan. Ask someone to help or let someone know when you will be working on the roof.