A roof is not a driveway. A drive is flat, solid and built to take weather and traffic on its face. A roof is a layered system: a covering that overlaps itself, an underlay beneath that, and battens and rafters beneath that again. The covering is designed to shed water that falls from above, in rain, not water fired at it from below, at an angle, with force.
That difference is the whole of the argument against pressure washing as a routine way to clear moss. This guide explains the three kinds of harm a hard jet can do: to the surface of the tile, to the laps and what lies beneath them, and to slate and stone. It then sets out what a safer approach looks like. For a comparison of every method, see our guide to moss removal methods; for the wider service, see roof cleaning and moss removal.
What a pressure washer actually does to a roof
A pressure washer squeezes water through a small nozzle, so a thin, fast jet leaves the lance. The cleaning comes from the energy in that jet: it cuts under dirt and growth and lifts them off. On a hard, sealed, level surface that works well. The surface is stronger than the jet, and the water simply drains off.
On a roof, three things change. The surface is a manufactured skin that is not meant to be scoured. The water does not run away over a flat plane; it runs into joints. And the person holding the lance stands on or beside a slope, aiming up it, which is the one direction roofs are not built to resist.
Why moss makes it tempting
Moss grips well, and a jet clears it quickly. The result looks impressive: a clean, even roof with no green left on it. That immediate result is a large part of why the method is so widely offered. The problem is what the roof looks like in the years afterwards, not on the afternoon of the clean.
What the cost guides say
MyBuilder's roof cleaning guide, updated 21 August 2026, lists pressure washing among the standard methods at £10 to £15 per m², with a caution that delicate tiles can be harmed. That is a short warning from a source that is in the business of pricing the work, and it is worth taking seriously. Our roof cleaning cost guide sets the price beside the rates for other methods.
Eroding the tile surface
Many roof tiles are made with a surface layer that does a job. Concrete tiles carry a thin coloured coating on their upper face; clay tiles have a fired skin that is dense and weather-resistant. Both are thin, and both are the first thing the jet meets.
Concrete tiles
A hard jet can strip that coloured layer from a concrete tile, and what remains is the coarser material beneath, which is rougher and more porous. The tile looks paler and patchy, and it behaves differently: a rough face holds water and gives spores something to grip. The roof that was cleaned to stop moss can end up growing it again more readily. Our answer on cleaning concrete tiles safely sets out the gentler route.
Clay and older tiles
Old clay tiles are handmade or machine-made to older standards, and their surfaces weather in ways that a jet can strip. A tile that has been softened by decades of frost and rain is more likely to flake, and flakes are a one-way loss. Once the face has gone, it is not coming back.
Why wear shows up later
Surface erosion does not usually leak on the day. It shows as a roof that ages faster, that stains sooner and that sheds granules into the gutters. By the time anyone notices, the connection with the earlier clean has been forgotten. This delay is part of why the method keeps being used: the cause and the effect are separated by years.
Driving water under the laps
A pitched roof keeps weather out by overlapping. Each tile or slate lies over the one below, and the overlap, called the lap, is what stops rain getting through. The lap is sized for rain falling or blown against the roof, and for water running down it. It is not sized for water forced up it.
Water travelling the wrong way
A pressure washer is normally pointed up the slope, because that is where the moss is. A jet aimed upwards pushes water underneath the tail of each tile, into the gaps and onto the underlay. Underlay is the sheet beneath the covering, intended to catch the odd drop that gets through. It is not meant to receive a flood.
What the water can do once it is in
Water that gets under the laps can soak the underlay, run to the battens and rafters, and in a hard clean reach the ceiling below. It can also find its way into the loft, where it dampens insulation and timbers. Often the first sign is a stain on a ceiling a day or two later, and by then the connection is easy to miss. Soaked battens dry slowly, and fixings can rust in the damp. Our loft-side checks always include looking for fresh dampness after any wash.
Wind-driven effects
The same thing can happen on a smaller scale with ordinary driven rain on a roof whose laps have been opened or whose surface has been roughened. A roof that has been over-washed loses some of its resistance to weather. The jet does not need to make a hole to do harm; it only needs to leave the covering slightly less able to do its job.
