The best way to control moss is by keeping the roof clean of debris, allowing more sunlight to dry the roofing material out quickly. Keeping trees trimmed back and the roof clean of debris can offer some relief from the moss. However some homes are situated perfectly for moss growth. If keeping the roof clean and the trees trimmed back isn’t enough, the use of organic herbicidal treatments should be a consideration.
In the past pressure washing was normal, everyone was doing it and it became an accepted practice for years. The problem with pressure washing is it’s extremely hard on the composition roofing material. Roofing manufacturers quickly realized the harm that pressure washing as done to the composition roofing material and will having your roof washed will void the warranty. Pressure washing not only rips live moss from the roofing material (which has a root-like structure) but water easily can damage the interior of the home through skylights or small exposed areas in the roof. The typical roof cleaning pressure washing machine uses 8-10 gallons a minute!
Probably the second most common approach to roof moss removal is the use of a stiff wire brush. While there is some moss that can be removed in this manner, the wire brush also pushes moss fibers into the edge of the roofing material. The following season when the moss is growing at a rapid rate again the moss is back, except now the roof is worn. Composition roof care simply does not entail using a wire brush on the roofing material year after year. It needs to be controlled annually.