Spray foam and lenders
Spray foam sits on a breathable membrane under slates. Why is that a different moisture question from foam on a felted cold roof?
Foam on a breathable membrane is a different moisture question from foam on a felted cold roof, because the membrane is meant to let vapour pass and the felt is not. Spray foam sitting on that membrane under slates can block the path the membrane was there to provide. The survey has seen foam on the membrane. It has not sampled the foam, has not measured moisture in the slates, and has not treated this roof as if it were a felted cold roof.
Last reviewed 24 October 2026. Written for buyers in England and Wales unless this answer says otherwise. No named surveyor has signed this page. It is general guidance, not a report on your house, and not legal advice.
What is the fuller explanation?
A felted cold roof already has a vapour-resistant layer rather than a breathing one. Foam applied to that felt changes a roof whose underlay was never meant to breathe. Foam on a breathable membrane changes the opposite idea: the underlay was chosen so vapour could leave, and the foam may close that route under the slates. Those are different build-ups. The visit can say which one it found. It cannot tell you the moisture content, and it cannot tell a lender what to decide.
Ask the seller for any installation record for the foam, and pass it to the solicitor with this distinction intact. If a lender asks about spray foam, describe the membrane and the slates rather than answering as if the roof were felted. A roofing specialist can inspect the build-up if the covering must be understood. The home survey has not tested the foam and has not certified either roof type as satisfactory.
What should you ask your surveyor?
- Is the foam sitting on a breathable membrane under slates, rather than on felt?
- Did you leave moisture in that build-up unmeasured?
- Have you kept this roof distinct from a felted cold roof?
What are the typical next steps?
- Describe foam on the breathable membrane, not foam on felt.
- Pass any foam installation papers to the solicitor.
- Do not tell a lender this roof was moisture-tested.
Costs vary with the building and with who you instruct. Where this page does not cite a published figure, ask the surveyor, a relevant specialist, or your solicitor for a written quote. Do not treat a forum average as the price.
Also asked about this
If the loft feels dry, has the membrane question gone away?
No. A dry-feeling loft is not a measurement of the membrane under the slates. The foam is still on a layer that was meant to breathe. The survey has not sampled that foam and has not treated the roof as a felted cold roof.
Does the same foam note apply if felt is found on another slope?
Only if that slope actually has felt. A felted slope is the other moisture question, and it should be written as felt. This finding is foam on a breathable membrane under slates. Do not copy one build-up onto the other.