Spray foam and lenders
Spray foam has been applied to the underside of a felted roof slope. Why is that a different lender question from foam on a slate roof with a ventilated void?
Foam under a felted slope is a different lender question because the foam sits against a covering that has no ventilated void the survey can describe. On a slate roof that still has a ventilated void, there is an air path and a gap in which the underside of the slates and the rafters can at least be discussed. Under felt, the foam closes the space against the membrane, so timber, felt and moisture cannot be separated by looking. The survey has recorded where the foam is. It has not tested the foam, has not sampled the timber, and has not made a lending decision.
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?
Lenders ask about spray foam because it can hide the roof structure and change how moisture behaves. The question is not the same on every roof. A felted slope with foam on its underside is one build-up: covering, then foam, with the rafters buried. A slate roof with a ventilated void is another: an air space remains, and the foam, if present, is not the whole picture of that roof. The home survey can describe which of those it found. It cannot tell the lender what to decide.
Put the actual build-up in front of the lender and the solicitor: felted slope, foam on the underside, no ventilated void seen. Ask the lender what they need, which may be a specialist report, rather than assuming the answer is the same as for a ventilated slate roof. The survey has not removed any foam and has not certified the felt. Do not cut the foam out on a viewing to create a void that was never there.
What should you ask your surveyor?
- Did you find foam directly under the felt, with no ventilated void?
- How does that differ from a slate roof where a void was still open?
- Have you left the lending decision out of the survey?
What are the typical next steps?
- Describe this as foam under felt with no ventilated void seen.
- Ask the lender what they need for this build-up, not for a different roof.
- Do not cut the foam away on a viewing.
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 felt outside looks new, does the lender question go away?
New-looking felt does not open a void under the foam and does not answer a lender. The survey still found foam on the underside of a felted slope, which is a different question from foam beside a ventilated slate roof.
Is foam on a ventilated slate roof automatically acceptable to a lender?
The survey has not said that either. A ventilated void can be described, and that is a different question, but the lending decision is still the lender's. This visit did not make it for either roof. No later decision about the lender was made, and this visit did not supply one.