The freeze-thaw mechanism
Moss is up to 95% water by weight. A square metre of saturated moss holds around 4 litres.
In Scotland the air drops below freezing on roughly 40–60 nights a year. Each freeze expands that water by 9%, prising apart the surface of the tile.
After 5–10 winters the protective sand coating on a concrete tile is gone — exposed aggregate is visible and the tile becomes porous. Once water gets inside the body of the tile, frost damage accelerates fast.
What 'too late' looks like
Surface flaking — small chips of sand coating in the gutter every autumn.
Visible aggregate (the grey concrete underneath the coloured coating).
Tiles that feel rough rather than smooth when handled.
At that point moss removal alone is no longer enough — you're looking at tile-by-tile replacement or a full re-roof in 5–10 years.
Clean before winter or after?
Before winter (Sep–Oct) is best for prevention. Remove the moss, apply biocide, and the tile goes into winter with no water-holding growth on it.
After winter (Mar–Apr) makes sense if you've already had a hard winter and want to assess damage. The frost lifts loose moss visibly — you'll see the worst affected areas straight away.
Don't clean in deep winter. Soft-wash solutions don't work below 5°C, and any water left on the roof can freeze the same night.
Biocide is the real long-term answer
Removing the moss without biocide treatment means it's back within 18 months — same problem, same frost damage cycle.
A single biocide application after cleaning keeps the roof visibly clear for 4–6 years on most Scottish homes. That's 5+ winters without water-holding moss on the tiles.
An annual top-up biocide (about 25% of the cost of a clean) extends that life further and costs far less than a re-roof.