Published on April 9th, 2015 | by Henri


Cleaning algae from roof

Q.  This past year, we noticed black markings coming down the roof. Would you have any idea what is causing that, and will a power-washer clean it up? I’ve noticed several homes with shingles on their roofs with the same black markings. Do you have a solution?

A   Do not use a power-washer to clean roof shingles; it could dislodge the mineral granules that protect the shingles from UV damage. The black streaks are caused by a type of algae that grows on the mineral granules of the asphalt shingles when they do not dry fast enough after rain or after snow has melted off the roof.

There are several ways to remove algae. You can buy special solutions that are sprayed on the roof; you can have the cleaning done by firms that specialize in cleaning roofs, or you can do it yourself. If that is your choice, spray the shingles with a solution of equal parts fresh Clorox bleach and water. Use a plastic garden sprayer and apply the solution on a windless day at the rate of 1 gallon per 50 square feet of roof. The purpose is not to overspray and have the solution run off the roof for a long time.

Wear rubber gloves and old clothing, as well as eye protection. Spray the solution from a ladder, as it is dangerous to walk on a roof, which may void any warranty left on the shingles.

If you have metal gutters, run water in them while applying the solution because it is corrosive. Soak all vegetation below the roof before spraying, cover it with plastic and rinse it thoroughly when you are done. Make sure there is no longer any solution dripping from the roof. It will take several weeks for you to notice any results.

Installing special zinc or copper strips at the ridge (or replacing an existing ridge vent with one specially made for this purpose) should prevent recurrence

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