The department of agriculture for Oregon on demossing roofs ie roof cleaning in the northwest.

If not controlled, moss will shorten the lifespan of a roof. Control measures include removing over- hanging branches that shade the roof, removing as much of the moss as possible, and using liquid spray or granular application of a pesticide. In many cases, where the pitch of the roof is not too steep, a sprinkling can rather than a tank sprayer can be used for applying the solution. This appli- cation method minimizes drift problems, but the applicator must not put too much solution on so that it runs from eaves or splashes on planted areas below roofs. Where this possibility exists, cover plants with plastic sheets to protect them. The application of these chemicals is most effective when moss is actively growing, and when it isn’t expected to rain for several day