Fix a crack in a ceiling — HomeGraft

Learn why a ceiling crack appeared, fix any loose board safely, then fill, seal and repaint for an invisible repair.

CK-01Work out what's causing the crackA single hairline crack, especially in a newer house, is usually just surface movement from heating and drying out - it's a filling job, nothing more. A crack running dead straight along a joint, or one that's grown over weeks, points to a board pulling away from the joist. Gently press either side of the crack and tap along it - a dull, drummy sound or a slight dip means the board isn't sitting tight. On a lath and plaster ceiling, fine cracks spreading in a star shape or a soft, sagging patch mean the plaster has broken away from the laths behind it, which is beyond a simple fill.

CK-02Protect the room and set up your stepsMove furniture clear or cover it with dust sheets, and sheet the floor - filler dust and old plaster flakes make a mess. Set up your steps or platform directly under the crack so you can work comfortably without overreaching. Wear a dust mask and safety glasses; working overhead means dust and debris fall straight down onto your face.

2 each · Dust sheets

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1 pack · Dust mask (FFP2)

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1 each · Safety glasses

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CK-03Check what's above and behind the ceiling firstCables for ceiling lights run across joists, and pipework for an upstairs bathroom or WC can run through the void above almost anywhere. Use a cable and pipe detector, but treat it as a warning, not a guarantee - detectors miss cables that are deep, in metal conduit, or behind a metal-backed board, and a metal joist frame confuses cheap detectors further. If you need to screw a loose board back up and have any doubt, switch off the lighting circuit at the consumer unit first. Before you drill anything near a bathroom above, find your stopcock (usually under the kitchen sink) and check it turns freely, so you can shut off the water fast if you hit a pipe.

1 each · Cable and pipe detector

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CK-04Identify your ceiling type before you fix anything back upYou only need this step if the board feels loose or drummy. Track the line of existing fixing screw heads with a magnet or detector - they mark the joists. Tap either side of that line: a solid thud is a joist, a hollow sound is the gap between them. If unsure, drill a small pilot hole in an inconspicuous spot near the crack - fine wood dust means a timber joist, grey metallic swarf means a light steel joist (common in flats, new builds and anything refitted since the 1990s). On a lath and plaster ceiling, don't drill into the laths themselves to test - they're thin and will crack further.

1 each · Magnetic stud/joist detector

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CK-05Re-fix a loose board before you fill the crackInto a timber joist, drive a 3.5 x 40mm drywall screw through the board into the centre of the joist, either side of the crack. Into a light steel joist, use a self-drilling wafer-head screw made for metal stud track - never a wood screw, which strips straight through 0.5mm steel, and never a plain plastic wall plug. If the ceiling is plasterboard fixed with adhesive dabs to a solid concrete soffit, a fixing needs to pass through the board and the dab void, so use one at least 60-75mm long into the concrete. On a lath and plaster ceiling, don't screw into a lath - find the joist behind it with your detector and fix into that instead; if a large area has dropped away from the laths, that's a re-boarding job, not a filling job. A metal joist or an adhesive dab alone will not hold a sagging board up on its own - if it keeps drooping after fixing, it needs a proper timber noggin or extra screws into the joists either side.

10 each · Drywall screws 3.5x40mm

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10 each · Self-drilling metal-stud screws

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6 each · Concrete/masonry screw fixings 60mm

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1 each · Cordless drill/driver

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CK-06Rake out and open up the crackRun a filling knife or scraper along the crack to remove any loose, crumbling plaster and to open it very slightly wider than the surface gap. This gives the filler something to grip rather than just sitting on top. Brush out the dust and vacuum the area so the filler bonds properly.

1 each · Filling knife

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1 each · Dust brush

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CK-07Bridge the crack with scrim tapeFor anything wider than a fine hairline, or any crack likely to move again, press self-adhesive scrim tape along its full length before filling. This stops the crack simply reopening through the new paint film. On a lath and plaster ceiling, use a wider fibreglass scrim, as the plaster surface either side is often slightly uneven.

1 roll · Self-adhesive scrim tape

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CK-08Apply the first coat of fillerUse a flexible ceiling filler for a plain hairline crack, or a standard multi-purpose filler pressed firmly over the scrim for a wider one, working it well into the gap to avoid trapped air. Slightly overfill, since it will shrink a little as it dries. Leave to dry fully - check the tin, but allow at least a few hours.

1 tube (300ml) · Flexible ceiling filler

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1 tub (1kg) · Multi-purpose filler

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CK-09Sand and apply a thin second coatOnce fully dry, sand the first coat lightly flat. Apply a thin second coat about 150-200mm wide, feathering the edges out with the filling knife so there's no visible ridge. Let this dry completely before sanding again.

1 each · