The short answer
Spray foam is not automatically bad, but sprayed onto the underside of an older pitched roof it carries real risks. It can trap moisture against rafters and battens, hide leaks and rot, and make mortgage lenders and surveyors cautious. Foam applied to the right structure by a careful installer can behave well, so the answer depends on your roof.
Why a roof is a fussy place for foam
Spray foam starts as two liquids mixed at the nozzle, which swell as they cure and set rigid. Applied under a pitched roof, it sticks fast to the tile backs, the timber strips that carry them, the membrane and the rafters beneath. That one move changes how the whole roof behaves.
A traditional pitched roof is built to breathe. Air moves through the loft, damp rises from the rooms below, and the timbers dry out in a normal cycle. Fill the rafter spaces with foam and that movement stops. Any moisture already in the timber, or any that finds its way in later through a displaced slate or a worn flashing, is sealed against the wood with nowhere to go.
A March 2023 RICS guide aimed at householders deals with foam, and it cautions that moisture can build up unchecked and harm parts of the building. That wording matters because it comes from the body that sets standards for surveyors, not from a roofer with something to sell. You can read the guide on the RICS website.
The risks, one at a time
It helps to separate the problems, because they do not all apply to every roof.
- Hidden timber decay. Foam stuck to the rafters stops anyone inspecting them. If the wood was damp when it was sprayed, or became damp afterwards, rot can build for years without a visible sign.
- Hidden leaks. A displaced slate or a cracked lead flashing used to show up as a stain on the loft side of the roof. Under foam, water can run sideways before it reaches the ceiling, so the stain appears far from the cause.
- Damage to the covering. Foam can glue itself to the back of the covering. When one needs replacing, lifting it may crack it and the neighbours.
- Lender and buyer caution. Certain lenders have refused loans on houses with foam under the roof, while others weigh each one separately. Our page on spray foam and mortgages explains what surveyors usually ask for.
- Vent paths blocked. Eaves and ridge ventilation designed to keep an old roof dry can be sealed over by foam.
Each of these is a risk, not a certainty. A roof that was dry, sound and well detailed before the foam went on has a far better chance than one that was already leaking.
Where spray foam can work well
A fair answer has to include the cases where foam does its job. Plenty of installers use it, and not every roof it has been fitted to is in trouble.
The best outcomes tend to share a few features. The roof was dry and in good repair before work began. The foam was applied in even layers by a trained installer, to the manufacturer's instructions. Ventilation was thought through rather than ignored. The owner kept the paperwork, including product data and any installer guarantee, so that a surveyor or buyer can see what was done.
Foam also suits some awkward shapes where board or quilt insulation is hard to fit, such as irregular cut roofs or rafters of uneven depth. The same bond that causes trouble on an old slate roof can be less of an issue on a modern roof with a sarking membrane and fully ventilated eaves.
Calling foam good or bad in general misses the point. The better question is whether this foam, on this roof, was fitted with the roof's construction in mind. Our page on spray foam and fire risk covers one concern people raise separately.
Warning signs to look for in your loft
You do not need to be a roofer to spot some of the early signs. Go up with a torch on a dry day and look at what you can see.
- Dark staining or a musty smell in or near the foam.
- Foam that has pulled away from the timber, or timber that looks black, soft or crumbly where it is exposed.
- Damp patches on the ceilings below, particularly after wind-driven rain.
- Slates or tiles that have slipped, with foam visible through the gap.
- Droplets under the covering, or on exposed metal fixings.
None of these proves that the foam is the cause. Each is a prompt to get the roof checked. A damp reading on exposed rafters, taken with a moisture meter, is more reliable than guessing, and a camera can reach spaces you cannot.
Anyone selling or remortgaging may find a buyer's surveyor raises the foam regardless. Having the answers ready saves weeks.
What to do if you already have foam
Having foam in the roof does not mean it must come out. Our guide to removing spray foam explains how we weigh it, and the decision usually rests on evidence rather than fear.
The first step is an assessment. We note the foam type, the state of the timbers where they can be seen, the state of the covering and the flashings, and whether the loft is ventilated. Where the foam covers a roof in dry, good order, leaving it can be a reasonable call, particularly if a surveyor or lender is happy with the evidence. Where timbers are damp or decayed, or the foam has hidden a leak, removal and repair are likely to follow.
If removal does turn out to be needed, UK cost guides give typical figures. MyBuilder's 2026 spray foam removal guide reports typical removal of around £20 to £50 per square metre, averaging about £1,500, with an assessment at about £100 to £200 and disposal at about £150 to £400. Those are national averages from one cost guide, not Empire prices. We give a fixed quote only once we have surveyed the loft. Our spray foam removal cost page goes through what moves the number.
Whatever you decide, do not try to cut or scrape foam out yourself. Dust, decayed timber and awkward access all make that a poor idea.
Our view on the balance
Foam is a tool that was used too freely on roofs it did not suit. For an older pitched roof, especially one covered in natural slate or stone tiles, the risks usually outweigh the benefits. For a newer, well ventilated roof that was dry when sprayed, they may not.
We would rather tell you to leave sound foam where it is than remove it for the sake of it. Booking a visit puts you under no obligation, and the written findings carry pictures of your own loft and slopes. If repairing it would be enough, we recommend a repair. Where the timbers are damaged, we explain which ones and why. If you want the hub for everything else on this subject, start at our spray foam insulation page.
