A jet wash is quick, dramatic and cheap to run, which is why it is so often offered. In a few minutes a green slope turns pale and the water running off it turns dark. The trouble starts when you look at what has been removed along with the moss. A roof covering is a weathered, working surface designed to shed water downhill, not to be blasted from a lance. This guide explains what pressure does to concrete tiles, clay, slate and old stone, how it forces water into the parts of a roof that are meant to stay dry, and why the damage often shows only after the firm has gone. If you have been offered a pressure wash, it should help you decide what to ask and what to turn down.
What a pressure jet actually does to a roof
A pressure washer narrows a flow of water to a small point and drives it at speed. On a car or a paving slab that is useful, because the surface is flat, sealed and built to take it. A roof is a different structure. It is a set of overlapping units, each lapping over the one below, held in place by gravity, nails, clips or mortar. The design relies on water running down the face and off the tail. It does not expect water to arrive from below, at force.
When a jet hits a roof at a low angle, two things happen at once. The force on the surface removes whatever is loose, which includes moss but also the weathered top layer of the covering. And the water that does not bounce away is pushed sideways into the laps, between the tiles and the battens, and up under the course above. Moss is removed, but the roof has been wetted from the wrong direction.
The operator usually works from a ladder or the ridge and sweeps the lance upslope, because that is where the growth is most easily lifted. Spraying upslope is the worst direction for a lapped roof, since it sends water towards the places a roof is designed to keep it out. A firm that sprays downslope is better placed, but it still puts the surface under a stress it was not built for.
The result can look good on the day. Clean tiles and a pale slope make a convincing before and after picture. The problems are the ones the picture does not show: a thinner surface, a wetter roof space and a loosened ridge, all of which turn up in the following months.
Concrete and clay tiles: stripped surfaces and faster regrowth
Most concrete tiles have a coloured surface layer over a body of denser grey material. That layer is thin. A pressure jet can wear through it, taking the colour with it and exposing a rougher, more porous face. The tile may look paler, patchy or streaked, and in some cases the damage is visible as a change in texture. This is not something a coat of paint repairs well.
The rougher surface causes a second problem. Moss and algae take hold more easily on a pitted, absorbent surface than on a smooth, sealed one. A roof that has been jetted can therefore grow moss faster than it did before, which is the opposite of what the customer paid for. The pattern of regrowth is covered in the reasons moss returns so quickly.
Clay tiles are tougher on the face, and a fired surface or glaze resists pressure better than a concrete skin. They are not immune. Old clay tiles, especially hand-made plain tiles, can have soft, weathered edges, and a jet can chip the lower edges, break nibs, and loosen tiles that are held only by a single peg or nail. Pantiles stay put through their weight and a couple of clips. A hard stream can shift them or force water under the roll.
MyBuilder's roof cleaning cost guide (2026) warns that a pressure wash can harm delicate tiles. Older and handmade tiles are exactly the kind that fall into that category. We think the safest rule is that any tile you would hesitate to replace is a tile you should not jet.
Slate: split edges, driven nails and water behind the course
Natural slate is a layered rock, split into thin sheets. It is hard on the face and weak across the layers, and it is held up by nails through two holes near the head. A pressure jet working across the tail of a slate can open the layers at the edge, causing delamination, which is a flaking away of the sheets. Once that begins it continues, and the slate gradually loses its tail.
The nails are the other concern. Slate roofs often rely on nails that are already ageing. If a jet shakes a slate loose, or forces water behind it, the nail can pull or the slate can slip. A slipped slate cannot always be seen from below. It lets rain run onto the batten and the underlay, meaning the layer under the battens that stops any water getting past the covering from reaching the loft. The battens are the timber laths that carry the slates.
Slate roofs that look sound can have a lot of hidden age. Repairs from previous years may be held in place only by a tingle, a small tab of copper or lead propping a replacement slate. A jet will catch and bend a tingle, and the slate it holds can come away.
Welsh slate on older Bath houses is often thin and well weathered. It was laid by hand to last, and it has lasted. It is also the roof least able to take punishment. A cheap jet wash can leave a roof that looks fresh and leaks in the first heavy storm. Our guide to soft washing roofs describes the gentler method that suits slate.
Where the water goes: underlay, battens, insulation and ridges
Water forced under the laps does not simply run out again. It lands on the underlay, runs along the battens and finds the lowest point. Modern breathable membranes are made to cope with a little moisture, but they are not designed to handle a stream from a jet. Older roofs may have bitumen felt that has aged and cracked, with no real resistance to wind-driven water. A roof with no underlay at all, which is common on older houses and sometimes deliberate, has nothing to catch what comes through.
