Eugene, OR
It's more than cosmetic. Moss holds moisture directly against the shingles, and as it grows it lifts shingle edges -- breaking the watertight seal that keeps water out from underneath.
A moss colony can establish within 6 to 18 months of spores landing, and left untreated it keeps growing and holding more moisture against the roof surface year over year.
Pressure washing a mossy roof can strip the protective granules off shingles and shorten the roof's life -- which is why moss should be removed by hand, not blasted off.
