The short answer
Plaster cracks for several reasons, and most of them are cosmetic rather than structural. The most common causes are drying shrinkage in new plaster, normal seasonal movement as a building expands and contracts with temperature and humidity, minor settlement as a house beds in, and the natural ageing of older plaster on lath or solid walls. These produce fine, hairline or stable cracks that are an aesthetic nuisance. A smaller proportion of cracks signal something more serious: structural movement, subsidence, failed lintels, or significant damp. The warning signs are cracks that are wide (more than a few millimetres), diagonal across openings, growing over time, or accompanied by sticking doors, sloping floors or external cracking. If a crack is widening or stepping through brickwork, get a professional assessment.
Cracking plaster worries people because it is hard to know whether you are looking at a decorating job or the first sign of a structural problem. The good news is that the majority of cracks are harmless. Here is how to work out which kind you have.
Plaster cracking — at a glance
- Most common causeDrying shrinkage and seasonal movement
- Usually harmlessHairline, stable, straight cracks
- Worth watchingCracks over 3 mm, diagonal, or growing
- Serious signsStepped cracks in brickwork, sticking doors
- When to actCracks that widen over weeks or months
The harmless causes — shrinkage, movement and ageing
The great majority of plaster cracks in homes are cosmetic. The usual culprits are:
- Drying shrinkage. Fresh plaster contains water, and as it dries and cures it shrinks slightly. This can produce fine hairline cracks, especially at the joints between plasterboards, around the edges of new work, or where a skim has been applied a little too thickly. These cracks are normal and are filled and painted over.
- Seasonal and thermal movement. Buildings are not static. Timber, plaster and masonry expand and contract as temperature and humidity change through the year. This constant gentle movement opens and closes fine cracks, often in the same places each year — typically above doors, at wall-to-ceiling junctions, and along plasterboard joints.
- Ageing of old plaster. In older homes, plaster on timber lath or applied directly to solid walls degrades over decades. Hairline crazing, fine map-pattern cracks and small areas of separation are part of the natural ageing of a material that has been on the wall for a century or more.
- Vibration and use. Slamming doors, heavy footfall, and nearby road or rail traffic all transmit small vibrations that can open fine cracks over time.
These cracks share a profile: they are usually narrow (hairline up to a millimetre or two), they do not run diagonally across openings in a stepped pattern, and they are stable — they are not visibly getting bigger month on month.
The cracks that matter — structural movement and damp
A smaller number of cracks indicate a problem with the building rather than the plaster. The causes to take seriously include:
- Subsidence and settlement movement. If the ground beneath a foundation moves — through clay soil shrinking in a dry summer, a leaking drain washing away support, or nearby tree roots drawing moisture from the soil — the building moves with it. This produces cracks that are wider at one end, run diagonally, often appear near windows and doors, and frequently step through the mortar joints of the brickwork outside as well as the plaster inside.
- Failed or overloaded lintels. The beam over a door or window opening (the lintel) carries the load above it. If it has corroded, cracked or was never adequate, the wall above can drop slightly, cracking the plaster around the opening.
- Wall tie failure. In cavity-wall homes, corroded wall ties can cause the outer leaf to bulge and crack, with horizontal cracking at regular intervals.
- Damp. Persistent moisture in a wall can cause plaster to crack, blister, blow off the wall, and stain. Damp-related cracking is often accompanied by a tide mark, a musty smell, peeling paint or salt deposits on the surface.
These cracks tend to be wider than a few millimetres, are often diagonal or stepped, may pass through both plaster and masonry, and crucially they grow over time.
How to tell the difference
You can do a lot of useful triage yourself before deciding whether to call anyone. Work through these questions:
| Question | Usually harmless if... | Worth investigating if... |
|---|---|---|
| How wide is it? | Hairline to ~2 mm | More than 3-5 mm, or widening |
| What direction? | Vertical or along a joint | Diagonal across a door/window corner |
| Inside and outside? | Inside plaster only | Steps through the external brickwork too |
| Is it growing? | Stable over months | Visibly wider over weeks/months |
| Other symptoms? | None | Sticking doors, sloping floors, damp tide marks |
Guidance only — a qualified surveyor should assess any crack you are unsure about. Sources: RICS guidance; building survey practice.
What to do next
Match your response to what you have found:
- Fine, stable, cosmetic cracks: these are a decorating job. Rake out the crack slightly, fill with a flexible filler, sand, and redecorate. Cracks at plasterboard joints and wall-to-ceiling junctions that keep reopening can be bridged with flexible jointing tape or a flexible decorator's caulk before painting.
- Cracks you are unsure about: mark each end of the crack with a pencil line and a date, photograph it, and check again in a few weeks. If it has not moved, it is almost certainly cosmetic. If it has grown, that is your trigger to seek advice.
- Wide, diagonal, stepped or growing cracks, or any crack with sticking doors, sloping floors or external cracking: get a professional assessment. A chartered surveyor or structural engineer can determine whether the cause is subsidence, a lintel, wall ties or something else, and whether monitoring, repair or underpinning is needed. Do not simply fill a structural crack and redecorate — the crack will return and you will have hidden the symptom rather than fixed the cause.
Above all, resist the urge to panic. Most cracks are the building breathing, not the building failing. The structured approach — measure it, watch it, and escalate only the ones that grow — keeps a sense of proportion while making sure the genuinely important cracks get the attention they need.
Frequently asked questions
Are hairline cracks in plaster normal?
Yes. Fine hairline cracks are extremely common and are usually caused by drying shrinkage in new plaster or normal seasonal movement of the building. They are cosmetic and can be filled and painted over. Cracks only warrant concern when they are wide (more than a few millimetres), diagonal across openings, or visibly growing over time.
When is a crack in my wall serious?
A crack is worth professional investigation when it is wider than about 3-5 mm, runs diagonally from the corner of a door or window, steps through the external brickwork as well as the internal plaster, or is getting bigger over weeks and months. Cracks accompanied by sticking doors, sloping floors or damp tide marks also justify an assessment by a surveyor or structural engineer.
How can I tell if a crack is still moving?
Mark each end of the crack with a fine pencil line and write the date next to it, then photograph it. Check again after a few weeks and again after a few months. A crack that has not moved is almost certainly cosmetic. A crack that has widened or extended past your marks is active and should be assessed by a professional.
Sources & further reading
- RICS — guidance on cracking and movement in buildings
- Historic England — practical building conservation guidance
- The Property Care Association — damp and structural advice
Figures on this page are typical UK ranges drawn from published sources and depend on your specific room. They are guidance, not a quotation.