Moss and leaks
Can moss cause a roof leak?
The short answer
Yes, it can, though moss is more often a contributing cause than the hole itself. A thick mat keeps tiles damp for longer, and loose clumps fall into gutters and valleys and block them, so rain has nowhere to go but over the edge. Moss alone rarely leaks, but moss sitting over a worn tile or a tired valley lining often does.
What moss does to a roof, how to tell a cosmetic patch from a problem, and what to do about it.
How a green coat becomes a wet roof
Moss is a sponge. It takes up water in a shower and gives it back slowly, so the tile under a thick patch stays damp long after its neighbours have dried. That constant dampness does two things. It wears at the surface of the tile, and it keeps the overlap between tiles wetter than the roof was designed to be.
On a sound roof that is a slow process, and a coat of moss may do nothing worse than look untidy. The trouble starts where the roof already has a weakness. A split tile, one that has dropped out of line, or a crumbling bedding joint lets in more water than a dry roof would, and moss that holds the water at that spot makes a small fault worse. If you have already noticed a mark on the ceiling below, our answer on damp patches on a ceiling shows how to read it, and what the shape of the stain can tell you.
Blocked valleys and gutters
The more direct route to a leak is blockage. Moss grows thickest on shaded slopes and along the lower edge of a roof, and it loosens and falls when it dries out, when birds pull at it and when it is cleaned off. Pieces wash down into the gutter or settle in a valley, the channel formed where two roof planes join, and there they catch leaves and silt.
A blocked valley is a classic cause of a ceiling stain, because rain that cannot run away climbs over the edge of the lining and reaches the roof timbers. A blocked gutter can overflow behind itself and wet the fascia, which is the timber board the gutter is fixed to, and then the wall below it. Both are cheap to prevent and expensive to ignore. Our page on valley repair explains what is done once a lining has been wetted for too long, and why a damp valley is worth checking early.
Does the age of the tile matter?
It does. A newer concrete or clay tile in good condition copes with moss for longer, since its surface is intact. An old, porous tile soaks up more of what the moss holds, and where it has begun to flake the moss settles into the cracks. Slate is different again. It is dense, so moss on slate is usually a sign of shade and damp air rather than a failing surface.
What we look at is whether the covering under the moss is still sound. At the survey we lift a few tiles where it is safe to, photograph what is underneath and set out in writing whether the roof wants cleaning, repairing or, rarely, more than that. Because no two slopes behave alike, you get a separate finding for each one. That written verdict is the part you can show anyone else who wants a second opinion before they quote.
Clean it off, or repair first?
The order matters. Cleaning a roof that has a cracked tile under the moss can push water into the crack, so any repair comes before the wash, not after it. Hard scraping or aggressive jetting can strip the surface of a tile and shift sound tiles on their nails, so the method should suit the roof. Our roof cleaning and moss removal service describes the options in more detail.
Moss removal is not a repair, and a clean roof with a broken tile will still leak. After cleaning, the roof is looked over again for loose, cracked or slipped tiles, and anything found is mended while the roofer is up there. A treatment to slow the regrowth can follow, though how long it holds depends on the shade and the damp around the house. Expect moss to return on a north-facing slope that sits under trees, whatever has been done to it.
What you can learn from the garden
You can learn a fair amount without leaving the ground. Look for a thick, raised mat of moss on one slope, ridges of green along the bottom edge, plants growing in the gutter, or a stain below the eaves. Binoculars help. Never climb up to look, and never lean a ladder against a gutter that is full of moss, since the weight can pull it from the fascia. A phone camera with a zoom lens also shows more than you might expect.
If you see any of these signs, ask for a survey. The photographs show moss, any missing or cracked tile, and the state of the valleys and gutters in a single set of pictures, so the verdict covers all of it together. Our answer on how to find a roof leak explains the tracing from inside the loft. The roof repairs page shows what we do once the fault is found, and the price is written down as a single sum.
Related questions
Moss and leak questions
Is a small patch of moss on a north slope a problem?
Usually it is a watch-and-wait matter. Shaded slopes grow moss first, and a small patch on sound tiles does little harm. It becomes a concern when it thickens, spreads to the valleys and gutters, or sits over a cracked or slipped tile that is letting in water.
Will a moss treatment stop the roof leaking?
No. A treatment slows regrowth but cannot seal a crack or renew a worn bedding joint. A leak has a physical cause, and finding that cause comes first. The treatment is for upkeep once the roof has been checked and any repairs have been made.