Algae Algae on roof

Published on March 26th, 2018 | by Henri


Moss on roof

Q.  I am almost certain you have covered this subject before, but we didn’t need information at the time. We live in New England and we had new shingles installed 14 years ago. Like so many homes around here, ours has developed ugly black streaks and now it appears moss is starting to form.

My husband recently saw an ad for power washing the roof. Is this a good idea?  Will it shorten the life of the roof? What questions should we be asking? Is there a way of preventing this from happening in the future? Why is this happening?

The ad mentioned that yearly maintenance was available. Love your column. Thanks for your help.

A.  Algae bloom on roofs because of moisture and lack of sun. Asphalt and fiberglass shingles are especially vulnerable.

Power washing a shingle roof should be avoided, as it may damage the shingles and void any warranty remaining.

There are firms that specialize on treating algae, moss and lichen. Some of these firms are doing it from the ground with high pressure hoses while others are getting on the roof to do so, and that’s where the risk of damage can be found.

Choose experienced professionals in order to avoid damage to the shingles.  Ask how long they have been in business, ask for recent and 2-year old references and check them out. Questions to ask of any reference you contact are how reliable the firm was in scheduling and showing up; did they do a professional job and left the premises clean; and what is they guaranty, and if there was need to invoke it, were they on top of it without hassle and without a variety of excuses, blaming someone or something else for the failure of the treatment.

There is no need for yearly maintenance; it should take several years for the algae to return, at which time the treatment can be repeated.

To prevent recurrence, you can install or have installed copper or zinc strips just below the ridge cap.

For those who want to do it themselves, Wet & Forget ( is an environmentally-friendly solution that is very effective.
Use a garden hose with a pressure nozzle, and follow directions, being fully aware of the risks of working from a ladder.

Algae, moss and mildew are not considered to shorten the life of shingles. I have seen 40-year-old asphalt shingle roofs covered with a thick coat of moss with no signs of deterioration.

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