Plaster Cracks: Causes, Types and How to Repair Them

Plaster cracks are common and usually not a sign of structural failure. Most cracks in UK homes come from natural settlement, drying shrinkage, seasonal movement, or poor preparation — not from the building falling down. The key is learning to tell the fine, harmless hairline cracks from the wider, recurring or stepped cracks that point to an underlying problem.

Why plaster cracks

Walls and ceilings move. Houses expand in heat and shrink in cold, floors flex under load, and older London properties — particularly Victorian and Edwardian terraces — settle over decades. Plaster is rigid, so when the surface behind it moves, the plaster cracks.

Common causes:

• Drying shrinkage — new plaster loses water as it cures; if it dries too fast it cracks
• Settlement — slight, ongoing movement of the building over time
• Seasonal movement — timber floors and roofs expand and contract with humidity
• Poor preparation — skimming over a dusty, weak or poorly keyed background
• Moisture — damp walls, leaks or condensation forcing the plaster to fail
• Impact or vibration — doors slamming, drilling, or structural work nearby

Types of plaster crack

| Crack type | Typical width | Likely cause | Concern level | |---|---|---|---| | Hairline | Under 1 mm | Drying shrinkage, settlement | Low | | Straight, vertical | 1–3 mm | Plasterboard joint, settlement | Low–medium | | Diagonal | 2+ mm | Differential settlement | Medium | | Horizontal | 2+ mm | Floor or lintel movement | Medium–high | | Stepped / following brickwork | 3+ mm | Structural movement | High — investigate | | Wide, recurring | 5+ mm | Ongoing structural movement | High — investigate |

Fine hairline cracks under 1 mm are usually cosmetic. Cracks over 3 mm, cracks that follow the brickwork in steps, or cracks that keep returning after repair are the ones to take seriously.

Hairline cracks

Fine, spider-web or straight hairline cracks are the most common type and are usually harmless. They often appear as new plaster dries or in older properties during dry weather. They're covered in detail in our separate hairline cracks in plaster guide.

Ceiling cracks

Cracks in a plastered ceiling can be caused by:

• Joist movement or deflection
• Old lath and plaster separating from the laths
• Water damage from a leak above
• Plasterboard joints opening

A ceiling crack that runs along a joint between two boards is usually just the joint tape failing. A crack that crosses the ceiling in a random pattern, or one accompanied by sagging, may mean the plaster has parted from its backing — see our water damaged ceiling repair and ceiling plaster repair pages.

Cracks around doors and windows

Cracks that appear above door frames and window openings are very common. They're usually caused by the lintel or the surrounding structure moving slightly. Fine ones can be filled; wider or diagonal ones above openings should be checked, especially in older properties.

When to call a professional

Get advice before repairing if you notice:

• Cracks wider than 3 mm
• Cracks that keep returning after being filled
• Stepped cracks following the brickwork
• Cracks accompanied by sagging ceilings or bulging walls
• Cracks after a leak, flood or structural work

A crack that reappears within weeks of filling is telling you the movement is still happening. Filling it again won't fix the cause.

How cracks are repaired

The right repair depends on the crack and its cause:

• Hairline cracks — open slightly, fill with a flexible filler, sand smooth, repaint
• Plasterboard joint cracks — re-tape the joint with paper or mesh tape and joint compound
• Larger cracks — cut back to a sound edge, fill in layers, and reinforce with scrim tape where movement is likely
• Recurring cracks — the cause (movement, damp, loose background) must be addressed first, or the crack will return

For cracks caused by failed or blown plaster, the only lasting fix is often to remove the loose material and re-plaster the area. See our plaster crack repair service and replastering costs for what's involved.

FAQ

Are hairline cracks in plaster normal? Yes. Fine hairline cracks under 1 mm are very common, especially in older properties and in newly dried plaster. They're usually cosmetic.

Why is my new plaster cracking? Usually because it dried too fast, was applied too thick, or went over a weak or poorly prepared background. See our plaster drying guide for how to dry new plaster without cracking it.

When should I worry about plaster cracks? When they're wider than 3 mm, follow the brickwork in steps, keep returning after repair, or come with sagging or bulging. Those need investigation.

Can I just fill ceiling cracks? Fine joint cracks can be filled. Wider cracks or a sagging ceiling may mean the plaster has parted from its backing and needs more than filler.

Do cracks mean subsidence? Not usually. Most cracks are from normal movement. Stepped cracks wider than a few millimetres, especially around doors and windows, are the ones to have checked.

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Not sure whether your cracks need a simple fill, a re-skim or a full replaster? Send London Plastering Service a few photos of the affected wall or ceiling and we'll advise on the right repair, with a free quote.