Cracks in plaster: which are cosmetic and which need a pro
Nearly every house has cracks in the plaster, and most of them are nothing to worry about. A few are early signs of movement in the building. The skill is telling them apart and keeping an eye on the ones you’re not sure about.
Guide · The Honest Fix
Filling cosmetic cracks is DIY. Diagnosing movement isn’t.
Tools and materials
- A pencil, ruler and phone camera (for monitoring)
- Scraper or filling knife and a flexible filling knife
- Flexible or fine-surface filler
- Self-adhesive scrim tape for plasterboard joints
- Fine sandpaper, dust sheets and a dust mask
- Flexible decorators’ caulk for junctions
- Primer or a mist coat, and paint
Why plaster cracks
Plaster cracks because buildings move a little all the time. Timber dries out, heating cycles expand and contract materials, and new plaster shrinks as it dries. Floors flex under foot traffic. None of that is a structural problem.
Cracks matter when they’re caused by the building itself moving: subsidence, heave, failing lintels or roof spread. Those cracks tend to be wider, diagonal and getting worse.
Usually cosmetic
- Fine hairline cracks, roughly the width of a pencil line or less.
- Straight cracks along plasterboard joints, often in ceilings.
- Cracks where walls meet ceilings, or where two walls meet.
- Cracks running out from the corners of doors and windows, if they’re fine and not growing.
- Cracks in new builds or new plaster in the first year or two as everything dries out.
- Cracks that open and close a little with the seasons but don’t grow.
Get it looked at
- Cracks wider than about 5mm. The Building Research Establishment (BRE) classes cracks up to that width as usually cosmetic.
- Diagonal cracks that are wider at one end, especially running from door or window corners.
- Cracks that you can see on both the inside and outside of the same wall.
- Stepped cracks in the brickwork or mortar outside.
- Doors or windows that have started sticking or no longer close, alongside new cracks.
- Cracks that keep growing, or come back quickly after filling.
- Bulging walls, sagging ceilings or floors that slope.
- New cracks near large trees, a bay window, a recent extension or a leaking drain.
Safety: asbestos and old ceilings
- Textured ceilings (Artex and similar): textured coatings applied before 2000 may contain asbestos, which was banned in the UK in 1999. Don’t sand, scrape, drill or fill into an old textured ceiling until it’s been tested. A sample can be tested by an accredited lab.
- Lath and plaster ceilings (common in houses built before the 1930s) can come down suddenly. A crack with sagging, a bulge, or plaster that moves when pressed gently needs a plasterer, and the room should be kept clear underneath.
- Subsidence is often an insurance matter. If you suspect it, tell your buildings insurer early, and don’t start repairs before they’ve seen it.
How to monitor a crack
- Draw a fine pencil line across each end of the crack and write the date next to it.
- Measure the widest point and photograph it with a ruler held against it.
- Check every few weeks. Note whether it has grown past your marks or got wider.
- For a crack you’re worried about, use a plastic crack monitor (a cheap tell-tale gauge) fixed across it for more precise readings.
- Keep monitoring through at least one change of season. Some movement comes and goes with dry summers.
How to fill a cosmetic crack
- Open it up slightly. Run the corner of a scraper along the crack to remove loose material and make a shallow V shape so the filler has something to grip.
- Brush out the dust and lightly dampen the crack with a damp brush if the filler instructions say so.
- Fill it with a flexible knife, pressing the filler in firmly. Leave it slightly proud.
- Let it dry, then sand flush. Deep cracks may need a second fill.
- For plasterboard joint cracks, cover the crack with scrim tape before filling, then feather the filler out wider than the tape.
- For cracks at wall and ceiling junctions, use flexible decorators’ caulk instead of filler. It moves with the building.
- Prime and paint. Use a mist coat on bare filler or new plaster.
Stop here: call a pro if…
- Any of the signs in “Get it looked at” above.
- A crack you filled has come back wider within months.
- Plaster sounds hollow or comes away in large pieces when you start raking it out.
- The ceiling is old and textured and hasn’t been tested for asbestos.
- A lath and plaster ceiling is sagging or bulging.
- You’re selling or buying and a surveyor has flagged movement.
DIY or Hire?
Filling a cosmetic crack: DIY. Filler, scrim tape and a little patience will fix most of them.
Working out whether a crack is structural: hire. The right person is a chartered structural engineer, not a builder or plasterer. A builder may quote for repairs before the cause is known. Paying for an independent report first often saves money, and your insurer may need one anyway.
Large areas of cracked or loose plaster: hire a plasterer. Filling and re-filling a wall that needs re-skimming is a false economy.
Common mistakes
- Filling a crack before knowing whether it’s still moving.
- Using rigid filler where walls meet ceilings, so it cracks again straight away.
- Filling plasterboard joint cracks without taping them.
- Sanding or scraping an old textured ceiling without testing it for asbestos.
- Painting straight onto new filler without priming, leaving a visible patch.
- Hiring a builder for repairs before an engineer has found the cause.
FAQs
Are cracks in a new-build house normal?
Fine shrinkage cracks in the first year or two are common as the structure dries out. Note them, and if you’re still in the warranty period, report anything wider or growing to the builder or warranty provider.
Should I see a surveyor or a structural engineer?
For cracks you’re worried about, a structural engineer. A building surveyor is useful for wider condition issues and buying, but structural movement is the engineer’s field.
Will my insurance cover cracks?
Ordinary settlement and shrinkage cracks usually aren’t covered. Subsidence often is, subject to the policy and excess. Check your policy wording and speak to your insurer before starting work.