Moss Control on Roofs

Effects

• If allowed to remain, moss can, after a time, cause a roof to decay. In itself, it will not harm a roof, but it collects and holds dirt and moisture, which in turn leads to rot and decay. In addition, the tentacle-like rhizoids which allow moss to cling to a roof may wriggle into tiny cracks in the roofing material; moss may also wrap itself around the edge of a shingle. Either of these scenarios can eventually cause a roof to leak.

Causes

• Moss thrives in shady, moist conditions. Water or the wind can introduce it by sweeping spores or a moss fragment into any area where moss can live. Once established, a patch of moss will also spread by asexual reproduction, with the established moss sending out new shoots, thus spreading the patch. When tree branches reach over a roof, or when the house is situated next to a tree that casts a shadow over the roof, it can grow moss.

Chemical Cures

• You can use one or both of two approaches to remove moss from a roof: chemical and physical. All but two of the chemical methods involve zinc in one form or another. You can purchase zinc sulfate, zinc chloride and zinc-copper sulfate mix at a hardware store in spray bottles or as a liquid that you pour into a backpack sprayer. Any of these produce results that last from one to five years, according to a research team at Oregon State University. Alternatively, you can nail zinc strips to the ridge caps at the peak of the house. Every time it rains, zinc leaches from the strips, coating the roof.

  Two other chemical solutions include a commercially available chlorine bleach especially formulated for moss removal, which the Oregon research team says will last about a year, or natural potassium salts of fatty acids, also available as a commercial product. However, natural potassium salts of fatty acids does not appear to work as well as the other chemical methods.

Physical Cures

• A high-powered pressure washer will clean off moss, but it may also damage the roof if you don't use it with great care. Alternatively, you can push the moss off with a flat-edged shovel, stiff brush or push boom. None of these methods prevents new moss from growing, so you must regularly remove the moss to prevent buildup.

Prevention

• To prevent moss from growing on your roof, cut back any overhanging branches and keep organic debris cleared away. If a nearby tree is so close that it throws a shadow over the roof, cut it down. If you prefer not to cut down a tree, you can prevent moss growth by washing your roof with a chlorine bleach solution once a year.