Moss can be removed almost year around as long as the roof and moss are not frozen. The best time for us to treat a roof or any flat work is when we have a stretch of dry weather. We have found our soft wash mix gets deeper into the pours of the surfaces in order to get a better kill at the roots. As far as the removal of moss, it is easier to remove and bag when the moss is completely and fully dry. We can still treat any surface that is wet but the best results have come from when we have dry surfaces. The optimal time of year to treat a roof in the Seattle area is anywhere between May through October.