Shedding Light on Roof Fungus

Mold grows on shingles in shady areas; a chlorine solution can remove it

House Doctor | Gary Dymski

November 20, 2003

Q. My daughter has a house in eastern Suffolk County, and there's a problem with her roof. It seems there's a black film on her asphalt shingles. What is this and how can I get rid of it? -Al Robinson, Commack

A. The black film most likely is a fungus growing in the shingles. If the roof is older, I'd bet it is in the shade most of the time, and moisture from rain and snow builds up. If the shingles are relatively new, the black stains are growing because the shingles contain finely ground limestone. The fungus loves limestone, so it creeps into rooftop shingles in hot, humid areas. While a roof in the shade can develop problems, including deterioration of shingles and rotting wood sheathing, those in the sun usually are safe.

This fungus is ugly, but you can take a few precautionary steps. First, trim overhanging tree branches to let in more sunlight. Once you've removed the fungus - we'll get to that in a moment - the extra sunlight should keep it from growing back. If the shingles are cracked and curled, they might need replacing.

To remove those black stains, use an algicide or a commercial wash designed for roofing and wood decks. Some of these washes contain chlorine bleach, while others contain oxygen bleach. Both are effective. Using too much chlorine bleach, however, can cause shingles to dry out prematurely and also may harm plants and shrubs.

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If you don't want to purchase a special cleaner, try using a pump sprayer to spread a 50-50 solution of chlorine bleach and water. Work slowly and wear rubber-soled shoes. Another solution contains 3 quarts water, 1 quart chlorine bleach, 1 cup of trisodium phosphate substitute and a couple of tablespoons of liquid dish soap. (TSP substitute is available at paint stores.)

If you have any fear about working on the roof, hire a roofing company or cleaning service.

Before you spray the roof, wet all plants and shrubs on the ground with water or cover them with plastic sheeting. This will protect them from the bleach solution.

After spraying the solution on the stained area, wait about 30 minutes, then rinse with cold water; a hose works fine. Do not use a power washer on the roof. This will remove granules from the shingles, and these granules contain UV inhibitors that keep the roof cool and help prolong the elasticity of the shingles.

Some newer shingles actually have small pieces of copper in the granules to prevent this fungus from growing. When washed over with rain water, the copper releases an ion that kills the fungus. Zinc also releases a residue that kills the fungus.

You can't add copper or zinc granules to your shingles, but you can add metal strips near the peak of the roof that will release the same oxide wash. You can get strips of copper and zinc at home centers or building supply stores. Trim them with tin snips, then slip them under the shingles near the peak or above the stain. Make sure some of the metal is slightly exposed, and use a dab of roof adhesive to hold them in place.