Moss on Your Roof

Mostly harmless, sometimes not — and never a job for a pressure washer.

Moss is the most visible "problem" a roof can have and usually the least urgent — but "usually" is doing some work in that sentence. What moss actually does: it holds a wet blanket against the tiles (slowing drying and feeding frost damage on tiles already becoming porous), it sheds in clumps that block gutters and valleys — the genuine harm in most cases — and in quantity it can lift tile edges and wick water under laps. What it almost never does is cause a leak by itself on a sound roof.

Context decides. Light moss on a healthy roof is cosmetic. Heavy growth on ageing concrete tiles — whose worn, porous surface is exactly why the moss thrives — is both a symptom of the covering's age and an accelerant of it. North-facing slopes and tree-shaded roofs carry more for the same age; that's aspect, not neglect.

One hard rule: never pressure-wash a roof. It strips the granular surface off the tiles (ageing them a decade in an afternoon), drives water under laps and into the felt, and the moss returns to the now-rougher surface faster than before. Roofs that have been jet-washed are visibly worse for it, permanently.

Features

What's included

Honest assessment — cosmetic versus genuinely harmful growth
Hand scraping and treatment, never pressure washing
Gutters and valleys cleared of shed moss
Tile condition under the moss reported while we're there
Regrowth honestly explained — aspect and shade decide

Repair options

Repair or replace

Where removal is worth doing, it's done gently: hand scraping and brushing from proper access, a biocide treatment to kill what remains and slow regrowth, and the gutters and valleys cleared of everything that came off. Copper ridge strips help modestly on some roofs and we'll say when they're worth it. And because moss removal puts eyes close to every tile, it doubles as an inspection — if the covering beneath is spalling or porous, you'll get that news straight, including when the honest advice is to leave the moss alone and put the money towards the re-roof the tiles are heading for. Related reading: tiled roofs.

Common questions

Frequently asked

A company offered to clean and coat my roof — good idea? +

Be careful. Roof "cleaning and coating" packages typically mean pressure washing (harmful, see above) followed by a paint-on coating that is cosmetic, unverifiable in benefit, and can trap moisture in the tiles. No coating restores a worn tile. If the roof looks tired enough to tempt you, spend the survey money instead — it may have years left as it is.

Why does my neighbour's identical roof have no moss? +

Aspect and shade, mostly — a south-facing slope dries fast and starves moss; north-facing and tree-shaded slopes stay damp and feed it. Tile age matters too: newer surfaces shed water before moss can anchor. Same street, different microclimates.

Will removing moss stop my gutters blocking? +

It removes the main supply for a few years, yes — moss is the number-one gutter filler on mossy roofs, ahead of leaves. Pair the removal with a gutter clear and the whole drainage path restarts clean. Regrowth is gradual, so expect a maintenance cycle rather than a permanent cure.