Moss looks harmless, even charming on an old cottage. On a roof, it's a slow problem.
It holds water
Moss works like a sponge. It keeps the tile surface wet for days after rain, long after the rest of the roof has dried. Concrete and clay tiles are porous, so that water works its way in.
Frost does the damage
In winter the trapped water freezes and expands. Over a few seasons this cracks the surface of the tile, breaks off edges and loosens the granules on coated tiles.
It lifts tiles and blocks gutters
Thick moss can grow into the gaps between tiles and lift them, letting wind-driven rain underneath. As it dies back, it washes into the gutters, blocks them, and the overflow stains your walls and soaks your fascias.
What to do about it
Remove it with a method that suits your tiles, then treat the roof with a biocide so the spores don't regrow. Avoid anyone who walks on the roof or blasts it with high pressure: that does damage of its own.
