Can you clean clay, concrete and slate roofs?

Can every type of roof be cleaned?

Yes. Clay tiles, concrete tiles and natural slate can all be cleaned. What changes from one roof to the next is how much moss grows on it and how easily its surface is damaged. The same rules apply to all of them:

• Low pressure, not a jet. A soft wash applies a cleaning solution at low pressure, and a biocide kills the moss, lichen and algae at the root. Leading UK roof tile manufacturers advise against pressure washing.
• No walking on the roof. Health and safety guidance lists slates and tiles among the roof materials likely to be fragile, and they can crack under someone’s weight. A hairline crack may not show at first and can open up after a frost. A professional works from properly supported roof ladders, crawling boards, scaffold towers or a cherry picker instead.
• Care with chemicals. Cleaning solutions are kept away from plants, animals and drains.

If you’re thinking of cleaning a roof yourself, don’t go up on the roof. 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.

At Washmates, every roof, whether it’s clay, concrete or slate, is cleaned with a soft wash and then treated with a biocide. We don’t walk on fragile tiles: 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. Every roof clean includes the gutters, fascias, soffits and any solar panels on the roof.

How is a concrete tile roof cleaned?

Many concrete tiles have a sanded or granular face. That rough surface catches dirt from rainwater and gives moss somewhere to root, so concrete tiles are often the first to turn green.

The surface is also what a pressure washer damages. A high-pressure jet can damage the tile’s finish and strip colour from concrete tiles, and a damaged surface can shorten the tile’s life considerably. Older concrete tiles are the most at risk: they can crack or lose their surface if they’re cleaned too hard.

Tiles in good condition usually cope with a careful clean; it’s old or weakened tiles that crack. A soft wash and a biocide clean concrete tiles without blasting the surface. See whether pressure washing damages a roof for what a jet wash does to tiles.

How is a clay tile roof cleaned?

Smoother surfaces, such as natural slate and many clay tiles, tend to grow less moss than rough-faced concrete tiles. Clay roofs still need cleaning when moss builds up or starts filling the gutters.

Clay is brittle. Clay tiles can break if they’re stepped on, and a high-power spray can damage them, so they’re cleaned with a soft wash and a biocide, from roof ladders, scaffold towers or a cherry picker.

Frost is the other thing to watch. Water that soaks into a clay tile and freezes can make its surface flake or crumble over time, and older tiles are the most at risk. Moss that keeps the tiles wet for longer gives frost more chances; see whether moss damages roof tiles.

A weathered colour or a few small cracks on an old clay roof are a natural sign of age, and don’t on their own mean tiles need replacing; a roofer can tell you which. Tiles that are flaking, crumbling or slipping are a job for a roofer before the roof is cleaned.

How is a natural slate roof cleaned?

Natural slate is smooth, so it tends to grow less moss than rough concrete tiles, but it is brittle. Slates crack under someone’s weight, and a hairline crack can go unnoticed until frost opens it up and the slate breaks, often in winter.

Slate is made of thin layers. Over time, water getting in can make the layers separate, until the slate no longer keeps the rain out. A high-powered jet can damage the surface of a slate and shorten its life, which is why slate is cleaned with a soft wash and a biocide, never a jet wash.

Slates can also slip when the nails holding them rust away. Slipped or split slates are a job for a roofer, and are best dealt with before a clean.

How do clay, concrete and slate roofs compare?

A rough guide: the age and condition of a roof matter as much as what it’s made of.

What about asbestos cement roofs?

Some garage and shed roofs, and some roof sheets and slates on older buildings, are made of asbestos cement. Asbestos was banned in the UK in 1999, so a building built or refurbished before 2000 may have it.

Left alone and in good condition, it’s low risk. The danger comes when it’s damaged, scrubbed, scraped or sanded, which can release fibres. Asbestos cement roofs are also always fragile and can’t bear a person’s weight. So:

• Never stand or walk on it.
• Never scrape or brush it, and don’t jet wash it yourself. Health and safety guidance says high-pressure cleaning is only appropriate in exceptional circumstances, by a specialist who collects the run-off.
• Don’t drill, break or disturb it. If you’re not sure whether a roof is asbestos cement, leave it alone and have it checked by an asbestos specialist before any work is done.

Should a roof be coated or sealed after cleaning?

Some companies offer to paint, coat or seal a roof after cleaning it. It’s a separate job from a clean, and it costs considerably more, so treat it as a separate decision.

Painting or coating roof tiles may invalidate the tile manufacturer’s guarantee, so check first, especially on a newer roof. On older and historic homes, conservation guidance warns against spray-on coverings: they make leaks hard to trace, stop tiles being reused, and anything added to the roof should still let it breathe.

A biocide is not a coating. It kills the moss, lichen and algae left on the tiles, so growth is slower to come back: a roof cleaned and treated with a biocide usually stays clear for two to four years. Shaded, north-facing roofs or roofs under trees see growth come back sooner.

What should you tell a roof cleaner about your roof?

A few details help any cleaner judge the right approach:

• What it’s made of, if you know: concrete, clay, slate, or something you’re not sure about.
• Roughly how old it is.
• Any cracked, slipped or missing tiles, or damp in the loft. Those need a roofer before a clean; see whether cleaning a roof can cause leaks.
• What any garage or outbuilding roofs are made of, so anyone quoting knows what they are.

Ask how they’ll clean it and whether anyone will walk on the tiles. You can see typical roof cleaning prices by type of house, and read whether roof cleaning is worth it on an older roof. When you’re ready, tell us about your roof and we’ll send you a fixed price, usually without a site visit.