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Home Maintenance

Keeping asphalt shingles algae-free
Scripps Howard News Service


July 11, 2005

Q: On many roofs with asphalt shingles I notice black streaks. New light-colored orange-red asphalt shingles were installed on our roof about six years ago, and now I notice the dark streaks forming. I went up to observe if there was any residue at the origin of the streaks, and there was none. The dark-colored material seems to come out of the shingles.

Do you know what causes these streaks, how they can be prevented and how to get rid of them once they form? - T.K., Elkhart, Ind.

A: The streaks are caused by airborne algae that forms on the surface of the shingles. Although the algae discolors the roof, it does not appear to cause any damage to the shingle itself.

When you see roofs with algae streaks, you will notice that the algae does not form below any of the metal fixtures that penetrate the roof, such as a furnace flue pipe or a roof vent. That's because the metal oxidizes when it rains, and these "metal oxides" wash down the roof's surface, preventing the algae from getting a foothold in the areas just below the metal.

The algae that has already formed can be cleaned using a commercial roof and deck cleaner or with a mild bleach and water solution sprayed on with a garden sprayer or a power washer. However, a power washer can also dislodge the color granules and damage the roof, plus the cleaning solution in a garden sprayer or washer makes the roof very slick and dangerous to walk on. Cleaning the roof's surface should be left to professionals.

After the shingles have been cleaned, they can be kept free of algae by installing a metal strip at the peak of the roof. Larger roof areas may require an additional strip of metal halfway down the roof.

Copper, aluminum and galvanized steel will all work to some extent, but zinc seems to be the preferred metal.

Metal strips will also protect wood-shake shingles. Zinc strips for roofs can be found at roofing dealers and home stores or can be ordered online.

You can order zinc strips online from Chicago Metallic by going to Click on retail, then exterior products, then roof protector strips. Or go to Shingle Shield's Web site at or to ZincShield at

When it's time to replace the roof's shingles, check into the new algae-resistant shingles available from most manufacturers. Small pellets of zinc, copper or both are mixed in with the color granules to protect against algae throughout the life of the shingle.

(Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home improvement questions at C. Dwight Barnett, Evansville Courier & Press, P.O. Box 286, Evansville, Ind. 47702 or e-mail him at d.barnett(at)



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