Yes, it can, and often does. A pressure washer is a good tool for a patio and a poor one for a pitched roof. The jet that lifts grime from paving can strip the coloured face off a concrete tile, score a slate, force water up into the roof space and blast out the mortar along the ridge. It also leaves the roots of moss and lichen in the tile, so the growth returns. This page explains what happens to each material, how to tell if a past clean has caused damage, and which methods work with the roof instead of against it.
Why a roof is not a patio
A roof covering is built to shed water, not to resist it coming from below. Tiles and slates overlap, with each course lapping the one beneath. Rain falls down the slope, runs over the laps and drops into the gutter. A pressure washer sends water in the opposite direction, or sideways, at force. Pointed up the slope, a jet can drive water between the laps and under the tiles, past the batten and onto the felt. The felt is meant to catch the odd drip, not a sustained spray, and water that gets behind it can reach the timber, the insulation and the ceiling.
The surface is also thin. Many of the properties that make a tile durable sit in its outer layer, and a hard jet is capable of removing that layer in a few passes. Once it is gone, it cannot be put back.
What it does to each material
Concrete tiles
Most concrete tiles have a pigmented surface layer over a rougher body. High pressure can erode that layer, leaving a mottled, patchy roof and bare concrete underneath. Bare concrete is more porous and rougher than the face it replaces, so it holds water and grows moss faster than before. You might clean a roof and find it greener within a couple of seasons. On old concrete tiles that have already lost some of their face, the effect is greater. Our comparison of cleaning slate, clay and concrete roofs sets out how the surfaces differ.
Clay tiles
Machine-made clay tiles with a smooth or glazed face tolerate cleaning better than the rest. Handmade and sand-faced clay is softer. A jet can scour the sanded face, open the pores and leave the tile more likely to absorb water and spall in a hard frost. Close-range spraying on a handmade tile can leave visible tracks or lines.
Slate
Slate is dense but brittle, and its edges are thin. A hard jet can chip the tails, split slates that are already cracked, force water up the laps in a way slate is not designed for, and scour the fine surface so that it weathers faster. Nails and hooks that hold slates can loosen, which is a problem on older roofs where they have been in place for decades.
Mortar, pointing and the ridge
Ridge, hip and verge mortar is the first thing to go. A jet sets off cracked joints and carries away weakened bed, and tiles that were held by little more than mortar can come loose. We have written elsewhere about the signs that ridge tiles are failing, and a roof that has just been pressure-washed is likely to show them.
Asbestos cement sheets
Do not pressure-wash asbestos cement. Garages and outbuildings built before the late twentieth century may have fibre cement sheets, and high pressure can damage them and release fibres. The HSE explains the issue on its asbestos pages, and our answer on whether an asbestos cement roof can be cleaned covers what to do instead.
Signs a roof has been damaged by pressure washing
If a roof was cleaned some time ago and you are unsure how, look for these signs.
- A patchy, uneven colour on concrete tiles, with lighter or darker streaks that follow the path of a jet.
- A rough, sandy feel to the surface of a tile when you run a finger across it, compared with a smooth tile elsewhere on the roof.
- Quick regrowth of moss, in less time than you expected after the clean.
- Missing mortar or fresh cracks along the ridge, hips or verges.
- Tiles that have moved or slipped, or small chips at the edges.
- A new damp patch on a ceiling after the work, or staining on the felt in the loft.
None of these proves a jet was used, but several together make it likely. A survey can distinguish new damage from old and record it with photographs.
Why the growth comes back faster
Moss, lichen and algae take hold in the pores and imperfections of a surface. A pressure wash blasts off the visible growth but leaves the roots, spores and organic film behind, and it makes the surface rougher. The conditions that favoured the growth, shade, damp and a rough, absorbent tile, are all still there. Many roofs treated by pressure alone look good for a few months and then green up again, sometimes faster than before. A biocide treatment, which kills the growth at the root and lets it die back, tends to last longer. Our guide to roof biocide treatments explains how and why.