Cracking slate
Natural slate is hard, but it is hard in the way glass is hard: strong along the face, brittle at an edge, and prone to split along its bedding. A slate roof that has stood for decades is full of slates held by nails that have aged, with edges that have worn thin.
What a jet does to a slate
The force of a hard jet can crack a slate that is already carrying a hairline flaw, and it can lift a tail and drive water behind it. Slates that have slipped slightly on their nails are easily displaced. A roofer may need to replace several after a wash that looked like a quick job. MyBuilder's slate roof guide gives a single slate repair as £60 to £120, and five to ten slates as £150 to £350, so a damaging clean adds up quickly.
The danger underfoot
Slate can crack under a boot as well as under a jet, which is why anyone working on slate keeps the weight spread and the footing considered. A wet slate is also slippery, and a roof sprayed with water and cleaning solution is slipperier still. That is a safety point for the person on the roof, not only a matter of the covering.
Why a gentler route works
Hand work and low-pressure treatment let the roofer see and feel each slate as it is cleaned. A cracked piece is noticed at once, not found later as a leak. That is a direct benefit of gentler methods: the cleaning doubles as an inspection.
Stone slates, pegs and the Cotswold roofs of the Five Valleys
Stone roofs in and around Stroud, from Painswick down to Uley, are among the least suited to a pressure washer. The slates are limestone, split by hand or frost, laid in diminishing courses and hung over the battens on wooden pegs. Nothing is nailed tight; the weight of the courses above holds each slate in place, and the pegs do the rest. Our guide to cleaning stone roofs covers the hand methods that suit them.
Pegs, laps and soft stone
A strong jet can knock a peg loose or push a slate out of line. Limestone is also softer than slate, and its face can be scoured by a hard jet, leaving a rougher, paler patch. The Cotswold District Council guidance from 2000 records that a stone slate roof can last a century or far longer with regular maintenance, and that annual scraping could double that life. Strong washing is not part of that record.
Protected buildings
Many stone roofs belong to listed buildings or stand within one of Stroud District's 41 conservation areas. Cleaning is not a free choice there: Stroud's conservation team treats every listed building, whatever its grade, as fully protected. Check with Stroud District Council's planning team before any wash on a protected roof. See also cleaning listed roofs.
On a stone roof, if the method cannot be done by hand or at low pressure, it is the wrong method for that roof.
Telltale signs a roof has been over-washed
If a roof has already been pressure washed, certain signs can show whether harm was done. None is proof on its own, but together they are worth a closer look.
Signs you can see from the ground
Look for tiles that are paler or patchier than they were, a rough sandy look where a smooth coloured face used to be, and granules or grit in the gutters. Look also for slates that sit out of line, or for a tail line that no longer runs straight. Moss that returns unusually fast can also hint at a surface that has been roughened.
Signs inside the house
A damp patch on a ceiling soon after a clean, a musty loft or damp insulation are all signs that water went where it should not. These can be minor, but they should be checked promptly, because timbers that stay wet for long enough deteriorate.
What to do if you see them
Photograph the roof and the stains with dates. Ask for a survey of the covering and the loft side. We inspect underlay, battens and laps, then report back with photographs before suggesting any repair. It may be that nothing is wrong. If something is, the repair is covered in a written quote.
Cleaning without the jet
None of this means a roof must stay green. It means the method has to match the covering. For most pitched roofs there are routes that avoid high pressure altogether.
Hand scraping and brushing
The roofer removes the moss by hand with plastic scrapers and brushes. It suits fragile coverings and stone, and it lets each slate and tile be checked as it is cleaned. The trade-off is that spores remain in the surface, so growth can return.
Soft washing and biocide
Low-pressure rinsing, together with a treatment that kills the growth, deals with the spores that scraping leaves behind. The roof is not blasted, and the moss dies back and washes away over the following weeks. Our guide to soft washing and biocide treatments explains the method and the results. To see what keeps feeding the problem, read why moss grows on a roof.
What we put on the quote
The written quote names the method for each part of the roof and the reason for it. Gutters are dealt with, debris leaves site, and the work carries a 10-year workmanship guarantee in writing. A roof that needs repair rather than cleaning gets a repair quote.