The consequences appear in the loft. Insulation laid between the joists takes up water and stops working. Timbers get wet, and if they stay wet they invite rot. Stains show on the ceilings below. Often the house owner sees none of it for weeks, because the roof space dries slowly and the damp shows only after the next heavy rain.
Ridges and hips are especially vulnerable. Bedded ridge tiles are fixed with mortar, and mortar that has aged is brittle. A direct jet can blow out the bed, loosen the tile and leave the ridge exposed. Under BS 5534:2014 a ridge tile needs a mechanical fixing too, since mortar is no longer trusted to do that job alone, yet a great many existing roofs still depend on old mortar. Pointing and flaunching, the mortar that caps a chimney stack, suffer in the same way.
Lead is the last victim. Flashings and soakers are folded and dressed into place, and a jet can lift an edge that had been lying flat. Once a flashing is lifted, rain can enter at a joint that was previously sealed. The gentler methods that avoid all of this are set out in moss removal by hand, brush and biocide.
Old stone tiles, listed roofs and fragile coverings
The stone tiles found on older buildings in and around Bath are the most delicate covering of all. Stone is porous, it takes up water, and its surface has often weathered into a soft, slightly crumbly skin. A jet will blast that skin off, leaving a pitted face that decays faster. Stone tiles are also heavy and hard to replace. A broken one may need a salvaged match, and there may not be one.
For a listed building the position is more serious still. B&NES Council wants listed-building owners to contact its planners first, and notes that works carried out without consent are an offence. Whether a particular cleaning method needs consent depends on what it does to the fabric, and a method that strips historic surface is the type of thing the council will want to hear about first. Please ask before you book anything that involves force.
The Bath Preservation Trust has also pressed for lime mortar and gentle treatment of historic stonework in its casework on listed houses. The principle that old fabric should be treated with the least intervention that works applies equally to the roof. A gentle clean, with hand work where needed, respects it. A jet does not.
Our advice on fragile coverings is simple. If the roof is old, handmade, stone or in a conservation area, do not accept a pressure wash. Ask what else the firm offers, and ask them to put the method in writing. The method we use on these roofs is covered in stone roof cleaning.
Reading an offer: what to ask before a jet wash
Sometimes the firm will say that pressure is fine if it is turned down low. It is true that pressure settings vary, and that a gentle rinse is a normal part of some cleaning methods. The questions below help you tell the difference between a controlled low-pressure rinse and a blast.
- What is the method, in everyday words, and in which direction does the water travel on the slope?
- What is the covering being cleaned, and has the firm looked at its condition before starting?
- Is a biocide used, and is it left to work, or is the roof simply sprayed clean?
- What happens to the moss and debris, and who clears the gutters?
- If a tile or slate is damaged during the job, who pays to replace it and how is it matched?
- Will the method and the price be set out in writing before the work begins?
A firm that answers these clearly and chooses a gentle method is a better bet than one that talks only about speed. A firm that is vague about direction, or about who covers breakage, is telling you something.
The price comparison also deserves care. The same MyBuilder guide (2026) gives typical UK rates of £10 to £15 per square metre for pressure washing and £15 to £20 for soft washing. The jet looks cheaper by the metre, and the saving disappears the first time a ridge has to be rebuilt or a loft dried out. Any firm's quote means more after a survey, and the wider picture is in what a roof clean typically costs.
What to do if a roof has already been jetted
If a pressure wash has already been done, begin by establishing what it did. Look at the tiles from the ground in good light. Pale patches, a rough or sandy look where the colour has gone, and chipped edges are signs of surface loss. Check the loft in the days after rain for damp patches on the timbers or insulation, and the ceilings below for new stains. Look in the gutters for grit or granules, which may be the surface of the tiles coming away.
If you find evidence, photograph it with the date and keep the paperwork from the firm. Take it up with them first, since they are responsible for the way they did the work. For any dispute about the work itself, a record of the method used and what you were told at the time is useful.
What repair is needed varies with the covering. A ridge that has been loosened needs rebedding or a mechanical fixing. Slipped slates need refixing. Flashings that have been lifted need dressing back or replacing. If the tile surface has been stripped, there is no simple way to restore it, though a sealing treatment may slow further regrowth. Where the damage is widespread the roof may be better re-covered, and our page on cleaning or replacing a roof helps with that decision.
Our survey costs nothing, and the pictures we take of your roof show you what we saw. Where a clean is all that is wanted, we use a gentle method and say openly what it will achieve. You get the price fixed in writing, and the work is backed by a ten-year written guarantee on workmanship. The service as a whole is described under roof cleaning, or you can ring 01225 285181.