When to Worry About Cracks in Walls and Ceilings

Most cracks in walls and ceilings are not serious and are caused by materials drying out or normal seasonal movement. A crack under 2mm wide is usually purely cosmetic and easy for a plasterer to fill and skim. You should start to worry and seek professional advice when a crack is wider than 5mm, is diagonal or stepped, or comes with sticking doors and windows.

Why walls and ceilings crack

Cracks appear in buildings for several common reasons that range from harmless drying to serious ground movement. Shrinkage happens as materials such as mortar, plaster and concrete dry out in a new home or a recently plastered room. These shrinkage cracks are usually vertical or horizontal near the ceiling or floor and are normally less than 2mm wide. Thermal movement is another common cause, as building materials naturally expand and contract when temperatures change throughout the year.

You will often see thermal movement cracks where an external wall meets an internal plasterboard wall, around lintels, and where boards join on a plasterboard ceiling. Normal settlement happens as the ground beneath a building slowly compresses under the weight of the structure. Settlement cracks can be vertical, horizontal or diagonal, and they usually stop forming once the building settles into its final position.

Subsidence and heave are much more serious forms of ground movement that require professional investigation. Subsidence is most often caused by trees and shrubs draining moisture from clay soils, defective drains leaking water into the ground, weak ground or past mining activity.

Cracks that are usually cosmetic

The building industry uses a standard crack damage scale that sets six categories of crack damage, from 0 to 5. Categories 0, 1 and 2 cover crack widths up to 5mm and can be regarded as aesthetic issues that usually need only redecoration. Hairline cracks up to 2mm are typically normal and are often caused by plaster drying, seasonal movement, temperature changes or normal settlement.

A crack of 2mm or less is generally considered cosmetic and unlikely to affect a property's structural stability. You can easily have these small cracks filled and painted over to restore the look of your room. A small crack might reappear after a year or so through thermal movement, which is not a worry unless it keeps getting wider. If a crack stays at 2mm or less, is not diagonal or stepped and shows none of the warning signs below, a plasterer can repair it.

Cracks that need a closer look

Categories 3 and 4 on the crack damage scale cover cracks of 5mm to 25mm. These are considered serviceability problems that affect weather tightness and the operation of doors and windows. Category 5 covers cracks over 25mm and is a stability problem likely to need structural intervention. You should contact a professional if a crack is horizontal or vertical and constantly more than 2mm wide, or if it has got significantly wider or longer.

Subsidence and heave cracks tend to be wide at one end and narrow at the other. Possible signs of subsidence include:

• Cracks wider than around 3mm.
• Diagonal cracks especially around doors and windows.
• Cracks that are wider at the top than the bottom.
• Cracks visible on both the inside and outside of the wall.
• Sloping or uneven floors.
• Doors or windows starting to stick or swinging open.

You can easily estimate the width of a crack without specialist tools because a pound coin edge is about 3mm wide. Write down the date and the exact width in a notebook so you can track it accurately. The number and distribution of cracks matter as well as the width of any one crack. A single hairline crack is rarely a problem, but multiple cracks spreading across different walls suggest a wider issue that needs investigating before any plastering takes place.

Ceiling cracks and what they usually mean

Hairline cracks often appear on ceilings under the joists and where boards join on a plasterboard ceiling. These are usually under 2mm wide and open and close slightly through the year. Sagging or visible bowing, rapidly widening cracks, discoloured patches showing active moisture, or bulging with spreading damp need immediate professional inspection.

Older houses often feature lath and plaster ceilings, which are made of lime plaster pressed onto timber laths. These can crack with age, and a plasterer will check how well the plaster is still holding before you decide whether to repair or replace a lath and plaster ceiling. Do not push or prod a cracked old ceiling to test it. If your home was built or refurbished before 2000, textured coatings such as Artex may contain asbestos.

The Health and Safety Executive says materials in good condition that are unlikely to be damaged are often best left where they are. Homeowners should not sand, scrape or drill suspect coatings and should seek advice from an environmental health officer at their council for damaged materials.

External cracks and render repairs

If a crack shows on both the inside and the outside of a wall, have the cause checked before anything is filled. Once any movement has been dealt with, the outside may need cracked render repairs as well as the plaster inside.

Planning Portal guidance for England says that where 25 per cent or more of an external wall is re-rendered, re-plastered or re-lined internally, the building regulations normally apply and the wall's insulation would normally have to be improved. Listed buildings need consent for any significant work, inside or out.

How plasterers repair cracks and what goes wrong

A small cosmetic crack can often simply be filled. For a crack that keeps coming back, a plasterer will usually widen it slightly to remove any loose debris, which gives the new plaster a clean edge to bond with. For plasterboard walls, they often apply jointing tape over the crack to bridge the gap and stop the crack returning through the new finish. They will then apply a fresh skim coat over the area to leave a smooth, flat surface.

