Algae Algae on roof

Published on July 26th, 2012 | by Henri


Algae stains on roof

Q. I have noticed, over a period of time, some ugly black stains on the roof of my 55-year-old Cape Cod here in N.J. The roof faces north. I thought there was a mildew problem. One of your columns indicated the availability of a solvent to rid one of mildew from around, on and outside the house – including the roof. Well, it did not work for me.

I must conclude that my diagnosis of mildew was in error, perhaps it was the backing tar on my asphalt shingles? And only on the north side? Do you have any suggestions on removing these hideous streaks? By the way, I have plenty of company with many northern roofs in my neighborhood afflicted likewise. Thanks.

A. There is no way that what you call the backing tar of your asphalt shingles can cause the black stains you have noticed on your house and on your neighbors’ roofs as well. The asphalt that binds the base material that makes the shingles is stable and does not “leak” through the mineral granules. The stains have to be caused by an algae that is quite common on northern exposures that get little or no sun.

If I read between the lines of your letter correctly, my guess is that you did not use the right proportion of the solution needed to remove algae from roof shingles (by the way, that solution is not a solvent; that would damage the shingles and void any warranty on them). The right mix for an effective treatment is three parts fresh Clorox bleach to one part water. That is the opposite ratio of what is usually recommended for other mildew treatments.

The solution should be applied by sprayer at the rate of one gallon per 50 square feet of roof on a windless day. Do not overspray to the point of run-off; just wet the affected areas. Try to do it from a securely set ladder as walking on a roof not only is always risky but can also damage it and void the shingles’ warranty.

Soak and protect plantings below with plastic sheets and run water continuously into metal gutters (vinyl gutters are not affected). Once any dripping from the roof has stopped, rinse the plastic, remove it and wet the plantings again to dilute any solution that may have fallen on them.

This is a summer operation; it takes several weeks of hot weather to see results.

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