Pressure washing is never recommended for cleaning your roof. Shingle manufacturers specifically indicate that pressure should never be used to clean your shingles. Pressure washing does not kill algae and, if left untreated, the moss and algae will return quickly to your roof. You also run the risk of causing irreparable damage to the shingles themselves. Pressure washing will loosen granules which can cause shingles to prematurely fail. The water pressure can also lift shingles, breaking the seal between shingles and allowing a path for future leaks.
Valentine Roofing always recommends moss removal by gentle agitation using a roof brush to prevent granule loss and shingle lift, followed by a moss and algae treatment to ensure they stay gone.