If you own a listed house or live in a conservation area in Bath, a green or blackened roof raises a question that other owners do not face. Can it be cleaned at all, and how? The risk is real. Old slate, hand-made clay tiles and weathered Cotswold stone tiles are softer, more fragile and harder to replace than modern coverings, and a rough clean can damage them for good. At the same time, moss left to build up can block gutters and valleys, and a roof that cannot be inspected hides its faults. This guide sets out the permission questions that come first, then the methods that suit historic coverings and those that do not, with the detail that matters on a Bath terrace: parapet gutters, lime-pointed ridges and lead. Nothing here is legal advice, and planning questions belong with B&NES Council.
Check what permission applies before any work
Start with the status of the building. Every roof in the city falls within the World Heritage Site, because its boundary follows the city's. The conservation area is a smaller territory, covering roughly half, and about 5,000 buildings carry a listing. Your house could be listed, in the conservation area, both or neither, and the answer changes what you need to ask.
For a listed house, B&NES wants to hear from you before any work starts. Its guidance makes clear that a listed building calls for a separate consent, apart from planning permission, and that going ahead without it is a criminal offence. The detail is in the B&NES listed buildings guidance, and its conservation team takes calls on 01225 394041, Tuesdays and Thursdays.
Whether a cleaning job counts as work needing consent depends on what is done and how, and that is a decision for the council, not for a contractor. A light wash that leaves the roof as it was is a different thing from stripping a surface or applying a coating that changes its appearance. Put the question to the conservation team in writing and keep the reply.
Some Bath streets are covered by Article 4 directions, which remove specific rights to carry out work without an application. None of the published examples mention roofs, but your own street's position is a matter for the policy map. We can describe a method, and we can explain consent in ordinary language, but the council decides.
Why historic coverings need a gentler approach
A modern concrete tile is made to a standard and replaced from stock. A historic roof is not. Welsh slate on a Georgian terrace was chosen and laid for the building. Clay tiles made by hand differ from one to the next in shade and form. Stone tiles from the Cotswold tradition are thick, uneven and no longer easy to source. Bath Preservation Trust casework gives examples: a Grade I square house with natural Welsh slate across the pitch and pantiles behind, and a terrace where the Trust supported an artificial slate replaced by natural Welsh.
What makes these coverings vulnerable is age, not just material. Old slate can be soft or laminated, with edges that flake. Old nails may have corroded, so tiles are held less firmly than they look. A stone tile can have a weathered face that holds its character, and any aggressive treatment strips that face for good.
It also matters what sits around the covering. A roof in Bath often depends on lead lining valleys, parapet gutters and abutments, on lime-pointed ridges, and on pointing deliberately softer than the masonry. Force and strong chemicals reach all of it. Water driven under laps can wet old timbers and plaster. A clean that damages the lime pointing or lifts a lead flashing creates a repair worth more than the cleaning.
The test for any method is simple: does it leave the roof as built, with its surface, colour and joints intact?
Methods that suit historic roofs
On an old roof the aim is to remove growth with the least intervention, and to accept that the result will look weathered, not new.
Careful assessment first
Before any cleaning, the roof is inspected so that loose, cracked or slipped coverings are known and any bats or nesting birds are noticed. Both are protected, and work stops if they are present. Cleaning over unsound slates can dislodge them.
Low-pressure soft washing
A soft wash loosens growth with a cleaning solution at low pressure, so water is not driven under the covering. It is the method we describe in soft washing roofs. MyBuilder's roof cleaning cost guide (2026) gives typical UK ranges of £15 to £20 per square metre, against £10 to £15 for pressure washing, which it says may damage delicate tiles. Your fixed quote follows the survey.
Targeted biocide
Fragile surfaces are better served by a treatment that kills the growth where it sits and leaves it to work loose and wash away over a matter of weeks, since nothing is scraped off. Read the data sheet for the specific product, and see our guide to roof biocide for how treatments work.
Soft brushing by hand
For thick moss on valleys, gutters and ridges, hand removal with a soft brush or by gloved hand protects the surface. Dead growth is gathered, not rinsed down the slope.
