Stone tiles, sometimes called stone slates, are the oldest roof covering in many of the villages around Bath. They are cut from layered stone, laid in diminishing courses and held with pegs or nails, and they are heavy, porous and irregular. That combination changes the cleaning question. On a modern concrete roof the aim is a clean, even face. On a stone roof, the lichen and the soft colour are part of its character, and the surface is a one-way resource: once grit and pressure have taken the weathered face off a stone tile, it does not grow back. This guide explains why heavy cleaning does so much harm, which growths matter and which do not, what light care is safe, and when the right decision is to leave a stone roof alone. It is one of the guides under roof cleaning, written for owners of stone-tiled houses, barns and outbuildings.
What a stone tile is, and why it is fragile on the surface
A natural stone tile is a thin slab split along its bedding planes. The stone is a sandstone or limestone, depending on the quarry, and it holds water in tiny pores. Unlike slate, which is dense and fine-grained, stone tiles are coarser, and their faces are naturally uneven. They are also thick and heavy, and the roof timbers beneath are built to carry that load.
Two things make the surface fragile. First, stone sheds in layers when it freezes. Water held in the pores expands as it turns to ice, and over many winters the face of a tile can blister and flake, a process known as spalling. A tile that is already starting to spall has little margin. Second, the outer face is harder than the stone behind it, having weathered over years. Break that skin and the softer stone beneath erodes faster.
The weight matters for cleaning, too. A stone roof cannot flex, so a person on it loads individual tiles, and a cracked tile in a heavy, pegged roof is awkward to replace without disturbing the courses around it. For that reason, work is done from boards, with the weight spread, and sometimes from a tower or cherry picker instead of the slope.
Stone roofs are also scarcer than they were. Two B&NES village appraisals, one for Wellow and one for Freshford, describe surviving stone tiles as rare in the first and still fairly common in the second. Either way, a roof like that is worth treating as a record.
How heavy cleaning harms stone tiles
Heavy cleaning means pressure washing, wire brushing, scraping and strong chemicals. Each does a particular kind of harm.
Pressure and abrasion
A pressure jet removes growth by force, and stone gives way before the lichen does. The weathered skin is stripped off, the surface becomes rougher and more absorbent, and the grain is opened up. MyBuilder's roof cleaning cost guide (2026) cautions against pressure on delicate tiles, and a stone tile is a clear case. The next frost then has more water to work on. Mortar bedding, pegs and the edges of tiles can also be washed out or dislodged.
Water driven into the roof
Many stone roofs have no underlay, or only an old one, and the laps rely on weight and fit. A jet driven upslope pushes water between the tiles and into the roof space. Timber that has stayed dry for generations can be soaked in an afternoon.
Strong chemicals
Acid-based and chlorine products attack the stone itself, leaving pale patches and etched surfaces, and they run onto lead flashings and metal gutters below. The trouble with household bleach has its own answer, bleach on a roof. Run-off also damages planting and can stain walls.
The pattern in each case is the same. The roof looks bright for a short time, then looks worse, and grows growth faster than before. The harm is cumulative and hard to reverse, and a stone tile once scoured is scoured for good.
Lichen, moss and which growths actually matter
Lichen is a partnership of fungus and algae, and it grows slowly on stone, often over many years. Grey, yellow and orange patches are a normal sight on an old stone roof. In most cases, thin crusty lichen does little harm and may even shelter the surface from weather. It is a matter of taste whether it needs to go. Many owners and conservation officers prefer it to stay, and on a listed building it forms part of what the roof is valued for.
Moss is a different matter. It forms a thick cushion, holds water against the stone for long periods, and gets into the joints between tiles. In winter that water freezes and presses on the tile edges. Moss also sheds into gutters and valleys, which is where it causes blockage; our page on moss blocking gutters sets out the knock-on effects. Heavy moss is worth removing, and gently.
Algae and black streaks are largely cosmetic on stone, appearing as darkened bands where water runs. They are not usually a reason to clean in their own right.
