ANSWER: A plague that infects homes in nearly 80 percent of the United States, including Chesapeake Bay homes - Gloeocapsa Magma.

Gloeocapsa Magma is a species of algae that causes black streaking and discoloration on asphalt/fiberglass shingles. The black staining is caused by the growth of algae and fungus spoors that land on houses. The streaks and discoloration you are now seeing is an accumulation of their dead, dark colored cells.

Trees do not have to be present for this air-born menace to set up camp on your roof. This alga produces its own energy source and feeds on inorganic materials, such as the materials in shingles. All they need to grow is the nutrients from both the dirt on your roof and the limestone granules on the shingles. Heat and moisture help them turn your whole roof black.

An alga is an organic material and feeds only on inorganic material. Fungus and mildew need organic material to survive. So if you can reduce the algae on your roof, you will also reduce the growth of fungus and mildew. Keeping your roof free of twigs, leaves, and other
debris will also help reduce the growth of fungus and mildew. Cleaning and controlling algae and fungus growth can add to the life of your roof.

Roof shingles come in a wide variety of quality levels. The higher quality shingles have been treated to prevent algae and fungus growth. This treatment consists of adding varying amounts of copper and zinc granules and petroleum distillates to the shingles.

If you clean the roof yourself, be careful of products that are just bleach or chlorine. Using these can leave your roof streaked. It also can dry out the shingles and cause premature cracking. Strong cleaning solutions will also kill grass and shrubbery. Look for deck cleaners that contain bleach such as oxygen bleach, a non-toxic powder that mixes with water.

Wait for a cool, overcast day to apply the oxygen bleach solution. Do not let the solution dry. After it has soaked for 30 minutes, brush the solution with a stiff broom or brush. Always aim the rinsing hose down the roof, not up under the shingles.

Resist the advice of your neighbor to rent or buy a pressure washer. This common method of washing and scrubbing may get rid of the algae temporarily, but it will also damage the shingles.

If you “don’t do” roof cleaning, (as our grandchildren say), call a qualified roof cleaning company. But you still need to ask if the products they use will cause streaking or grass and shrub damage. Roof cleaning companies can also provide preventative maintenance services that can prevent future algae and fungus growth.