A skim is a thin finishing coat, a few millimetres thick, applied over sound plaster or plasterboard. A replaster means hacking off failed plaster and applying a backing coat plus a finish coat, or fixing new plasterboard and skimming it. Both methods introduce water to the walls, and the fresh plaster must dry at its own pace. Turning the heating up to speed up the process will cause the new plaster to shrink too fast and crack.

Things go wrong when tradespeople take shortcuts to finish the job quickly. Common problems include:

• Skimming over loose, damp or unsealed painted surfaces so the new coat cracks or peels.
• Applying modern gypsum or cement on old lime walls which traps moisture.
• Agreeing cash-only deals with nothing in writing.
• Accepting quotes that omit preparation, ceilings, scaffolding or waste removal.
• Dry sanding or scraping old Artex without checking for asbestos.

When to call a plasterer, a structural engineer or your insurer

If damage looks serious, active or structural, your buildings insurer may be the first call. You should check your policy before arranging major work to see exactly what you are covered for. A structural engineer can assess the cause of the movement and provide a detailed report on how to fix it.

Do not rush into repairs, because cracks from suspected movement should not simply be filled and painted over. Any structural repairs should closely follow the engineer's, surveyor's or insurer's recommendations. Once the structural issue is solved and the building is stable, you can hire a plasterer to make the interior walls look new again.

Plastering is not a licensed trade, meaning there is no legal register like the one for gas engineers, so evidence of past work matters. A CSCS card is for construction sites, not proof of quality on domestic work. TrustMark is the government-endorsed quality scheme, and FIS is the trade body for the finishes and interiors sector. Under the Consumer Rights Act 2015 work must be done with reasonable care and skill, and if it is not, the customer can ask for it to be put right or seek a partial or full refund.

What crack repairs cost

Plasterers price per job after seeing it, based on the area in square metres, the state of the background, ceiling height and access. The price also depends on the preparation needed, such as stripping wallpaper, removing loose plaster, sealing painted surfaces or dealing with Artex. Ceilings are usually priced separately, most plasterers have a minimum charge for small jobs, and London and the South East cost more.

The table below shows typical plastering costs. These are typical published prices for 2026 and your local quotes will differ.

If your cracks are caused by ground movement, you will need a structural engineer. The Federation of Master Builders outlines the costs of getting a professional assessment. The table below shows typical structural engineer costs. These are typical published prices for 2026 and your local quotes will differ.

Common questions

What are the signs of subsidence cracks?

Subsidence cracks tend to be wider than 3mm and are often wide at one end and narrow at the other. They are frequently diagonal or stepped, especially around doors and windows, and are often visible on both the inside and outside of the property. You might also notice doors or windows starting to stick, swinging open on their own, or sloping and uneven floors.

Why do plasterboard walls crack in straight lines?

Plasterboard is often fixed using a method called dot and dab, which means the plasterboard is stuck onto dabs of adhesive on the blockwork beneath. Thermal movement causes the building materials to expand and contract at different rates. This movement can open straight hairline cracks along the joints between boards, and these are usually under 2mm and cosmetic.

When should I worry about hairline cracks in plaster?

Hairline cracks up to 2mm are typically normal and are caused by plaster drying, seasonal movement or normal settlement. Get advice if a crack is constantly more than 2mm wide, is diagonal or stepped at any width, is wider at one end than the other, shows inside and outside, runs along the damp proof course line, or gets noticeably wider or longer over time.

Can I repair a cracked Artex ceiling myself?

The Health and Safety Executive warns that textured coatings such as Artex may contain asbestos in any building built or refurbished before 2000. Work on them does not normally need a licensed contractor but must be done by people trained to work safely with asbestos. The HSE also says not to try to repair or remove asbestos materials yourself without training, so do not sand, scrape or drill a suspect coating, and tell any plasterer about it before they quote.

Next steps for dealing with cracked walls

If you have found a crack and need to decide what to do next, follow this simple checklist.

• Measure the gap against the edge of a pound coin to see if it is wider than 3mm.
• Write down the date and the exact width in a notebook so you can track any changes.
• Look closely to see if the crack is diagonal, stepped, or wider at one end than the other.
• Test your doors and windows to see if they are sticking in their frames.
• Call your buildings insurer and a structural engineer if the crack is over 5mm or shows signs of subsidence.
• Ask tradespeople if their quote includes VAT, materials, waste removal and floor protection.
• Check the public register to ensure your chosen tradesperson is a registered waste carrier.

A quote is a fixed price, while an estimate is a rough guess. Citizens Advice suggests getting three written quotes, a written contract, paying deposits of no more than 25 per cent and paying by card. They also point out that you might be able to cancel within 14 days of agreeing a contract. When you are sure your cracks are purely cosmetic and ready to be filled, you can find a plasterer near you to get your walls completely smooth and ready for paint.