Most cracks in ceilings and plaster are cosmetic: fine lines caused by plaster drying out, timber floors flexing, or the joints between plasterboard sheets moving slightly. They’re worth keeping an eye on but rarely mean anything structural. The ones to take seriously are cracks that keep widening, come with a sag or bulge, show water staining, or line up with cracking in the walls below.
This guide explains how to tell the difference, how old lath-and-plaster ceilings behave differently from modern plasterboard, and when to get someone to look.
Why do ceilings crack?
A ceiling is a thin, brittle skin fixed to timber joists that are always moving a little. Timber shrinks and swells with the seasons, floors flex when people walk across them, and the house itself moves with temperature. Plaster can’t stretch, so it cracks where the stress concentrates.
The common causes, roughly in order of how often we see them:
- Drying shrinkage. New plaster, skim coats and timber all shrink as they dry. Hairline cracking in the first year or two after building work or replastering is normal. NHBC’s guidance on cracking in new homes notes that shrinkage cracks commonly appear on ceilings beneath the joists, are usually a constant width and normally less than 2mm.
- Plasterboard joint movement. Straight cracks along the lines where boards meet, or around the edge of the ceiling where it meets the wall.
- Floor deflection. Bouncy floors above — long spans, notched joists, or heavy furniture and baths — flex the ceiling below.
- Thermal and seasonal movement. Ceilings under cold lofts or flat roofs move more, and cracks may open in winter and close in summer.
- Water. A leak from a bath, shower tray, pipe or roof softens the plaster and makes it crack, stain and sag.
- Structural movement. Less common, but cracks that continue from the walls into the ceiling, or appear alongside sticking doors and sloping floors, can reflect movement in the building itself.
Lath and plaster vs plasterboard ceilings
How a ceiling cracks depends a lot on what it’s made of. In Yorkshire’s Victorian and Edwardian stone terraces, many original ceilings are still lath and plaster; most houses built after the Second World War have plasterboard.
| Lath and plaster | Plasterboard | |
|---|---|---|
| How it’s built | Lime plaster pushed through thin timber strips (laths) nailed to the joists; the plaster “keys” behind the laths | Factory-made gypsum sheets screwed or nailed to the joists, joints taped and skimmed |
| Typical harmless cracks | Fine, irregular “map” cracking across the surface; cracks along the line of joists | Straight lines along board joints; cracks at the wall–ceiling junction; nail or screw “pops” |
| Warning signs | Sagging or a bulge, a ceiling that moves when pressed gently, cracks that open up suddenly | Sagging between joists, wet or brown staining, boards dropping at the edges |
| Main risk | Plaster keys break and the ceiling loses its grip on the laths — sections can come down | Water damage or poor fixing; heavy items loaded on the loft side |
Lath and plaster is heavy, and its weakness is the “nibs” of plaster that hook over the laths. Over a century, vibration, leaks and well-meaning work in the loft can snap them. When enough break, the ceiling sags and eventually a section can fall. Old ceilings often look worse than they are, though, and conservation guidance from bodies like SPAB stresses that repair is often possible — reattaching loose plaster with screws and washers, for example — once any cause such as a leak has been dealt with.
Plasterboard ceilings are lighter and more forgiving. Straight cracks along joints are usually the tape or skim failing rather than anything structural, and a “nail pop” (a small round bump or crack over a fixing) is cosmetic.
When are cracks in a ceiling a problem?
Most cracks are harmless if they are:
- hairline, or under about 1mm wide
- straight along board joints, or following the line of a joist
- stable, with no change over months
- in new or recently replastered work
They are worth investigating if you see:
- sagging, bulging or a dip in the ceiling, especially with a crack running around it
- a crack that keeps widening or gets longer, particularly over a few weeks
- water stains, brown tide marks, soft or powdery plaster, or a crack that appeared after a leak
- cracks continuing from a wall into the ceiling, especially diagonal ones from the corner of a door or window
- cracks along a ceiling–wall junction that open up on an external wall, with gaps at skirtings or sticking doors nearby
- a ceiling that moves when you press it gently with a broom head (do this carefully, and stop if it feels loose)
A sagging lath-and-plaster ceiling should be treated as a safety issue. Keep people out from under it and get it looked at promptly.
Cracks in plaster walls: the same rules apply
Plaster on walls cracks for many of the same reasons. Fine, random cracking in a new skim coat is shrinkage. Cracks at junctions — where an extension meets the house, around door frames, or where a stud partition meets masonry — are usually differential movement, two materials moving at different rates.
Plaster that sounds hollow when tapped has lost its bond with the wall behind. That’s common in older houses and isn’t automatically a problem, but large hollow areas may need replastering. Plaster that is crumbling, salty or stained at low level points to damp rather than movement; our damp checker can help you narrow that down.
The cracks that matter in walls are the ones that are tapered, diagonal, stepping through brickwork, visible on both faces of the wall, or growing. Our guide to what causes cracks in walls covers wall cracks in detail, and the cracks in walls checker shows where a crack sits on the BRE damage scale.
What does a crack in the ceiling mean when you’re buying?
A surveyor will note ceiling cracks and comment on whether they look like normal shrinkage and joint movement, or something that needs further investigation. In older houses we’re particularly looking at:
- whether lath-and-plaster ceilings show signs of sagging or loss of key
- whether cracking or staining sits under a bathroom, tank or roof valley, suggesting a past or current leak
- whether ceiling cracks line up with wall cracking or distortion, which points to movement in the structure
- whether the ceiling has been overboarded (new plasterboard fixed over an old ceiling), which can hide what’s underneath
A Level 2 survey is a visual inspection and won’t lift floorboards or open up ceilings — here’s what a Level 2 survey can’t see. If the house is older, altered, or has original ceilings throughout, a Level 3 survey gives more room to dig into causes.
What should you do about cracks in your ceiling?
- Photograph the crack with a ruler or tape in shot, and date it.
- Mark the ends lightly in pencil and check every few weeks. A crack that stays put is very different from one that is growing.
- Check above. Look for leaks from the bathroom, tank or roof, and check the loft for anything stored on the ceiling joists of an old lath-and-plaster ceiling.
- Fill and redecorate hairline shrinkage and joint cracks — a flexible filler or decorator’s caulk at wall–ceiling junctions often works better than rigid filler. Joint cracks that keep returning may need re-taping.
- Get professional advice if there’s sagging, staining, widening, or a link to cracks in the walls.
Who should you call?
For a sagging ceiling that’s clearly about to fail, a competent plasterer is the immediate fix — but it’s worth knowing why it failed first, or the new ceiling may go the same way. If you’re not sure whether a crack is cosmetic, a leak, or structural movement, an independent inspection answers that without anyone having repair work to sell you.
Our building defects investigation is designed for exactly that: establishing what’s causing a defect and what a proportionate response looks like. We’re based in Huddersfield and cover around a 90-minute radius across Yorkshire, Greater Manchester and Derbyshire, with a fixed fee quoted for the property. Get in touch with a photo and a short description of what you’re seeing.
Sources and further reading
- Cracking in the home — NHBC
- Lime or clay plaster ceilings — SPAB
- Repairing lime plaster ceilings — The Building Conservation Directory
- Assessing cracks in houses — BRE