Cracked and sagging ceilings
Most ceiling cracks are in the finish rather than the structure. The exceptions matter, because a ceiling is the one element that announces a problem in the floor above it before anything else does.
Why is my ceiling sagging in the middle?
In older properties this is usually lath and plaster failure, where the plaster has separated from the laths. It is a finishes problem rather than a structural one, but the ceiling can come down and should not be left.
Lath and plaster reaching the end of its life
Traditional lath and plaster works by pushing wet plaster through gaps between timber laths so it forms nibs that hook over the back. Those nibs are what hold the ceiling up. Over a century or more they become brittle, and vibration, damp or a bumped floor above breaks them.
The failure looks like bulging or bellying rather than clean cracking. The plaster separates from the laths and hangs in a shallow dome, sometimes over a large area, and it will feel spongy if pressed. That is a finishes failure and the ceiling is genuinely at risk of coming down, but the structure above it is usually untouched.
The choice is normally between overboarding and taking down and replacing. Overboarding is quicker but adds weight to joists that may be marginal, so on an older floor it is worth knowing what the joists can take before adding a layer of plasterboard across the whole ceiling.
When the ceiling is reporting the floor above
A straight crack running along the line of a joist, or a series of parallel cracks at joist spacing, usually indicates the floor above is deflecting more than the ceiling finish can accommodate. The finish cracks where it is restrained. That points at the floor rather than the ceiling.
Cracking around the perimeter, where the ceiling meets the walls, often means the floor above is moving relative to the walls, or that the walls themselves are moving. It is the same evidence as a gap opening between skirting and floor, seen from below.
New cracking that appeared after work was done above is worth taking seriously. A new bathroom, a heavy floor finish or notched joists all reduce the margin the floor was working to, and the ceiling is often the first place it shows.
Working out which one you have
The pattern distinguishes them fairly reliably. Bellying and sponginess without much cracking suggests plaster nibs failing. Straight cracks following joist lines suggest deflection above. Cracking concentrated at the perimeter suggests relative movement between floor and walls.
What happened recently narrows it further. Ceilings that have been quietly cracking for decades are usually finish related. Ceilings that started within months of work being done above are usually reporting the consequence of that work.
Where the floor above is implicated, the assessment is a floor assessment rather than a ceiling one: span, joist size, what was notched, and what load the floor now carries against what it was built for. Replacing the ceiling without establishing that, ideally through a structural inspection report, is how people end up doing the work twice.
What we check.
The points our calculations resolve for a project like this.
- Whether the ceiling is bellying and spongy, indicating plaster nib failure
- Whether cracks follow joist lines, which points to deflection in the floor above
- Whether cracking is concentrated where the ceiling meets the walls
- Whether the cracking followed work carried out above, such as a new bathroom
- Capacity of the existing joists before adding weight by overboarding
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
We agree the survey, drawings or data we need and any site access required.
Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
Why is my ceiling sagging in the middle?
In older properties this is usually lath and plaster failure, where the plaster has separated from the laths. It is a finishes problem rather than a structural one, but the ceiling can come down and should not be left.
What do cracks in straight lines across a ceiling mean?
Cracks following joist positions generally indicate the floor above is deflecting more than the ceiling finish can take. The issue is the floor rather than the ceiling.
Can I just overboard the ceiling?
Often yes, but it adds weight across the whole floor. On older joists that are already working hard, it is worth checking capacity before adding a layer of plasterboard.
My ceiling cracked after a new bathroom was fitted upstairs. Why?
Commonly because joists were notched for pipework, or because the new floor and fittings are much heavier than what they replaced. Both increase deflection, and the ceiling below is usually where it first shows.
Related in structural problems & symptoms.
Tell us what you are building.
Send the drawings or describe the project. We confirm scope, deliverables and a realistic timescale.
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