Plaster Cracks: Cosmetic or Structural? When to Worry

You notice a line across the ceiling, or a diagonal crack running up from a door frame. The first question is always the same: is the house moving?

Mostly, no. But it is worth knowing how to read what you see.

Common cosmetic cracks

Hairline cracks. Fine lines, often thinner than a pencil line. They appear as plaster dries, as the house heats and cools, and where old paint has become brittle.

Joint cracks. Straight lines along ceiling edges or where plasterboard sheets meet. Often caused by movement between boards or a missing scrim tape.

Corner cracks. Where two walls meet. Seasonal movement and shrinkage are the usual cause.

Cracks above doors and windows. Small, often diagonal. Common in older homes where a lintel has settled slightly.

These are normal and can be filled and painted over.

Signs it may be structural

Call a surveyor or structural engineer if you see:

• Cracks wider than around 5mm
• Cracks that keep growing or reopen after repair
• Cracks that run through plaster and brick on the outside as well
• Sloping floors or doors that suddenly stick
• Stepped cracks following mortar lines in the brickwork
• Gaps opening between walls and ceilings

Subsidence, heave and failing lintels are rare, but they are not decorators’ problems. Get the cause assessed first.

Look at the pattern over time

Mark the ends of a crack with a pencil and a date. Check a few weeks or months later. A crack that has not moved is much less worrying than one that has.

How to repair a cosmetic crack

1. Rake out the crack with the corner of a scraper to make a key.
2. Remove dust and any loose plaster.
3. Fill with a flexible filler suited to cracks, not a rigid one.
4. For repeat cracks, bridge the gap with scrim or mesh tape.
5. Sand smooth when dry, then prime before painting.

For ceilings and joints, a flexible decorator’s caulk often outlasts powder filler.

Why the same crack keeps returning

A rigid filler cannot follow movement, so the line reappears. In older houses with lime plaster, the wall breathes and shifts with the seasons, and modern fillers can fight that. Our coving, cornicing and plaster repair service uses materials suited to the wall, which matters particularly in period properties.

Do not paint over damp-related cracks

Cracks with staining or a musty smell point to moisture. Fix the source first. Our damp and mould guide explains what to look for.

Once the wall is sound, we can repair and redecorate. Get a quote and tell us what you have found.