Moss, along with algae and lichen, commonly grows on roofs throughout the United States. Moss prefers environments that are wet, humid, and shaded, though moss will grow on just about any roofing material, including wood, metal, asphalt, clay, and concrete. (see ref 2)
Moss alone will not damage your roof. However, the water trapped by moss on your roof will accelerate wear and tear. This may lead to leakage, which may deteriorate wood structures in your ceiling and damage other items in your home. In addition, leakage is likely to result in mould and mildew growth, which presents significant health risks, including skin and eye irritation, respiratory problems, and asthma attacks. Furthermore, moss takes away from the beauty of a well maintained roof. (see ref 1)
Moss has a shallow root system and thus cannot simply be wiped or washed off your roof. To remove moss, scrub the moss with a stiff bristle scrub brush. Be sure to scrub down the roof, as scrubbing up the roof may break the bond between shingle layers. Once the root systems have been disconnected from the shingles, you can clean the loose pieces of moss from your roof using clean water and a power washer. (see ref 1)
Moss can be prevented from growing on your roof in the future by installing galvanised ridge caps or copper strips, available at most roofing supply companies. When installing copper strips, slide the strips under the shingles, but be sure to leave at least 4 inches of copper exposed to the sky. When it rains, copper molecules will run down the roof, creating an environment that is poisonous to moss, algae, and lichen.
Moss is extremely slippery, especially when wet. Attach the bristle scrub brush to a pole to limit the amount of movement necessary to clean the moss. In addition, wear shoes with excellent traction and sit on the roof, rather than stand, to lower your centre of gravity and reduce your chances of falling. Finally, keep in mind that roofs can become very hot in warm weather. Try to avoid cleaning your roof on particularly hot days where the likelihood of you passing out or becoming disoriented is highest. (see ref 1)