Use this guide for common hairline and shrinkage cracks in plaster, especially around doors, windows and ceiling joints. Have a filling knife, flexible filler or decorator's caulk, fine sandpaper, a dust sheet and matching paint ready.
Step by step
Decide whether it is cosmetic
Fine cracks of up to about 1–2 mm that do not grow are usually settling or drying movement. Get advice from a surveyor or structural engineer before filling if a crack is wider, runs diagonally from window or door corners, is wider at one end, or doors and windows start to stick. If you rent, report such cracks to your landlord.
Open up and clean the crack
Run the corner of a scraper along the crack to remove loose plaster and make a small V groove so the filler can grip. Brush out the dust and wipe the area with a slightly damp cloth. Protect the floor and furniture with a dust sheet.
Choose the right filler
Use ready-mixed or powder filler for stable cracks in walls. For cracks that reopen, such as where the ceiling meets the wall, use a flexible filler or decorator's caulk that can move slightly. Wear eye protection when working overhead.
Fill in thin layers
Press the filler firmly into the crack with the filling knife, drawing it across the crack rather than along it. Leave it slightly proud of the surface. For deeper cracks, fill in layers and let each one dry as the packet says.
Sand and prime
Once fully dry, sand the filler flush with fine sandpaper on a block, wearing a dust mask. Wipe off the dust. Spot-prime the repair with thinned emulsion or primer so it does not show through the top coat.
Paint to finish
Paint the whole wall or ceiling, or at least to a natural break, for an even finish. Two thin coats hide repairs better than one thick coat. Keep a little leftover paint labelled for future touch-ups.
Ready-to-use checklist
- Crack checked for signs of structural movement
- Dust sheet and eye protection
- Scraper and flexible filling knife
- Filler or flexible caulk suited to the crack
- Fine sandpaper and sanding block
- Dust mask
- Primer and matching paint
Practical tips
- Photograph a crack next to a ruler and date it: if you see it grow over a few weeks, get it checked before filling.
- Hairline cracks in new plaster or new builds are common as the building dries, so wait a few months before redecorating.
- Use a stable step ladder or platform for ceilings rather than reaching from a chair.
Common problems
The crack came back a few weeks after filling.
It is probably a joint that moves with temperature or loading, such as a ceiling line. Rake out the old filler and use a flexible caulk, or bridge the area with scrim tape and a skim of filler before painting.
The repair shows through the paint as a patch.
The filler absorbs paint differently from the wall. Sand it flush, apply a coat of primer or thinned emulsion over the repair, then repaint the whole wall section.
There are cracks with brown staining around them on the ceiling.
Staining suggests water from a leak above, such as a bath, pipe or roof. Find and fix the leak first and let the area dry fully before filling and using a stain-block primer.
This guide gives general information, not personal legal, financial or medical advice. Rules and prices change, so check the current position with the official service before acting.
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