Roofs and flat roofs
A blister in the felt was not opened. Is it a leak, trapped vapour, or still unnamed?
Still unnamed. A blister in the felt was seen and was not opened, so the survey has not called it a leak or trapped vapour. Blisters can form when air or moisture is held between layers, and they can also sit over a path that lets rain in. Cutting one is a destructive check this visit did not do. From the surface the surveyor can note size, position and whether the felt nearby is split or holding water. Those clues do not choose the cause. The report records an unopened blister. The cause remains unnamed.
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 blister may be air or vapour under a sheet that is still continuous, which is not the same as a hole. It may also crown a failed bond over a deck that has already taken water. Opening it would show the layers, and it could create a leak if it is left unfinished. The survey leaves that cut to a roofer who can make it good. Until then, naming it a leak or harmless vapour both go past the evidence.
Tell your solicitor the flat roof has an unopened blister and that the cause is unnamed. A contractor asked to quote should see the roof, not inherit a diagnosis the survey refused to make. A lender should not be told there is a proved leak at that spot unless the report found one. The survey has not specified stripping the roof and has not cleared the felt. If the defect needs a name, a roofer can open the blister and make it good. This inspection stopped at the surface.
What should you ask your surveyor?
- Where on the roof is the blister, and is the felt split or only raised?
- Does the ceiling below show a stain in line with it?
- Was standing water noted around the blister, or was the surface dry?
What are the typical next steps?
- Leave the cause of the blister unnamed until it is properly opened.
- Do not tell a lender it is a proved leak or proved trapped vapour.
- If it must be named, have a roofer open it and make good what this visit left closed.
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
Will a blister always become a leak?
The survey has not said so. Some blisters stay as they are and some split. This one was not opened and was not given a future. Watching the felt does not finish the diagnosis. A roofer can advise once the layers are seen. The report's position is simply that the cause is still unnamed.
Can you press the blister to feel whether it holds water?
Pressing is not opening the layers and can damage aged felt. The survey did not open the blister and did not treat a push as a test. A soft crown can be air, vapour or water, and you would still not know which. Leave it intact for a roofer if the cause needs a name.