A slate roof looks tough, and in one sense it is: good natural slate keeps water out through generations of weather. But the roof as a whole is a set of thin, overlapping pieces held by a few nails each, and that is where cleaning goes wrong. A slate can snap under a boot. A nail that has rusted thin can let go when a jet of water hits the course. A tingle, the small strip of metal that holds a replacement slate in place, can be knocked out of line. This page covers how a slate roof is cleaned without those results: what is checked before anyone steps on the slope, how moss and lichen are taken off, which methods are kept away from slate, and how to tell a roof that wants cleaning from one that wants repair first. It sits alongside our wider roof cleaning and moss removal service, and it deals only with slate.
Why slate breaks when it is cleaned badly
Slate is a layered metamorphic rock, split into thin sheets. It resists water and frost well, but it is not flexible. Point load from a heel or a knee will crack a slate across its width, often without any sound that reaches the ground, and the crack may stay closed until the next heavy rain. That is why a roof can look fine on the day it is cleaned and leak a week later.
The fixings matter as much as the slate. Each slate hangs from nails driven through two holes, with the course above lapping over the heads. Over a long life, iron or poorly galvanised nails corrode, and the slate begins to slide. The condition is known as nail sickness, and we cover it on our page about nail sickness in slate roofs. A roof in that state is held more by friction and habit than by its fixings. Any sideways force from a brush or a jet can finish the job.
So the first rule is that cleaning a slate roof is a condition check with a cleaning job attached. Before moss comes off, we want to know whether the slates are sound, whether they are still tightly nailed, and whether the lead at the chimney and valleys is intact. Moss can be hiding a problem, because a thick pad sometimes wedges loose slates into place. Take the moss away and the slate moves.
That is also why a roof ladder or crawling boards are used on slate. They spread the weight across several courses so no single slate takes it. Walking on the bare slope, even in soft shoes, is how most cleaning damage happens, and it is avoidable.
What we check before any cleaning starts
The survey comes first, and on slate it is closer to an inspection than a quote. From the ground and from a ladder at the eaves we look at the pattern of the roof: are the courses still straight, or are there gaps and dropped slates? Are there tingles dotted across the slope, which suggest the nails are failing generally? Are there rust-coloured streaks beneath the slates, a sign of iron in the stone or of corroding fixings?
Then we go up with boards and look at the slates that matter. We lift a few at the edge of a course to see the nail heads and the battens beneath them. If the nails are sound and the battens are dry and firm, the roof can be cleaned. If the nails are thin or the battens soft, we tell you, and we recommend the repair first, because cleaning a failing roof makes it fail faster.
We also look at what is around the slate. Lead flashings at the chimney and abutments should be tight and unsplit. Valleys should be clear of debris. The gutters should be able to take what the cleaning will send down them, a point we explain on our page about moss blocking gutters.
Finally, we look at the type of slate. Welsh slate is dense and generally takes careful cleaning well. Softer or more laminated slates, and some imported ones, can flake on the surface, and a slate that is already delaminating will lose more of its face if it is scrubbed. In that case we tell you what to expect before we start, and we match the method to the slate rather than to a standard procedure.
Hand work and soft washing, the methods suited to slate
On a slate roof the safest first step is dry. Thick moss is lifted off by hand or with a soft tool, working from the ridge downwards so debris does not catch under the laps. The moss is bagged and taken off the roof rather than swept into the gutters, where it would sit and block the outlet. A stiff metal scraper has no place here, because it chips the edge of the slate and scores its face.
What the hand work leaves behind is the thin film of algae, lichen and spores that stain the slate dark. The usual way to deal with that is soft washing, which means applying a cleaning solution at low pressure and letting it do the work over time, then rinsing gently. MyBuilder's roof cleaning cost guide (2026) gives typical UK figures of £15 to £20 per m² for soft washing against £10 to £15 per m² for pressure washing, so soft washing is not the cheaper option per metre. It is chosen because it protects the surface.
