Age and type of house
One bay of a lath-and-plaster ceiling sags, and the next bay is flat. Does the sagging bay mean the whole floor structure above has failed?
The sagging bay of lath-and-plaster does not mean the whole floor above has failed. One bay has dropped and the next bay is flat, so the defect is local to that ceiling. Lath can lose its key, and plaster can sag, without the joists having failed as a floor. The survey can record the bay it saw. One patch is not the whole ceiling, and it is not a verdict on the floor structure.
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 lath ceiling hangs from the joists by plaster that was pushed through the laths. Where those keys have broken, that bay can belly while the next bay stays flat. The pattern points to the ceiling finish in the bay you can see. It does not, on its own, show rotten joists or a failed beam across the room.
The floor is a separate question, answered only where boards were lifted or joist ends were visible, if they were at all. Do not invent a price for the plaster, and do not give the laths a lifespan. Ask what was seen from above. Until then the sagging bay stays a ceiling comment, not a condemnation of the whole floor.
What should you ask your surveyor?
- Was the sag confined to one bay, with the next bay still flat?
- Did you see the joists from above, or only the underside of the lath ceiling?
- Are you separating this ceiling finish from the floor structure?
What are the typical next steps?
- Keep the comment on the one sagging bay, not on the whole floor.
- Do not assume every lath ceiling in the house has failed with it.
- If the joists need to be seen, agree an opening rather than guessing from below.
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
Can one sagging bay stand in for the rest of the ceiling?
No. The survey recorded one bay, and the next bay was flat. The rest of the ceiling has not been condemned from that sag.
Does a sagging lath ceiling prove the joists are rotten?
No. The sag is the ceiling finish. Rot in the joists would need those timbers to be seen, which the shape of one bay does not do.