Where this starts
WHAT IT IS MADE OF DECIDES EVERYTHING, and it can usually be established without touching it. LATH AND PLASTER is thin timber laths nailed across the joists with plaster pushed through the gaps, forming keys that hook over the back of the laths — the plaster is held by those keys and by nothing else. PLASTERBOARD is sheet fixed with nails or screws directly to the joists, and the board is held by its fixings.
THE FAILURE MODES ARE NOT COMPARABLE. When lath-and-plaster keys break — from vibration, from a leak, from a century of thermal cycling, from somebody laying loft insulation on top of it — the plaster is held up by friction and by the ceiling below it, and it can release suddenly over a large area. When plasterboard fixings fail, the board sags between fixings and stays there; it is heavy, but it is one sheet at a time and it announces itself.
AGE AND LOCATION give a strong hint. A building from before roughly the middle of the twentieth century very likely has lath and plaster in its original rooms, sometimes with plasterboard over it from a later refurbishment — which is worse, because the plasterboard adds weight to a ceiling whose keys are already tired. Tapping gives the other hint: lath and plaster sounds solid and dead, plasterboard sounds hollow between joists.
WATER IS THE OTHER HALF. A ceiling that has been wetted — from a leak, a bath overflow, a burst pipe — has usually lost strength permanently whatever it is made of: plaster keys soften and break, plasterboard's gypsum core loses its integrity and the paper liner delaminates. A ceiling that dried out and looks fine after a leak is not the same ceiling it was.
The one thing not to do: push it back up or put a screw through the bulge. On lath and plaster that is exactly the force that breaks the remaining keys, and on wet plasterboard the screw pulls straight through.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
Lath-and-plaster keys have broken
Most commonA broad bulge or a dropped area on a ceiling in an older building, solid rather than hollow when tapped, often with fine cracking and a gap visible at the wall junction.
How to check: The age of the building and the sound when tapped. Solid and old means the plaster is held by keys, and keys break.
Water damage from a leak above
Most commonSag centred on a stain, often with a tide line, below a bathroom, a tank, or a roof defect, and the affected area soft or discoloured.
How to check: Whether there is a stain and what is directly above it. A wetted ceiling has lost strength whatever it is made of.
Plasterboard fixings pulling through
CommonSag between joists with the lines of the joists still straight, sometimes with visible fastener heads or popped filler, in a newer ceiling or a re-boarded room.
How to check: Whether the joist lines are still straight. Board sagging between them is a fixing problem, not a structural one.
Weight added on top
CommonSag under a loft where insulation, boarding or stored belongings sit directly on the ceiling or its joists, sometimes with the ceiling dropping where the load is.
How to check: What is in the loft directly above. A ceiling was never designed to be a floor.
The joists themselves deflecting
Less commonThe joist LINES dipping rather than the board between them, sometimes with a springy floor above, and a sag that follows the span rather than the sheets.
How to check: Sight along the ceiling. A dip that follows the joists is structure; a dip between them is finish.
Vibration or impact from work above
Less commonOnset following a loft conversion, a new bathroom, or work involving repeated hammering or a floor being lifted, on an older ceiling.
How to check: Whether anything was done above recently. Lath and plaster keys are broken by vibration as readily as by anything else.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
It is the only check on this page that has to come first. A failed lath-and-plaster ceiling releases a substantial weight at head height with no further warning.
- 2
Lath and plaster fails suddenly and over a large area; plasterboard sags and stays. The age of the building and the sound when tapped usually settle it without touching anything.
- 3
A wetted ceiling has lost strength permanently, and the leak has to be found anyway or whatever is repaired will be wetted again.
- 4
A dip following the joists is a structural question about the floor above; a dip between them is a finish question about the board and its fixings.
- 5
Boarding and storage placed on ceiling joists is one of the commonest causes and one of the easiest to reverse, and a ceiling was never designed to be a floor.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Immediately, where a ceiling in an older building has a broad bulge or a dropped area, since lath and plaster fails suddenly and a partial collapse is a serious injury.
- Where the sag follows the joist lines rather than sitting between them, which is a question about the floor structure above rather than about the ceiling.
- Where there is any staining, because the leak must be found before anything is repaired and a wetted ceiling rarely recovers its strength.
- Where the room is used by children or by anyone who could not move quickly, which changes the timescale regardless of what the ceiling is made of.
- Where the ceiling is in a building of historic interest, since a lath-and-plaster ceiling may need to be repaired in place rather than replaced.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.