Does jet washing damage roof tiles?
Jet washing can damage roof tiles. A lance held close strips the surface finish off a concrete tile and takes the valley mortar with it. The tile is solid concrete, so a stripped face opens no hole, but a stripped valley does. The surface does not come back. Grit in the gutters after a clean is that finish.
Does jet washing damage roof tiles?
It can, and what a lance takes off is the surface of the tile rather than the body of it. A pressure lance held close to a concrete tile takes the outer finish off along with the moss. The concrete under it is solid, so stripping the face does not open a hole in the roof, but the face has changed and it does not change back.
On the tiles a lance takes the look off. In a valley or along a ridge it takes off what was keeping water out.
Marley, which makes concrete and clay roof tiles, says on its own support page, opened on 5 September 2026, that a high powered jet wash "can damage the surface of the slate or tile".
Pressure is not one setting, and from the ground you cannot tell what a roof has had. If you are holding a quote for a clean, ask what pressure they will use, how far back they will hold the lance, and what happens to the bedding along the ridge and the valleys.
What is the surface finish on a concrete tile?
It is the outer face of the tile. Take it off and what is underneath is paler and rougher than the slope next to it. On the DIYnot forum, a poster wrote in August 2020 of noticing "a fair amount of the granular tile grit coating in the gutters", and put it down to natural wear and tear.
On Mumsnet, a buyer who posted on 11 December 2023 wrote that the sellers had had the roof power washed, that it had blasted the protective coating off the tiles and the cement out of the gullies, and that metal was visible.
The same buyer wrote on 13 December that the tiles would originally have been glazed red, that the colour had been blasted off and they had gone a sandy colour, and that this can apparently make them more absorbent. A concrete tile is the same material all the way through its thickness, so a tile that has lost its face still sheds water, and no source on this page puts a figure on what the change costs.
Redland and Marley are two of the common concrete tile names on British houses. A clay tile takes its colour from the fired clay itself, and Welsh slate has no applied finish at all, so on those roofs the thing to look for is not a colour change but chipped edges and cracked slates.
Why do valleys and mortar suffer before the tiles do?
Because mortar is softer than a concrete tile face, and a lance cuts the softer material first. A valley is the channel where two slopes meet, and on houses where mortar is packed along both sides of it to close the gap, that mortar is sand and cement. Nothing holds it there but its own set, and it cracks and drops out with age whether or not anybody cleans the roof.
The Mumsnet buyer names both losses in the same sentence. The wash took the coating off the tiles and the cement out of the gullies. The buyer could see the tile damage from the ground floor and the exposed gullies from the loft conversion window.
Bedding mortar along a ridge and at a gable edge goes the same way on its own over the years, and a lance finishes that job in an afternoon. The tile it was holding is then sitting on nothing.
It may not leak on the day, and that is what catches people out. A stripped valley and a ridge with no bedding left both look tidy from the ground.
Is grit in the gutters a sign of damage or of age?
It can be either, and the timing tells you which. Grit that turns up in the gutters within days of a clean came off in that clean. Grit that has been collecting for years is the tile surface ageing, which happens on any concrete roof in time.
Both readings sit on that Mumsnet thread. The buyer blamed the power wash for the sandy colour. The reply on 13 December 2023 took the tiles to be concrete and said that concrete tiles are glazed and coloured when new, that flipping one over shows the true unglazed colour underneath, and that they all lose the finish over time to a rough concrete texture as the colour fades. The same reply doubted very much that pressure washing a roof to clean it would damage the tiles.
The DIYnot poster who found grit read it as natural wear and tear. On a roof nobody had washed that is a fair reading. On a roof that was washed a few days before, it is not.
Either way it needs clearing, because it does not stay in the gutter. It works down to the outlet and sits in the bend at the top of the downpipe. A gutter that spills over in the first heavy rain after a clean is worth opening up.
We do not offer roof cleaning
We do not offer roof cleaning and we do not sell moss removal. Our work is roof repairs, new roofs and reroofs, flat roofs and GRP roofs, dry verge and dry ridge systems, UPVC roofline, gutter cleaning and exterior painting. We cover Crawley, Brighton, Horsham, Reigate, Redhill and Dorking.
