How to Fix a Sagging Lath and Plaster Ceiling

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.

Ceiling repairs as part of a renovation

A sagging ceiling is often one item on a longer list. Where the same project includes structural work, new layouts or other rooms, it is usually cheaper to sequence the ceiling within that renovation and building work than to treat it as a standalone job, because the access, making good and decoration all overlap.

What lath and plaster ceiling repair costs depend on

There is no honest per-metre figure, because the method is decided by what the survey finds. The drivers are consistent, though: the area involved, whether the ceiling can be overboarded or has to come down, the height and access, how much wiring or pipework needs sorting in the void, and whether original cornice or a ceiling rose has to be protected, packed out or refitted.

A small localised repair with plaster washers and a skim is a different job from a full take-down and reboard of a large reception room, and a quote should say which method is being priced and why. Where part of the ceiling is sound and part is gone, a mixed approach — re-fix and patch the sound area, reboard the failed area — is often the sensible middle course.

Fixing things to a lath and plaster ceiling

A lath ceiling that is staying put still needs care when anything is fixed to it. Screws into laths alone hold almost nothing — the lath flexes and the plaster around it crumbles. Anything with weight, from a pendant light to a pull-up bar, must be fixed through the plaster and laths into the joists, and the joist positions found with a detector or a small pilot hole rather than guessed.

Heavy items such as ceiling roses with large pendants, track lighting and hanging chairs deserve a spreader or a fixing that bridges two joists. If a planned fixing lands where the keys are already failing, that patch is repaired first — otherwise the new load finishes what gravity started.

Living with the ceiling while you decide

A ceiling with a few cracks and a solid note under the knuckle can usually wait for a planned repair. One that is visibly bowing, shedding dust or crumbs along a crack, or moving under gentle upward pressure should be treated as a near-term job, and the room below used with that in mind.

Photographing the ceiling in raking light every few months gives a useful record: a crack that is stable over a year is a different proposition from one that has widened or lengthened between photographs.