Published on August 6th, 2013 | by Henri


Moss growing on roof

Q.   In the last two years, moss has appeared on the asphalt shingles on the east side of my roof. There are tall maple trees on this side of the house. Is there any product I could use to remove this moss and also to prevent it from forming again? Is this moss harmful?

A.   Moss (and lichen) develops under moist conditions. Your asphalt shingles, shaded as they are by the maple trees, do not dry fast enough, or at all, and provide the ideal medium for growth of either or both of these two parasites.

There are several commercial products that claim to remove moss from roofs; one of them is Wet and Forget Moss Mold Mildew & Algae Stain Remover.

Moss can also be removed by picking it off but, considering that it is not only dangerous to walk on a roof, but it is also not good for the roof itself I do not recommend it. Moss does not seem to hurt asphalt shingles; my 50-year-old storage shed asphalt shingles (they don’t make shingles that last this long any longer) are almost entirely covered with moss and are still quite serviceable.

Good results have also been obtained by spraying the affected areas with a solution made of equal parts Clorox bleach and water. One gallon will treat 50 square feet of roof. Use a plastic garden sprayer. Work from a ladder on a windless day. But first cover any plantings under the eaves with plastic after soaking them. Spray only enough of the solution to wet the affected areas but avoid run-off. If you have metal gutters and downspouts, keep water running through them with a garden hose while treating the roof.

When you are done with the spraying, do not rinse the roof but thoroughly rinse the gutters and downspouts, the plastic covering the plantings, the plantings themselves and the surrounding areas to dilute any of the solution that may have dripped from the roof or the downspouts. It may take a few weeks before you see results as yellowing of the dead moss. This does not mean that the moss will come off the roof; this may occur over time as heavy rains wash the roof off.

For a longer lasting solution, install zinc or copper strips at the top of the roof just below the cap shingles. Rain releases some of the metal’s ions which are poisonous to any growth as can be seen below some flashing or other zinc-coated metal structures on house and barn roofs. You can buy zinc strips on line at, and rolls from hardware and box stores.

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