The short answer
Moss can damage a roof, though rarely fast. A light film on sound slate or tile is often cosmetic for years. The harm comes where moss packs valleys, gutters and overlaps, holds water against porous or ageing tiles, and keeps the surface wet through frosts. Thick moss that lifts tiles or sends water behind the covering needs removing, and the roof needs checking underneath.
When moss is only cosmetic
A thin, patchy growth of moss on a sound roof is mostly a matter of appearance. Good natural slate is dense and takes up very little water, and a modern concrete or clay tile in good condition sheds rain whether or not it wears a green coat. If the roof is watertight, the lead is sound and the gutters run clear, a little moss on the north slope is not an emergency.
That does not make it harmless for ever. Moss is a living mat, and it holds moisture long after rain has stopped. It is slow, and what it does depends on what it is sitting on. Moss on a healthy roof is something to watch rather than panic over, and something to deal with before it thickens. When the time comes, roof cleaning and moss removal covers how we go about it.
The test we use on site is simple. We look from the pavement first, then from a ladder or the scaffold, and ask whether the moss is a surface film or a thick cushion. A film can be left a while. A cushion that stands proud of the tile edge is already doing work you cannot see.
How moss causes real damage
The damage comes from water being held where a roof is designed to shed it. Each tile or slate relies on a lap, where each row of tiles sits over the one below, to carry rain down and off. Moss grows into those laps and the gaps between tiles. Once it is thick enough, water has nowhere to go but sideways, and it can be pushed up and under the covering instead of running off.
Moss also keeps the surface wet. A tile that stays damp takes up water, and in a cold spell that water freezes and expands in the pores of the material. Over several winters the face of an ageing tile can flake or spall, which means the surface layer breaks away. Concrete tiles and softer clay are the ones most at risk. Dense natural slate resists this far better.
The weight is a smaller matter than people expect, but a saturated mat is heavier than a dry one. On an older roof, where the battens, the thin timbers each tile row hooks onto, have tired, extra load and constant damp do not help. Moss that has been left for a long time can also lift the edges of tiles and slates slightly, and a lifted edge catches the wind.
The places where moss does most harm
Not every part of a roof is equally exposed to harm. The worst places are where water is meant to be channelled.
- Valleys. Valleys, the lead-lined troughs between two slopes, are the first place to look. Moss and leaves gather there, so rain backs up and creeps over the lining.
- Gutters and downpipes. Moss that washes off the slope, or is scraped off in a careless clean, collects in the gutter and can block the outlet. An overflowing gutter soaks the eaves timbers. Our guide to moss blocking gutters covers that in detail.
- Behind chimneys and against walls. Flashings, the lead tucked into the brickwork around a stack, stay wet when debris piles up behind a chimney.
- The ridge and hips. Moss under the edges of ridge tiles can hide cracked bedding, so the growth hides a fault.
On a roof with parapet or box gutters, the lead-lined channels behind a stone parapet, a build-up of moss and silt is a common reason for a blocked outlet and a damp ceiling below.
Why the covering and the age of the roof matter
Moss does not behave the same way on every material. Natural slate, being dense, usually comes through years of moss with little harm to the slate itself. The fixings and the lead around it are the weak points. Clay tiles vary. Old handmade clay can be porous, and a mat of moss keeps it wet. Concrete tiles are the most likely to lose their surface to a combination of moss and frost, especially when the roof is elderly.
Age is just as important. On a young roof, moss is an early sign that the surface is holding damp, and it can be cleaned and treated without any risk. On an old roof, a thick moss cover can be hiding a lot: slipped tiles, cracked tiles, failing mortar, lead lifting at a stack. Cleaning such a roof is still worth doing, but the survey before it matters, and so does the question of how much working life the roof has left. We cover that choice on our page about whether to clean or replace an old roof.
One further point is method. Blasting moss off with a pressure washer can strip the surface and force water beneath the laps, and one cost guide for roof cleaning warns that high pressure can mark fragile tiles. Gentler methods suit an ageing covering better.
What to do about moss on your roof
If moss is showing, begin with binoculars from the garden, then go into the loft on a bright day. Daylight showing through, damp stains on the underside of the roof or wet insulation tell you more than the moss itself does.
On a roof with sound tiles and thin growth, removal and a biocide treatment, the wash that kills spores and slows regrowth, is usually all that is needed. How long that lasts is covered on our page about how often a roof should be cleaned. If the moss is thick, we clear it by hand where we can, check the laps, lead and gutters as we go, and report what we find with photographs.
The survey is free, we photograph the roof for you, and the price we quote afterwards is fixed in writing. If cleaning would only hide a roof that needs repair, we tell you and quote for the repair instead. Typical UK prices for cleaning are on our roof cleaning cost page. And if you are preparing a house for sale, see cleaning a roof before selling.
