Sagging Lath and Plaster Ceilings

Assessing from below and above

From below, look along the ceiling at a shallow angle in raking light. Bowing between joists, long cracks running with the laths and a hollow sound all indicate lost keys. Gentle upward pressure that produces visible movement means the ceiling is being held by friction rather than by its keys.

Wherever access allows, look from above in the loft or through a lifted floorboard. Broken keys are visible as a bed of loose plaster crumbs sitting on top of the laths, and that is the clearest evidence available.

Overboarding a partly sound ceiling

Where the ceiling is largely flat and only locally cracked, plasterboard can be fixed through the old ceiling into the joists and skimmed. Fixings must reach the joists — screwing into laths achieves nothing — and the joist positions need marking accurately first.

Adds weight, so the joists and their bearings need to be sound · Loses around 15mm of ceiling height, which matters under low period ceilings · Cornices, roses and picture rails need packing out or removing and refitting · Not appropriate where the ceiling is already visibly sagging

When the ceiling has to come down

A ceiling that is bowing, moving under hand pressure, or shedding plaster is beyond overboarding. Taking it down is dirty work — a century of soot and dust sits above it — but it gives access to wiring, allows insulation and sound deadening between joists, and produces a genuinely flat result.

In listed buildings and where original decorative plasterwork survives, specialist repair rather than replacement may be required, and that decision needs to be checked with the local conservation officer before anything is disturbed.

Do the services first

An open ceiling is the cheapest access a house ever offers. Rewiring, downlight positions, extract ducting, speaker cable and pipework should all be planned before boarding starts, because cutting them in afterwards means patching a brand new ceiling.

Frequently asked questions

Is a sagging lath and plaster ceiling dangerous?

It can be. A section of lath and plaster is heavy, and once the keys have gone it fails suddenly rather than gradually. A visibly bowing ceiling should not be left over a bed or a sofa while a decision is made.

Can a lath and plaster ceiling be saved?

Yes, where the keys are largely intact. Localised re-fixing with plaster washers and screws into the joists, followed by repair and skim, is a recognised method and preserves the original fabric.

How much height do you lose overboarding a ceiling?

Typically the board thickness plus skim, around 15mm with 9.5mm board or 20mm with 12.5mm. Under an already low period ceiling that can be enough to affect door heads and cornice lines.

Do I need to tell building control about a ceiling replacement?

A like-for-like ceiling repair generally does not require an application, but fire separation between flats and any structural alteration to joists does. It is worth checking before starting where the property is divided.