The solution is a biocide, a treatment that kills the moss and lichen at the root. It works slowly, which is a feature: the growth dies, loosens, and weathers off over the following weeks. We cover the pros and cons in our page on soft washing roofs. The rinse is the part that needs judgement on slate. Water is directed down the slope, never up under a lap, so it runs off with the dead growth and does not get behind the slate onto the battens and underlay.
Where plants, trees or walls sit close, they are protected and the run-off is managed, since biocide should not reach beds or ponds. The treatment is chosen and applied with that in mind.
Why pressure washing and harsh chemicals are kept off slate
Pressure washing is the method most often offered on the doorstep, and it is the one we avoid on slate. MyBuilder's cost guide itself cautions about pressure on delicate coverings, and slate belongs in that group.
What a jet does to a slate roof
A strong, narrow jet can scour the face of a slate, particularly an older one, leaving a rough face that holds moisture and grows moss again sooner. It can also drive water upwards under the laps, past the slate and onto the underlay. On a roof laid without an underlay, as many older Bath roofs were, that water goes straight into the loft. And it can push on slates whose nails are already weak, shifting them or snapping the tail.
What strong chemicals do
Household bleach and similar products are sometimes poured on roofs as a quick fix. They can discolour the slate, rust the fixings, kill the planting below and corrode the lead and metal gutters they run over. We say more on our page about using bleach on a roof. The treatments used by roof cleaners are chosen for the surface they are going on.
A third risk is the clean-looking result. A roof that has been blasted can look bright for a season, which makes it seem like good work. The damage shows up later as flaking, leaks at the laps and a surface that regrows quickly. If someone offers to clean your slate roof with a pressure lance and no inspection, that is the point to ask them what they will do about loose slates.
Lichen, black streaks and what cleaning will not change
Not every mark on slate is moss. Lichen, the crusty grey-green or orange growth, grips the surface tightly and can leave a faint shadow even after it has died. Black streaks, usually running down from the ridge or from a chimney, are a different growth, and our page on black streaks on a roof. Both respond to biocide treatment, but slowly, and the final look appears over weeks.
Cleaning also has limits that are better stated at the start. It will not restore the colour of slate that has weathered naturally, and it will not undo rust stains from fixings; those are marks on the stone, not growth on top of it. It will not make a delaminating slate sound. And it will not fix a roof that already leaks at a valley or flashing, although it may reveal where the leak is.
A fair expectation is a roof that is clean, free of moss, with an even look that continues to improve over the following weeks as dead growth weathers away. Where a weathered roof has patches of different shades, they usually blend as the treatment settles.
Slate that has been well kept and cleaned gently will often carry moss less quickly than rough, scrubbed slate. Shaded slopes, and those below overhanging trees, still grow it fastest. Trimming back branches does as much for the next clean as the clean itself, and we are happy to point out where that applies on a given roof.
Cost, access and the Bath factor
Typical UK ranges reported by MyBuilder's roof cleaning cost guide (2026) put slate roof cleaning at £500 to £1,000, against £400 to £750 for a tiled roof, and say that cleaning typically starts from about £300. Slate sits higher because it needs more care, more time on boards and more checking. The fixed figure comes after the survey, setting out the method, the access and what is included.
Access shapes the price more than people expect. A low slope over an extension can be reached by ladder and boards. A steep three-storey Bath terrace may need scaffold or a tower, and a parapet behind a stone wall needs a safe route across the roof. Where scaffold is needed, it is itemised separately in the quote. Some roofs cannot be cleaned economically for that reason alone, and we tell you clearly rather than suggest a method that cuts corners.
Bath adds its own considerations. The city boundary is the World Heritage Site boundary, the conservation area covers roughly half the city, and listed houses are everywhere. Cleaning is not normally a change that needs consent, but a listed roof deserves a conversation with B&NES Council before anything is applied, and our page on cleaning listed roofs goes through how that works. Tiled roofs, by contrast, call for a different approach altogether, which we set out in our guide to cleaning clay and concrete tile roofs.
Where the survey finds nails or slates that need attention, we price the repair separately and tell you whether it should come before the clean. Spring or early autumn tends to suit a slate clean best, when the moss is moist enough to lift without scraping the slate.