Moss is one of those things that's easy to ignore because it doesn't look urgent. It's not a missing tile, it's not a leak, it's just green fuzz — surely it can wait? The problem is what moss is actually doing while it sits there.
What moss actually does to a tile
Moss holds moisture against the surface of the tile, essentially keeping it damp for far longer than it would otherwise be after rain. On concrete tiles in particular, that constant dampness accelerates frost damage — water gets into tiny surface pores, freezes, expands, and gradually breaks the tile down. It's a slow process, which is exactly why it's easy to underestimate: the damage from this year's moss shows up as a cracked tile two or three winters later, by which point nobody connects the two.
Heavy moss growth also holds debris and seeds, which encourages more growth, and clumps of it regularly break off and wash into the guttering, contributing to blockages that cause their own separate problems.
Why we don't pressure-wash roofs
A lot of "roof cleaning" services rely on high-pressure jet washing, which does remove moss quickly — but it also strips the protective granule coating off concrete tiles and can force water underneath them, which shortens the roof's life rather than extending it. We remove moss by hand and with low-pressure methods specifically to avoid that trade-off, then clear the gutters as part of the same visit since that's usually where the dislodged moss ends up.
If your roof has visible green or dark patches, particularly on north-facing slopes or under overhanging trees, it's worth dealing with sooner rather than later. See our roof cleaning & moss removal service for how we approach it.