Yes, moss is the single biggest contributor to early roof failure in the Willamette Valley.  Moss is a menace to roofs! It grows quickly, preventing water from flowing downhill to the gutters, it lifts the shingles allowing water to penetrate the roof's exterior, it collects in and plugs up gutters and downspouts, and it decomposes the felt that the shingles are made of. We recommend annual inspection for moss and treatment with a commercial herbicide as soon as it is noticed.