A sensible test is to ask what the growth is doing. If it keeps water against a tile, clogs a valley, lifts an edge or sits where the roof meets a wall, it deserves attention. A thin, patchy crust on an otherwise dry slope can reasonably be left. After the survey we can tell you where your roof stands, and we do not invent work.
Light care that is safe on stone
The safe approach is built around doing as little as will solve the problem. For a roof with moss and no more, it usually comes down to four steps.
- Remove the bulk of the moss by hand, with gloved fingers or a soft tool, working with the lay of the courses and lifting rather than dragging. Moss is bagged and taken down, not swept onto the roof below.
- Leave the lichen where it is unless it is thick enough to hold water or cover the edge of a tile.
- Where treatment is wanted, apply a biocide suitable for stone at low pressure, and let it work slowly over weeks. A biocide kills growth at the root without scouring the face, so the stone is not touched mechanically.
- Clear the gutters, valleys and outlets by hand before and after, so debris does not stay on the roof.
Soft washing is the nearest equivalent to this on a modern roof, and MyBuilder's guide (2026) gives typical UK figures of £15 to £20 per m² for it. On stone it is applied sparingly and without pushing water upslope. No rinse is aimed at the laps, and run-off is managed so it does not hit the wall face or the beds.
The pace is part of the method, and it is worth agreeing at the outset. A treated stone roof may take a season to look different, as dead growth weathers away on its own. Owners used to an instant result can find that unsettling, but it is a sign that nothing has been stripped.
Access follows the same principle: boards or a platform that spread the weight, and no stepping on a tile that is not supported by a batten or a lath beneath. On a steep or high stone roof, a tower or a hired platform at the eaves is often the better way in, because it lets the cleaner reach the lower courses without touching the slope at all.
Consent, character and the stone roofs near Bath
The city boundary doubles as the World Heritage Site boundary, with a conservation area over roughly half of it. Stone tiles turn up on older buildings there and in the villages to the south. Where a house is listed, the decision on consent rests with B&NES Council. Cleaning by itself is not usually the sort of change that needs consent, yet repair, relaying or a switch of material does, and work carried out without it is an offence. Ring the conservation team before anything is booked; cleaning listed roofs sets out how that conversation goes.
Concrete re-covering is singled out for criticism in the Freshford and Sharpstone appraisal. That is why matched materials count on stone roofs, and it is why a cleaning visit that finds a failing section often leads to a conversation about like-for-like repair. Two related guides, stone roof tiles and stone tile roof repair, take that route further.
Reconstituted stone slates exist too, and they are cast in imitation of the natural kind. The Bradstone Cotswold range, for instance, is about 84 kg/m² laid, according to its technical data sheet. They clean differently from the real thing because they are cast with a coloured surface, so ask which you have if you are unsure.
One surveying firm says stone tiles can easily last 100 years, and some have reached 300. That is a single source and depends on exposure, so treat it as indicative. The point for cleaning is that these roofs have already lasted and the job is not to shorten their life.
Cost, expectations and when to leave a stone roof alone
The cost guides do not publish a separate figure for stone-tiled roofs. MyBuilder's roof cleaning cost guide (2026) gives typical UK ranges for other coverings, with cleaning in general from about £300, tile roofs at £400 to £750 and slate at £500 to £1,000. Stone is priced on survey because the care it needs, the access and the weight of the work vary so much. Our page on roof cleaning cost explains what moves a quote. Your fixed price follows the survey.
On expectations, a stone roof cleaned lightly will not look new, and it should not. The aim is to stop moss doing damage while keeping the weathered look. Some patches will stay darker, some lichen will remain, and the roof will keep the unevenness that makes it recognisable.
There are times when the right advice is to do nothing. A roof that is dry, with thin lichen and no moss in the valleys or gutters, does not need cleaning. A roof with spalling tiles or slipped courses needs repair, and cleaning would only make it worse. A roof with a historic character that an owner wants kept as it is can simply have its gutters and valleys cleared each year.
If cleaning is the right call, we recommend it, set out the method in the quote and keep to it. If you are unsure, a visit to look at the roof itself settles it, and you receive the photographs. A photograph of the north slope, where moss and lichen are usually thickest, is a good thing to have ready before that visit.