Does moss damage roof tiles?

How does moss damage a roof?

Moss doesn’t eat into roof tiles the way rot eats into wood. The harm comes from what it does to water on the roof. The main effects are:

• It holds water against the tiles. Moss soaks up rain like a sponge. Water that would run off the roof stays in contact with the tiles for much longer, so they spend more of the year wet.
• Wet tiles are more exposed to frost. When water that has soaked into a tile freezes, it can flake or crack the tile’s surface. Older and more porous tiles are the most at risk, and the longer a tile stays wet, the more often that can happen.
• It blocks gutters and valleys. Moss comes loose in wind and rain and washes down into the gutters, downpipes and the valleys where two roof slopes meet. When they block, the gutters overflow, and water can back up onto the roof.
• It builds up where rain should run off. Thick clumps grow along tile edges and in the gaps where tiles overlap. On a roof that’s already in poor condition, that can help rain find its way underneath.

On a sound roof, a light covering of moss is mostly a matter of looks. It’s the thick, long-standing moss, and moss that is getting into the drainage, that is worth removing.

Which roofs are most at risk from moss?

Moss needs damp, shade and something to grip. It grows most on:

• Roofs under or near trees, which drop leaves on the roof and keep it shaded.
• North-facing slopes, which get little sun and stay damp for longer.
• Low-pitched roofs, which shed water more slowly than steep ones.
• Rough-faced concrete tiles. Their surface catches dirt from rainwater, giving moss somewhere to root, so they are often the first to turn green.
• Older, more weathered roofs.

Smoother surfaces, such as natural slate and many clay tiles, tend to grow less moss, though lichen and algae still grow on them.

What about lichen and black streaks?

Moss isn’t the only growth on a roof. Lichen forms flat, crusty patches, often grey, green, orange or black, that grip tightly to the tile surface. It is hard to shift by brushing or jetting, so it needs a biocide to kill it.

Dark streaks running down a roof are usually caused by algae rather than moss; see what the black streaks on a roof are. A soft wash and a biocide treat moss, lichen and algae in the same clean.

What are the signs that moss is causing damage?

From the ground, look for:

• Gutters full of moss, or water spilling over the gutters in heavy rain.
• Moss built up in the valleys or along the bottom edge of the roof.
• Cracked, slipped or flaking tiles, or bits of tile surface in the gutters.
• Damp or water marks in the loft, especially after heavy rain.

Moss in the gutters is something a roof clean deals with: every Washmates roof clean includes clearing them. If you see cracked or slipped tiles or damp in the loft, get a roofer to check the roof before it’s cleaned. Our guide to whether roof cleaning is worth it covers when to call a roofer first.

How should moss be removed from a roof?

Gently. The aim is to remove the moss without damaging the tiles it’s growing on:

• Don’t pressure wash. A high-pressure jet can crack older tiles, strip their surface and force water into the roof space. See whether pressure washing damages a roof.
• Don’t walk on the tiles. Older tiles and slates crack underfoot. A professional works from properly supported roof ladders, crawling boards, scaffold towers or a cherry picker instead.
• Don’t scrape fragile tiles. A professional scrapes only sound tiles, with care, from roof ladders or crawling boards; scraping can chip old tiles and slates.
• Never scrape or brush an asbestos cement roof. Some older roofs, garages and outbuildings are covered in it. If you’re not sure what your roof is made of, leave it alone and ask.
• Treat the roots. A biocide kills the moss and its spores, so it’s slower to come back. Keep cleaning chemicals away from plants, ponds and drains.

At Washmates, every roof is cleaned with a soft wash, then treated with a biocide. The work is done from roof ladders, scaffold towers or a cherry picker, normally by a team of two, and a three-bedroom semi usually takes a full day. We clean roofs in dry or mild weather and light rain, never in heavy rain, frost, ice or high winds. Every roof clean includes the gutters, fascias, soffits and any solar panels on the roof.

Is it safe to remove roof moss yourself?

Not from the roof itself. Falls from height are the most common cause of fatal accidents at work in Great Britain, and that’s among people trained and equipped for it. Working on a sloping, mossy roof from an ordinary ladder is far riskier.

From the ground, you can keep the gutters clear (if they’re low and easy to reach), cut back overhanging branches and keep an eye on the roof each autumn. Anything that means climbing onto the roof is best left to someone with the right access equipment and insurance. You can see typical roof cleaning prices by type of house in our cost guide.

How quickly does moss come back?

A roof cleaned and treated with a biocide usually stays clear for two to four years. Moss comes back faster where the conditions that grew it are still there: shade, damp and trees overhead.

You can slow it down by trimming back overhanging trees, keeping the gutters clear and, if you want to go further, having a roofer fit copper or zinc strips along the ridge to slow regrowth. A quick look each autumn tells you how the roof is doing, and autumn is a good time to clean it; see the best time of year to clean a roof. If the moss is building up, tell us about your roof and we’ll send you a fixed price, usually without a site visit.