Getting Rid of Moss

House Doctor | Gary Dymski

August 22, 2002

Q. The north-side roofs of my garage and house are covered by a thick, velvety moss that looks ugly and, I fear, might be harmful to the shingles. What can I do to get rid of the moss? - John R. Windorf, Miller Place

Q. While recently cleaning out the gutters, I noticed a green lichen growing on my asphalt shingles. The growth seemed to be damaging the surface of the shingles. This damage is occurring on a portion of the roof that is shaded by a large tree. Is there anything I can do, short of removing the tree, to solve this problem? - Tom Mowdy, Oakdale

A. You have similar problems. First, trimming overgrown tree branches is a good idea. Your shingles need sunlight to help the roofing system (shingles, sheathing and framing) dry properly. You don't have to chop down the tree, Tom, but you can provide more sunshine by having it trimmed. And, John, I bet your roof is shaded by some overgrown branches, too.

Once you've eliminated the overgrowth, you should clean your roofs. There are several commercial roof cleaning products on the market, many of which contain zinc sulfate. Follow the manufacturers' directions for these cleaners. Some cleaners can be sprayed on the roof from the ground, but in cases of extreme buildup, you'll likely have to climb the roof to apply the cleaner. If you do work on the roof, wear shoes with soles that will grip the shingles, and be careful. Before using a cleaner, carefully scrape away moss, lichen and algae. Use scrapers and flat-edged shovels, but work slowly so as not to damage your shingles.

House Doctor Recent columns

After cleaning the shingles, install metal flashing made of zinc or copper to keep the growth from returning. Rainwater and melting snow washes over the zinc or copper flashing and leaches small amounts of the metal into the shingles. This can inhibit the growth of moss, lichen and algae.

Two commercial products - Z-Stop (800-845-5863) and Shingle Shield (800-323-7164) - might help prevent your shingles from growing green in the future. Both products are zinc strips that fit under the shingles. (Shingle Shield also makes a cleaner.) If you'd rather skip commercial products, you can replace your metal flashing with new zinc galvanized flashing, or you can string copper wires at strategic spots on the roof. The copper wire will provide the same protection as the zinc strips.

One other bit of advice: If you have gutters or downspouts made of copper, zinc can be corrosive. Try a copper-based roof cleaner instead.