Roofs surrounded by trees and with little direct sun grow moss at a speed that surprises owners. Shade keeps the tile damp for more hours a day, and it is that permanent moisture that feeds the moss and the lichen.
The problem is not cosmetic. Moss holds water against the tile, and held water is what finds its way through the laps. On older clay tiles, lichen roots can lift the piece slightly and create a gap where there was none.
We cleaned the whole roof at low pressure, checked the tiles piece by piece in the areas where the moss had the strongest hold, and applied a biocide treatment at the end. The treatment is not permanent, but it puts a good deal of time between now and the next clean.