Spray foam and lenders
Can a surveyor see rafter decay once closed-cell foam is stuck to the underside?
Often no. Closed-cell foam can cling to the rafters and the felt so the timber faces are hidden. The surveyor should mark the roof structure as not inspected where they could not see it. Hidden does not mean rotten. It means unknown.
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?
RICS explains that surveyors may be unable to report on the timber when foam covers it, and that some foams can trap moisture if the roof cannot breathe. The visible clues at the edges still count: a stained ceiling, a soft rafter foot near the eaves, a failed gutter. Those can be inspected. The middle of the hidden slope cannot.
A borescope through the foam is sometimes offered. Ask what it can actually see, and whether the holes themselves cause leaks. It is not a standard Level 2 item. If the only honest inspection is to strip the covering, that is a major job. Get a roofer’s method and price. Do not describe the roof as sound in your mortgage application if the report says it was not seen.
What should you ask your surveyor?
- Which timbers were hidden, and which eaves or ceilings could you still see?
- Are you marking the structure not inspected?
- Would a few borescope holes change your advice, or would they be theatre?
What are the typical next steps?
- Repeat “not inspected” to anyone who asks if the roof is fine.
- Investigate further only if the lender or your own decision requires it.
- Do not fill a lender’s form with a guess about timber you have been told was hidden.
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
Does open-cell foam hide the timber less?
It can be easier to see through, and it is still a limitation if it covers the faces. Ask the surveyor what they could see in this loft, not what a product brochure claims.
If the ceiling is dry, is the timber fine?
A dry ceiling is encouraging and incomplete. Water can sit in a rafter without staining the plaster below. That is why the hidden timber is a limitation.