So we are not the people to ring for a clean. A slipped tile, a valley with no mortar left in it, or a ridge that has been washed loose, is a repair, and repairs are our work.
What to check after a clean, and when to have the roof looked at
Start in the gutters and work up. Grit at the bottom of a downpipe is the easiest thing to find and the clearest evidence of what came off the tiles, and dead moss and bits of mortar end up there with it. Do the rest from the ground on a dry day with a pair of binoculars. An upstairs window across the road shows you more than a ladder does, and it is safer.
Do not go up on the roof to look. Section62, posting on MoneySavingExpert in December 2022, warned that walking around on a roof to brush it, or to pressure wash it, risks cracking tiles and slates, and that "Most of the time there is more risk of damaging the roof from trying to remove the moss than there is from the moss itself."
Run your eye along the ridge line. The mortar along the top should be one continuous line. Gaps in it, or a ridge tile sitting at a different angle to the ones next to it, mean the bedding has moved.
Look into the valleys for bare metal in the channel or loose mortar sitting in it.
Then look across the tile faces themselves. Pale streaks running the way the lance moved, or a band of tiles lighter than the rest, mean surface has come off unevenly. Photograph it, so that next year you can tell whether anything has moved.
On the next wet day, take a torch into the loft and look at the underside of the valleys and at any point where the roof meets a wall. Daylight where there should be none, or timber that is dark and damp to the touch, is worth a phone call.
If a clean has left grit in the gutters or bare patches on the tiles, we give free estimates and advice, and we only charge if we do the work, so leave a name and a number on the short form further down this page and we will tell you what it did.
Sources
Every page and thread below was opened and checked on 5 September 2026. Where a page shows its own date, that date is given with it.
Marley, Can I clean or jet wash my roof tiles, manufacturer support page. No publication date shown on the page. https://www.marley.co.uk/support/problems-and-maintenance/can-i-clean-or-jet-wash-my-roof
DIYnot, roof cleaning (moss). Thread started 21 August 2020, quoted posts 21 and 22 August 2020. https://www.diynot.com/diy/threads/roof-cleaning-moss.551336/
Mumsnet, Surveyor said all was fine but we need a new roof. Thread started 11 December 2023, quoted posts 11 and 13 December 2023. https://www.mumsnet.com/talk/property/4961001-surveyor-said-all-was-fine-but-we-need-a-new-roof
MoneySavingExpert, Is moss harmful to roofs. Thread started 4 December 2022, quoted post 5 December 2022. https://forums.moneysavingexpert.com/discussion/6408717/is-moss-harmful-to-roofs
The forum entries are what individual posters wrote on public threads. The Marley entry is the manufacturer's own support page.
Common questions
- Does moss coming back mean the clean was done badly?
- Not on its own. Moss comes back wherever the roof is still shaded and damp under whatever drops off the trees above. On the DIYnot thread in August 2020 one poster got the advice to respray if it showed signs of growing again. If you know who made the tiles, look up that maker's own cleaning page before anybody starts. Judge the work by what the tiles and the mortar look like afterwards, not by how long the green stays away.
- What is left on the tile once the surface finish has gone?
- Concrete, and it is the same concrete all the way through, which is why a stripped patch still sheds water and looks different from the slopes nobody touched. It does not grow back, so there is nothing to put right on the face itself: spend the time on the bedding along the ridge and the mortar in the valleys instead.
- What should you look for on the tiles after a clean?
- Look at the tile faces for colour that is uneven where the lance travelled, and for edges that are chipped or broken. Then look along the ridge line and into the valleys for missing mortar, and in the gutters for grit. Going up on the roof to look is how people get hurt.
- Does the moss actually need removing?
- Not always, going by what homeowners say about it. On the MoneySavingExpert thread in December 2022 one poster wrote that moss is only a problem if and when it blocks the gutter, and that growth which is not excessive can be left alone. Another poster on the same thread put the risk of taking moss off above the risk of leaving it where it is. Those are homeowners writing on a public thread, not a technical authority, and it is the roof in front of you that decides. Whether the gutters are running clear is the part of it you can see from the ground.
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