Roofs and flat roofs
What should you make of an intact blister in flat-roof felt?
Treat it as trapped air or moisture, not as proof the covering has already failed. A blister can sit unsplit or open later, and a home survey can record one it could see and whether the surface has cracked, but it cannot date the felt from the blister or promise a leak. A blocked outlet is a separate comment.
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?
Blisters worry buyers because they look like the roof is boiling. Many are old, stable and still waterproof. The condition sentence should say if the blister is intact, foot-worn, or already split so the layer below shows. A survey from a window may see the dome and miss a split on the far side. It should not say the whole roof has failed because one bubble was visible, and it should not ignore a blister that has opened.
A roofer may recommend dressing a split blister, or recovering the roof if the felt is generally at the end of its useful covering. That judgement needs a look at more than a photograph in the survey. There is no RICS rule that one blister equals a new roof. Ask for the quote to say how many blisters were open. Do not walk on the blister to test it. Foot traffic is how sound blisters become holes.
What should you ask your surveyor?
- Was the blister intact or already split?
- How much of the flat roof could you see?
- Are you separating this from a blocked outlet?
What are the typical next steps?
- Ask a roofer to inspect before you treat one blister as a new roof.
- Do not tread on the flat roof to pop a blister.
- Read the outlet and the upstands as their own items.
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
Is a ripple the same as a blister?
A ripple may be a lap or a wrinkle. A blister is a raised void. The survey should use the word it can justify.
Does solar paint stop blisters?
The survey cannot promise that. Paint is not a repair of an open split.