Age and type of house
A lath-and-plaster ceiling sags in a 1930s bedroom and the joists above were not seen. What has the sag not proved about the joists?
The sag has not proved anything about the joists, because the joists were not seen. It shows that the lath-and-plaster ceiling in this 1930s bedroom is distorting. Laths can lose their grip and plaster can drop while the joists above remain. The joists could also be in trouble, and that was not checked. The survey must not turn a sagging ceiling into a joist failure, or into a clean bill for the joists. Neither finding was available from the ceiling alone.
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?
Lath and plaster is a heavy ceiling fixed from above to the joists. Age, vibration and failed key all let it belly. The failure can be in the plaster and the laths. It can be in the fixing. It is not, without a view from the void, a measurement of the timber. The surveyor did not see those joists. They should say so. A crack or a sag is a reason to look, not a diagnosis of rot, beetle or an undersized joist.
If the void can be reached safely, a further look at the joists is a separate access question, not a reread of the sag. Do not hack the ceiling down yourself. Ask your solicitor what the seller knows about previous ceiling collapses or repairs. A timber specialist, if you instruct one, needs to see the joists rather than the bulge. The home survey has not inspected those joists and has not named a timber defect in them.
What should you ask your surveyor?
- Was there any view of the joists above this ceiling, or none?
- Is the sag recorded as a ceiling defect only?
- What would need to be opened before the joists could be described?
What are the typical next steps?
- Do not treat the sag as proof the joists have failed.
- Do not treat it as proof the joists are sound either.
- Arrange a view of the joists before anyone specifies timber repairs.
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 floor above feels firm, are the joists cleared?
No. A firm floor is how that floor felt underfoot. It is not a sight of the joists over this bedroom, which were not seen. The sag still has not proved their condition. Firm footing is not a sight of the joists over this ceiling.
Should the ceiling be screwed back as a joist repair?
Not on this evidence. Screwing a ceiling is a ceiling job, and it may not be the right one. The survey has not seen the joists and has not specified a repair to them. Nobody has specified a joist repair from a bulge alone.