Algae Algae on roof

Published on September 5th, 2013 | by Henri


Algae takes time to form, and to kill

Q.    I enjoy reading your informative column. The problem I’m having is with fungus on the shingles on the northern side of my roof. The CertainTeed shingles were put on in 1994 and until about 5 years ago, they were OK color-wise with no black streaks at all.

Three years ago I took off 2 shingles and sent them to CertainTeed for testing because I believed there was a defect in the shingles. They naturally wrote back stating that there was no manufacturing defect and the stains were caused by fungus, and the type shingles I had purchased were not fungus resistant.

Now here is the question I have: Why did it take so long for the fungus to show up as black streak marks? A few other factors: there are no trees near the house to block the sun, and the roof does get a fair amount of morning sun as it doesn’t face directly north but northeast. And lastly, I put one of those zinc strips along the ridge of the roof but that has done little if anything to ease the problem.

A.    The black streaks are an algae growth with a mile-long name. These algae develop under moist conditions and it is possible that the morning sun is not strong enough, or stays long enough on the shingles, to dry them completely from morning dew or rain.

It may have taken several years for these black streaks to develop because moisture (including melting snow) may have penetrated the asphalt binder, and it does not dry as quickly as it did before. This is not a manufacturing defect — simply a sign of aging. How long ago did you install the zinc strip? Once the algae have taken hold, it takes a long time for the ions in zinc or copper to kill them.

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