Roofs and flat roofs
Blisters line up along a day joint in the felt and the rest of the roof is smooth. Why is that a joint note rather than a failed deck everywhere?
It is a joint note because the blisters follow one day joint and the rest of the felt is smooth, so the survey is not describing a failed deck across the whole roof. A day joint is where one spell of felt work meets the next. Blisters lined up on that line point to the lap: poor bond, trapped moisture or a ridge in the joint. The survey has seen that pattern. It has not cut a blister, tested the deck, or found the boards rotten from edge to edge.
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 form when air or vapour is held under the sheet and the covering lifts. When they follow one joint and the field either side is smooth, the note belongs to that joint between two days of work, not to a deck that has failed everywhere. The boards could still be damp under the line. The survey cannot see through the felt, and recording the line is not the same as opening it.
Ask a roofer to advise on that joint, which may mean dress it down or cut and patch the line, once they have seen it. Do not let a whole-roof replacement be priced as if the survey had found a failed deck everywhere. Tell the solicitor the note is the day joint. The home survey did not open the blisters and did not condemn the deck beyond that line.
What should you ask your surveyor?
- Do the blisters follow one day joint, with smooth felt on either side?
- Did you open any blister, or is the note visual only?
- What, if anything, could you tell about the deck under that line?
What are the typical next steps?
- Read the blisters as a day-joint note, not a failed deck everywhere.
- Ask a roofer to look at that joint before anyone prices a whole roof.
- Do not treat unopened blisters as a test of the boards.
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
Should every blister on a flat roof be read as a rotten deck?
No. A blister is a lift in the felt. These follow a day joint while the rest of the roof is smooth, so the note stays on the joint. The survey did not cut the felt and did not find rot from edge to edge. A local deck fault is still possible under that line, and only an opening would show it.
If the blisters are firm underfoot, has the joint healed?
Firmness underfoot is not a repair of the lap. The survey saw blisters along the day joint and did not press them as a test, and it did not bond the felt back down. A blister can feel firm and still be a poorly made joint. The rest of the roof being smooth does not close the line.