Moss on the roof: what are the risks?
By the Toiture Wash team · Updated on 20 October 2026 · 2 min read
Short answer. A little moss on a healthy roof is not an emergency. The risk comes when it settles in: it holds water on the roof covering, creeps into the overlaps, blocks the gutters, and water that freezes in winter weakens porous materials. When the moss forms cushions or the gutters overflow, it is time to remove it.
Moss, lichens, algae: three different things
- Moss forms green cushions, mostly at the tile overlaps, at the ridge and on north-facing slopes. It holds water like a sponge.
- Lichens are flat patches, grey, yellow, white or black, clinging tightly to the surface. They grow slowly but cling on tightly: they are the ones that take the most work to clean off.
- Algae leave dark streaks or a greenish film, mostly on smooth surfaces and in damp areas.
All three like the same things: damp, shade and time.
What moss really does
- It keeps the roof damp. A roof that dries badly turns green again faster and ages faster.
- It hinders the flow of water. Lodged in the overlaps, it can make water rise between the tiles in driving rain.
- It blocks the gutters. Pieces that come off end up in the gutters and downpipes, which overflow.
- It makes the effect of frost worse. In a porous material full of water, frost can make the surface break up over the years.
What it does not do
Moss does not pierce a healthy roof overnight, and it does not make tiles fall. When a leak appears, it most often comes from a broken tile, a damaged ridge or a blocked box gutter: a moss removal will not repair it. Have the roof covering checked in that case.
When should you act?
- when cushions of moss cover the joints between the tiles;
- when lichens spread over a good part of a slope;
- when the gutters overflow or fill with moss;
- before fitting solar panels on a roof that has turned green;
- before selling or letting.
The right time of year: When should you remove moss from your roof?
What not to do
- Scrape an old tile with a pick or a knife: you damage it.
- Pour on bleach: it is corrosive to materials and harmful to the garden.
- Climb onto a wet roof alone: moss is slippery, and the fall is the real danger.
The methods, their advantages and their limits: Which moss removal method for your roof?
Frequently asked questions
Is moss on a roof dangerous for the house?
Not straight away. Left for years, it keeps things damp, blocks the gutters and can weaken porous tiles through frost.
Are lichens worse than moss?
They are harder to remove and mark the surface of the tile; moss, for its part, holds more water.
My roof has black patches: is it moss?
Often these are lichens or algae. A photo is enough for us to tell you: send it to us with your request.