Shade from tall firs, months of drizzle, and mild winters make the South Sound one of the best places in the country to grow moss, and one of the worst places to ignore it on a roof.
Moss works like a sponge. It holds water against your shingles long after the rain stops, lifts the edges of tabs as it grows underneath them, and drops debris into the gutters. Over a few years that means curled shingles, clogged downspouts, and leaks that start at the roof edge.
The fastest way to ruin a composition roof is to pressure wash it. High pressure strips the protective granules off the shingles along with the moss, and those granules are what shield the asphalt from UV and weather. Once they're gone, the shingle ages years in a single afternoon.
A safe cleaning uses hand tools and low pressure to lift moss and needles, pays extra attention to valleys and the north-facing slopes where growth is heaviest, and finishes with clean gutters and downspouts. Trimming back branches that overhang the roof helps it dry faster and slows regrowth.
Most South Sound homes benefit from a roof cleaning every one to two years, depending on how much shade the roof gets. If you can see green from the street, it's time.