Roofing

Published on August 15th, 2013 | by Henri

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Why asphalt roofs turn black

Q.    I have noticed over the years that asphalt shingle roofs turn black — sometimes almost across the entire roof and at other times just in areas that appear streaked. This is more noticeable on lighter colored roofs, especially white ones. This usually takes place sometime after 10 years from the installation. Is this a moisture problem under the roof? How can this be prevented?

A.    No, it isn’t caused by interior moisture but by an algae that grows onto the mineral granules of the asphalt shingles because the shingles do not dry fast enough after getting wet or after snow has melted off the roof. Observe the fact that this algae grows on unheated buildings like garages and barns.

There are several ways to remove algae — the growth responsible for the black streaks. You can spray the shingles with Wet N Forget or spray them with a solution of equal parts fresh Clorox bleach (it contains some effective chemical ingredients not found in most other bleaches) 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. 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, and it 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 before spraying the roof, cover it with plastic and rinse it thoroughly when you are done and there is no longer any solution dripping from the roof. It will take several weeks for you to notice any results.

You can also have a contractor specializing in roof cleaning do the job for you. Installing special galvanized 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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