We live in a conservation area with Victorian houses over 100 years old. Several local houses have been reroofed in recent years, using synthetic slates which are now growing moss around the joins and overlaps. My neighbour's roofer says this is a common problem, which you don't get on real slates. My neighbour wants to clean them but roofer said they scratch easily which makes the problem worse. Anyone got any suggestions on how to clean them?