Roofs and flat roofs
An intact blister sits in the felt of a flat roof over a bay. Why should that blister not be burst on a viewing to see what is inside?
Leave the blister unburst. It is a raised pocket in the felt over the bay, and the felt around it is still continuous. Bursting it on a viewing puts a hole in that covering so you can look inside, and rain can then enter a roof that had not been opened. The pocket may hold air, moisture or both, and popping it does not date the fault or tell you the deck condition. The survey left the blister intact. It has not sampled the inside and has not called the bay roof failed because of one unopened blister.
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?
Felt blisters when a layer lifts, often from trapped vapour or a poor bond. While the skin is unbroken the covering can still keep rain out, even if the blister shows the felt is not perfect. Opening it to satisfy curiosity turns a closed defect into a leak path you made. A survey records the blister it could see. It does not invite a buyer to knife it on a second viewing, and it does not become more accurate because the pocket was burst.
Ask a flat-roof contractor to advise whether the blister should be left, locally repaired or taken as part of a wider recovering. That advice belongs in their quote, not in a cut made on the day you view. Tell the solicitor the report notes an intact blister, not an opened-up bay roof. The survey has not specified a new felt system and has not said the bay structure is decayed. Keep people off the blister so it is not split by a foot.
What should you ask your surveyor?
- Did you leave the blister intact, without opening the felt?
- Is the blister on the bay roof only, or are there others?
- Was any stain recorded on the bay ceiling under it?
What are the typical next steps?
- Do not burst the blister on a viewing.
- Ask a flat-roof contractor what an intact blister implies for this bay.
- Keep the note as an unopened pocket, not a failed roof.
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 blister is soft underfoot, has the deck failed?
Softness under a blister was not a deck inspection. The survey did not open the felt and has not said the bay deck has failed. A contractor needs to advise before that conclusion is drawn. Asking about it again later still leaves this blister point outside what the survey decided.
Should the bay felt be replaced because one blister is visible?
Not from one intact blister alone. The survey recorded a raised pocket and left it closed. It has not specified a full replacement of the bay covering. The home survey itself did not pursue this felt question any further than the visit.