Can a sagging lath and plaster ceiling be saved?

Sometimes. Where most of the key survives and the sag is local, the ceiling can be re-secured through the plaster into the joists with washers or screws and adhesive. Where the nibs have gone across a whole room, overboarding or taking it down is the honest answer.

Is it safe to sleep in the room?

Not under a ceiling that has visibly dropped. A belly means the plaster is no longer supported by the laths, and collapse is triggered by ordinary vibration. Use another room and get it looked at rather than watching it.

Should the loft insulation come out?

Weight and traffic on top of an old ceiling both break the key, so taking storage and heavy boarding off that bay is sensible while it is assessed. Insulation is reinstated properly once the ceiling is secure, supported so it does not bear on the plaster.

Why is overboarding sometimes refused on an old house?

Because it adds weight to laths and joists that may already be at their limit, hides the condition behind it, and buries cornice detail. On a building where the fabric matters, re-keying or careful renewal is preferred to boarding straight over.

High risk

Updated 30 August 2026

Sagging Lath And Plaster Ceiling

An old ceiling has dropped, bellied downwards, or sounds hollow and gritty when tapped.

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What's happening?

A lath and plaster ceiling is held up by nibs. Wet lime plaster was pushed through the gaps between riven timber laths, and it squeezed out on the far side and set into thousands of small mushroom-shaped keys hooked over the timber. That is the entire fixing. When those nibs break — from vibration, from a leak, from a century of drying and movement — the ceiling stops hanging on the laths and starts hanging on itself, its paint and its paper. It goes on looking like a ceiling while carrying nothing, then bellies down, then comes away in a slab. This is the one plaster fault that regularly injures people, because a square metre of lime plaster on lath is heavy and it lands all at once rather than crumbling.

What usually causes it?

Age and vibration breaking the plaster nibs that hook over the laths, so the key is progressively lost
A water leak from above soaking the laths and softening the lime plaster around them
Nail sickness — the cut nails holding the laths to the joists corroding until the laths themselves drop
Loft insulation, boarding or storage laid straight onto the ceiling joists, adding weight and disturbance
Someone working in the loft standing between joists, punching through or shocking the key loose
Heavy fittings screwed into the ceiling rather than into the joists
An earlier repair that skimmed or overboarded the surface without addressing the lost key behind it

Is it dangerous?

Yes. This is the highest-risk plaster fault in a house and it is treated as urgent. A ceiling that has visibly bellied is already carrying itself, and there is no reliable warning between belly and collapse — the trigger is often something ordinary such as a door slamming or someone crossing the floor above. Stop using the room, keep children and pets out, and do not put a bed, a cot or a sofa under it while you decide. Do not push it back up. Older ceilings often also have textured coating over them, and if that is disturbed HSE's rule is to stop work immediately and assume asbestos until it is identified, which matters as much as the sag itself.

Can I fix it myself?

No. There is no safe amateur version of working under a ceiling that is already unsupported, and propping it is not the same as fixing it — a temporary support carries the load while you decide, and that is a job for someone who does it. What you can do is stop making it worse: keep off the floor above where possible, take weight out of the loft above that room, and photograph the sag against a straightedge or a laser line with the date so the movement is measurable. If plaster or dust is dropping, clear the room rather than sweeping under it.

When should you call a plasterer?

Call a plasterer or a builder now if any part of the ceiling has dropped, if it moves when the floor above is walked on, or if plaster dust is falling. Call before work starts if the ceiling carries decorative cornice or a ceiling rose worth keeping, because saving those changes the method. If the ceiling has a textured coating, it needs identifying before anyone disturbs it, and SPAB's advice is the starting point on any older building where the fabric itself matters.

What will the plasterer actually do?

Assess how much key is left by tapping and inspecting from above where access allows, and prop what is unsafe
Decide between re-securing the existing ceiling and taking it down, based on the extent of key loss
Re-key sound areas with plaster washers or screws and adhesive where the ceiling is worth conserving
Overboard or take down and reboard where the key has gone, fixing into the joists rather than into the laths
Take down the old ceiling with dust control and, where a textured coating is present, only after it has been identified
Reinstate cornice, ceiling roses and mouldings, and finish in a plaster compatible with the rest of the building

Frequently asked questions

We’re happy to answer all your questions.

Sources & References

Cracks Across A CeilingCaution Artex Ceiling You Want Removed Or SkimmedHigh risk Crumbling Lime Plaster In An Old HouseCaution Water Stain On The Ceiling But No Obvious LeakCaution

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