Can pressure washing ever be right?
There are narrow cases where a low-pressure rinse is acceptable: rinsing off dead growth after a treatment, or washing a smooth glazed tile with a wide fan nozzle held well back. What we rule out is the hard, narrow jet used close to the surface to blast moss off. Ask any cleaner what pressure and nozzle they use, and how far from the tile, and what they do about the ridge. Vague answers are a warning.
The cost figures also give a hint. Published UK guides report pressure washing a roof at £553, £638 and £723 for small, average and large roofs, against £748, £863 and £978 for hand removal with biocide. That is a modest gap, and the gentler method is the one that protects your tiles. Our page on roof cleaning costs covers the numbers in more detail.
| Method | What it does to the roof | Risk level |
|---|---|---|
| Hard pressure jet | Strips surface, drives water under laps, loosens mortar | High |
| Low-pressure rinse | Washes off dead growth without force | Low |
| Soft washing with biocide | Kills growth, releases over weeks | Low |
| Hand removal of thick moss | Clears gaps and gutters directly | Low to moderate |
Questions to put to anyone who offers to clean your roof
A few direct questions sort careful cleaners from careless ones. Ask what pressure they use and whether they use a jet at all on tiles. Ask what distance they keep from the surface and which nozzle they fit. Ask how they treat the ridge and hip mortar before they begin, and what they do if they find it loose. Ask whether they check the tiles for cracks first and photograph the roof before and after. Ask what happens to the water, moss and debris. An answer such as "we just blast it off" tells you what you need to know.
Where water goes when a jet drives it in
It is worth following the water, because the harm is often out of sight. A jet pushed up a slope enters the laps, runs behind the tile and wets the batten and the felt. Felt that is overlapped to shed rain from above can let water pass when it is driven from below. In the loft, the first signs are dark damp patches on the underside of the felt, wet insulation and a smell of damp. The water can take days to show on a ceiling, which is why a leak appearing a week after a clean is often linked to it. Where the felt is old or torn, the water runs straight through. On a roof with no felt at all, as is the case on many older houses with exposed rafter undersides, a jet can throw water directly into the house.
What repair looks like if damage has already been done
If a past clean has left the roof in a poor state, the repair depends on what was lost. Loose or missing ridge and verge mortar is re-pointed or re-bedded, ideally with a mechanical fixing, as our page on the fixing rules for ridge tiles explains. Cracked or chipped tiles are replaced individually, which is simple on most roofs where matching tiles are available. Wet felt or insulation in the loft is checked and dried or replaced as needed. A stripped concrete tile cannot be restored, but it can be treated to slow the regrowth, and where the roof is badly eroded, a coating or eventually a new covering becomes the topic.
The honest approach is to look first and recommend second. If the surface damage is cosmetic and the roof still sheds water, we say it can stay, and note it for the future.
What to use instead
The method most roofers prefer today is a mix of hand work and chemistry. Thick moss is lifted from the laps and valleys by hand, the roof is treated with a biocide approved for the job, and a gentle rinse follows, or nothing at all, leaving rain to do the work. Our page on what soft washing is describes the low-pressure approach, and the comparison of moss removal methods weighs scraping, washing and biocide against each other.
If a cleaner has already pressure-washed your roof and damage resulted, collect photographs and keep any paperwork. Whether a tile guarantee has been affected is covered in our answer on whether roof cleaning can void a tile warranty.
What we do instead on your roof
We start by identifying the covering and looking for weak points, then pick a method that fits, and photograph the roof so you see the condition before and after. We do not recommend a hard jet on a pitched roof. Where a repair is needed, such as a ridge re-bed or a few replacement tiles, we say so first, because cleaning over damage hides it.
The result is a fixed written quote for a method matched to your tiles, and the work carries our ten-year written workmanship guarantee. For more on the service, see roof cleaning in Basingstoke. If you would like a second opinion on a roof that was already cleaned, the survey is free, and asking for a look at the tiles on 01256 213956 or through WhatsApp is all it takes.
