A roof built before the twentieth century was never meant to be airtight. Stone slates, clay tiles and natural slate were laid over timbers with gaps at every joint, and air passing through carried away the damp that rose from the house. Lime mortar, soft stone and softwood rafters all cope with moisture by absorbing it and then letting it go. Spray foam interrupts that cycle. It sits tight against the structure, holds moisture where it lands and gives it little route out. In a plain modern roof that can be managed, and a roof of that kind leaves room for a ventilated detail or a controlled vapour layer. In a house with solid walls, a Bath stone parapet or a roof of heavy stone slates, the margin for error is smaller. This guide explains why older roofs are more sensitive to foam, what listed building consent means in Bath, and what we recommend if foam has already been sprayed into a historic roof. Nothing here replaces advice from B&NES Council on your own building.
How an old roof deals with moisture
A traditional roof is a system, and every part has a job. The covering sheds the rain. The gaps between slates, tiles or stone slates let air move under them. The timbers, usually softwood, swell a little in damp spells and dry again when the air moves. Lime-based mortar and plaster in the walls take up moisture and release it slowly. Nothing in the arrangement is sealed, and nothing needs to be. Even the ridge and the eaves play a part, because they are where air enters and leaves, and a roof whose air paths are blocked will behave very differently from the one its builders had in mind.
That open character is the reason such roofs have lasted. Moisture that rises from the house, or blows in as spray, is carried out through the same gaps that let it in. A soft material such as lime does not trap the damp. It spreads it through the thickness of the wall and lets it evaporate from the surface.
The principle is familiar from stonework. At 7 Walcot Buildings the Trust asked for repointing "in a lime mortar to allow the historic stonework to 'breathe'", and a roof deserves the same thinking. Anything that stops a historic building from breathing shifts the moisture into whatever part is least able to take it, which in a roof is often the timber or the stone.
That is why we treat an older roof as a whole before touching any part of it, and why we are wary of insulation that seals one layer off from the next. A repair that suits a modern roof can do quiet damage when it is copied onto a historic one, and the damage often shows only years afterwards.
Why foam is a particular risk in a breathable roof
Spray foam grips whatever it touches, whether that is rafters, the back of slates or the inside face of a parapet. In a roof made of many small units laid with gaps, that creates three problems.
It covers the wood
Old rafters, purlins and wall plates are often the original timbers, and they hold up a heavy covering. Foam over them hides their condition and can hold damp against them. Our guide to spray foam, condensation and roof ventilation covers how condensation forms behind it.
It fills the joints
Foam expands into every gap. On a stone slate roof or a roof of old clay tiles, it can lock slates together, glue them to battens and bridge the lap between one course and the next. Taking a slate off later may then break it, and the foam can stain or mark the underside, which matters when the roof is being preserved rather than renewed.
It is hard to reverse
Foam is not a material that comes away cleanly. Removing it from historic fabric takes care, and the more irregular and soft the surface, the greater the chance of damage to what it clung to. The methods are described in how foam comes off a roof.
Bath roofs: World Heritage, the conservation area and listing
Bath carries three layers of designation, and they are easy to confuse. World Heritage status was given in 1987, and the UNESCO boundary follows the city boundary. A conservation area managed by B&NES takes in roughly half of that ground, at 1,486 hectares inside a 2,900 hectare site. Listing is separate and attaches to single buildings, of which the city holds around 5,000. For insulation, listing is the layer that changes what you may do, because it brings a consent process that an ordinary house never faces.
Roofs in and around Bath vary a good deal. Welsh slate over the main pitch and clay pantiles at the back are recorded in a Trust response about a single Grade I address on St James's Square. Council appraisals of villages to the south of the city mention clay tiles beside surviving Cotswold stone tiles, which the Wellow appraisal calls rare and says should be preserved. Each of these coverings is laid on timbers and with detailing that expects air to move.
Many Bath houses also have features that are especially sensitive to trapped moisture. A mansard pairs a near-vertical lower pitch with a shallow upper one, and the narrow voids that result are easy to fill with foam. A parapet gutter, which is sheet lead dressed into a channel behind the parapet wall, runs beside roof timbers that need to stay dry. A valley between two slopes gathers water, and if the roof has foam in it, a leak there may go unseen. We explain these terms again wherever they matter on a job.
Listed building consent and foam
If your house is listed, contact the council before any work that touches its character. In this city the body to ask is B&NES Council, and its guidance makes two points: the two permissions differ from each other, and unauthorised works to such a building count as a criminal offence. The council's own page is here: B&NES listed buildings.
We are not able to tell you whether a particular insulation job needs consent, because that depends on the building, the grade, the extent of the work and how it affects the historic fabric. The safer course, in our view, is to ask the council before spraying and not afterwards. Consent is also a practical protection. If the council has agreed a method, you have a record of why it was chosen.
The same question arises when foam is to be removed. Taking foam off timbers or stone slates in a listed roof is work to the building, and it is worth asking B&NES whether consent is needed before we start. We help with that conversation. Where removal is straightforward and does not alter appearance or fabric, the council may take a different view from work that involves replacing slates or opening up the structure.
Where a house is unlisted yet stands inside the conservation area, fewer rules bite, although the council still watches how roofs appear from the street. Some streets have Article 4 directions that take away certain permitted development rights; it is sensible to check the council's policy map. Our spray foam guarantee page explains what installer paperwork and guarantees can and cannot do for you when something goes wrong.
If foam is already in a historic roof
Owners tend to find out in one of three ways: a survey on purchase, a leak or damp patch that leads to someone looking in the loft, or a conversation with a mortgage lender. Whichever it is, start by establishing what was done. We want to know the foam type, how much was sprayed, where, and when. If there are papers from the installer, they are worth finding.
Then a person needs to look. A survey will tell you whether the timbers are dry, whether the foam has glued itself to slates or stone, and whether the roof can come out of the foam without harm. That is a different question on a nineteenth-century slate roof from a modern one, and it is why a spray foam survey is the usual starting point.
Our approach in older buildings is to remove only what is necessary and to protect what is original. Where the foam sits on the face beneath the covering and the timbers are sound, removal may be a case of careful scraping and clearing. Where the foam has fastened itself to stone slates, we may be able to lift them in sections and clean them, and we will tell you before we start if we expect to lose some. Replacement slates or tiles are matched in material, size and colour to what is there, because a historic roof should remain a historic roof.
Afterwards, insulation can go back to suit the house: within the rafter depth with an air gap kept open above, or laid at ceiling level. Those choices are weighed in our guide to alternatives for lofts and roofs.
Mortgages, surveyors and older houses
Foam has made lenders wary. RICS published a consumer guide in March 2023 that tells owners to confirm with the lender that such products are allowed under its policy. A BBC investigation reported on 20 November 2024 found that some lenders refuse to lend against a house with foam in the roof, and others judge each application separately. Policies may have moved since then.
For an older house the concern is greater because surveyors who value historic buildings look at moisture and timber condition closely. A buyer's surveyor who sees foam in a Georgian or Victorian roof may ask for the timbers to be checked and for evidence that the work did no harm. Without that, a sale can stall.
This is not a reason to panic, and foam does not mean a house is unsaleable. It does mean that a clear record helps. An independent survey that shows dry timbers is far more useful to a buyer than an assurance, and if work is needed, a documented repair is better than none. We look at the sales side separately in our guide to selling a house with spray foam. The full range of our foam work sits under spray foam insulation.