Methods to avoid on a listed or conservation roof
Some approaches are common on modern roofs and wrong for historic ones. If a contractor proposes any of the following on a protected building, stop and ask the conservation team before you agree.
- Pressure washing. A hard jet can strip the face from soft slate, clay and stone, open joints and force water under laps. Our page on pressure washing roofs goes through the damage in detail.
- Wire brushes and scrapers. They scar the surface and leave a rough face that holds more growth.
- Strong or untested chemicals. Harsh products can stain stone, attack lead and affect the pointing. Household bleach is a poor choice for the same reasons, as our page on bleach on roofs explains.
- Coatings and sealants. A film changes how a roof looks and breathes, and on a listed building a change of appearance may need consent. See our assessment of roof coatings.
- Cement-based repairs during cleaning. On a Walcot Buildings house the Trust wanted lime used for pointing, observing that cement may have added to the damage on a chimney stack.
None of this means a historic roof must stay dirty. It means the method is chosen for the covering, not for the speed of the job.
Roof by roof: slate, stone, clay and lead
The covering decides the detail. A short guide to the common Bath types follows, and these are places to begin the discussion, not rules.
Natural slate
Slate is dense and tolerates a careful soft wash and treatment well, but old slates may be soft, and nails may have corroded. Pushing on a slipped slate can fail it, so the survey checks first. Our page on cleaning slate roofs covers the detail.
Stone tiles
Weathered stone is the most delicate. In the villages south of Bath, B&NES's own appraisals single out surviving Cotswold stone tiles as rare and worth keeping. Cleaning is by the lightest method, and often the answer is to leave lichen on a sound surface.
Clay tiles and pantiles
Hand-made clay is porous, so moisture and frost matter. Gentle washing is suitable, but a coating would trap water, which is another reason to treat before sealing is even considered.
Lead and lime work
Lead flashings, valleys and parapet gutters are checked and left undisturbed. A parapet gutter is the drain formed by lead laid between the roof slope and a wall that rises above it. Lime-pointed ridges are brushed, not blasted.
What we do and how to plan a clean on a protected roof
On a protected Bath roof we begin with a survey. We get up on the roof and photograph your own slate, stone or tiles, and note loose coverings, the state of the lead and any signs of bats or nesting birds. You see the photographs and a written plan of the method before work is agreed.
From there the order is straightforward.
- You confirm the building's status and ask B&NES whether the proposed work needs consent.
- We agree the method in writing, with soft washing, treatment or hand removal chosen for the covering.
- We repair what the survey found, matching materials, so natural slate stays slate and stone stays stone.
- Nothing is left behind: the area is cleared and the waste is carried away.
Where cleaning shows a fault, we stop and tell you before going further. A leaking valley or failed lead gets repaired as a repair, not hidden. Where repair works, we repair. If the house is listed, check with the conservation officer before any cleaning starts, as some methods may count as works that need consent.
Access deserves a plan of its own. A tall house may call for scaffold or a tower, and how it is erected matters on a listed front, where fixings into stone or pointing are best avoided. Boards sit clear of the slope, ladders are padded, and loose slates are never stepped on. Debris is bagged and carried off instead of being washed into a lead gutter or onto the pavement.
It is worth agreeing in advance what happens if the survey finds something unexpected. A hidden split in a lead valley, a rotten batten under a slipped slate or a loose ridge stone should be reported with photographs before any extra work is priced, so that you and, where needed, the council can see the reason for it. That keeps the job within what was discussed and what was permitted, and it avoids a repair being carried out on the roof that nobody had approved.
Timing helps too. A dry spell lets a treatment work and lets the roof be inspected without wet slate underfoot, and the survey photographs give you a record to show the council or an insurer if they ask later what condition the roof was in.
Treatments also matter for animals and plants, and owners often ask whether the product is safe once dry. We answer that in our guide on whether biocide is safe for pets. This work sits within our wider roof cleaning service. If you have a listed house and want a plan before you speak to the council, we can give you a clear description of the method to take with you.