There are over 12,000 different moss species in the world. The moss currently residing on your roof is probably Dicranoweisia cirrata and Bryum capillare. The Pacific Northwest with its consistent rain and shady trees provides an ideal setting for moss to thrive. Moss needs shade to grow, so by allowing sunlight to reach the roof it will help to keep the moss at bay. However, we don’t recommend cutting down all your shade trees just to halt the moss invasion, but you may be able to trim trees so sunlight can filter to the surface of the roof.
The problem with moss is it clings to asphalt roofs trapping soil and debris, which in turn retains water and keeps the roof moist. Although many homeowners like the cottage look of green moss dotting their roof and overhangs, it is important to keep your roof clean. Moss will lift shingles, allowing water to flow under the roofing shingles. Water on a roof needs to flow downhill unimpeded and moss prevents this from happening.
To begin cleaning your roof, think first about hiring a professional. There is a reason why insurance costs are high for roofing employees, because it is a dangerous profession and should be approached with safety in mind. But no matter who does the cleaning, using a pressure washer on your roof will generally void the manufactures warranty if not done perfectly. Unless your perfect, it better to follow a three step process:
1) Clean or gently blow off loose debris such as leaves and branches from the roof.
2) With light rain in the forecast, apply zinc sulfate over the entire roof, concentrating the majority of it along the ridges.
3) After a few weeks, using a scrub brush, gently loosen the moss from the roof. Once completed, blow the debris off the roof and clean your gutters.
A couple of quick notes. Don’t use products such as Tide with Bleach, they will kill the moss but putting degreasers on your oil based roof is a recipe for disaster. Also, use chemicals carefully and sparingly. That may mean treating the moss twice with a small amount of moss killer, versus applying a large amount and having much of it wash away in a heavy downpour.