The short answer
Sometimes. A few small patches of moss on an otherwise healthy roof are mostly cosmetic. A thick, spreading mat is a problem, because it holds water against the tiles, blocks gutters and valleys, and can lift the edges of tiles as it grows. It rarely causes a leak on its own, but it speeds up wear on the tile surface and can hide cracks. The sooner thick moss is removed, the less damage it does.
Cosmetic or damaging: how to tell
The test is thickness and spread. Thin green patches on the shaded side of the roof, with tiles still dry between them, are a nuisance rather than a risk. A deep, spongy cushion that stays wet days after the rain, that has crept up from the eaves or that fills the gaps between tiles is doing harm. Moss that you can see growing in the valleys and at the gutter line deserves attention soonest, since that is where water has to flow.
This answer is about tiles; slate has its own risks, covered in our answer on whether moss damages slate roofs.
Weight is a smaller factor than people fear. A saturated mat does add load, but a normal tiled roof copes with it. The damage comes from the moisture, not the weight. You can read about how moss affects leaks on our page about whether moss can cause a roof leak.
Moss also tends to grow where the roof is already under stress. Look along the bottom courses, where water collects, and around the ridge, where the pointing or dry-fix parts can crack. If moss is thick in one small area and absent elsewhere, that can point to a spot where water is running or collecting for a reason, such as a slipped tile or a damaged gutter, rather than general damp.
What moss does to different tiles
Concrete tiles are the most affected. Their surface is slightly porous, and moss holds water against it, so in winter the damp can freeze, expand and flake off the surface layer. Over several seasons that wears the tile thinner. Clay tiles are more resistant, but moss can still sit in the joints and lift tile edges. Natural slate is dense and takes moss less readily, though a heavy cover still holds water at the overlaps.
Moss can also work between tiles and into the gaps at the bottom edge, where it blocks the airflow that helps the roof dry. Over time that keeps the battens (the timber strips the tiles hang from) damp, which is not what you want. Our page on cleaning slate, clay and concrete roofs goes through how each material is treated.
There is a practical consequence for any repair. Tiles under a thick moss cover are hard to inspect, because you cannot see hairline cracks or the state of the nibs that hold each tile to the batten. That is one reason we like to clear the growth first when a roof is being surveyed. Otherwise the inspection is partly guesswork.
Moss, lichen and algae
Not every green or dark patch is moss. Algae shows as dark streaks and a green film, mostly on the lower slope. Lichen forms flat, crusty patches, often grey, yellow or orange, that grip the tile much harder and are slow growing. Moss is the soft, raised, cushion-like growth. The three often sit together, and each wants a slightly different treatment. We cover them on the lichen, algae and black streaks page.
Identifying which one you have matters because a method that clears moss may not shift lichen. Photos from the survey show what is there, which means the right treatment is chosen first time.
Black streaks are mostly algae and are a cosmetic issue at first. They do show that the surface is staying damp, though, and moss often follows. Treating early, while the growth is thin, is the easiest time.
What to do about it
If the moss is light, keep an eye on it and look at the roof once a year. If it is thick, spreading or sitting in the gutters, have it removed. Removal is done by hand with a brush or scraper, and the roof is then treated with a biocide to kill what is left. Do not scrape wildly or use a hard jet on concrete tiles, as that strips the surface.
The free survey includes photos of each slope, so you can see what is growing and whether any tile has been damaged beneath it. A fixed written quote follows. Where repair works, we repair, and if the moss is the only issue we say so. See our page on roof cleaning in Basingstoke for more.
In Basingstoke, many roofs built in the last fifty years are covered in concrete interlocking tiles, which are the kind most prone to surface wear from moss. It is not a reason to panic, but it is a reason not to ignore a thick mat for years. Light, regular attention keeps the problem small.
Every roof is different. A free survey with photos settles it for yours.
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