Our home is 10 years old and the asphalt roof shingles are starting to stain badly on the north exposure. Is there something that we can use to clean off the staining and treat the shingles to prevent further staining?-Tim

Hi Tim,

Your stains are probably caused by the airborne spores of a blue-green algae known as Gloeocapsa Magma, which is often mistaken for mold or mildew. While doing little harm to your shingles, the black streaks caused by the algae are unsightly.

Algae can be killed and the stains removed using chemical cleaners, though they usually returns over time. Use a cleaner specially made for killing algae, or by spraying a mixture of one part bleach to four parts water with TSP (trisodium phosphate) or other detergent added in a garden pump-up sprayer. Allow the bleach to remain on for 15 to 20 minutes, then rinse it off with a hose.

Note that bleach may kill or damage plants, so wet down or cover any shrubs or flowers that will come in contact before starting. Repeated use of harsh chemicals or pressure washing can damage or shorten the life of your shingles.

Once the roof is clean, consider installing strips of zinc or copper—which are toxic to algae—along your roof below the ridge. Molecules of the metal will then wash down the roof each time it rains to keep new algae from forming. A better solution is to specify algae resistant shingles, which have copper granules embedded in them, the next time you have your roof replaced.

For more information, check out our article on How to Remove and Prevent Algae Stains on Asphalt Shingle Roofs.

Good luck with your project,

Tags: , algae, mildew, roof, stain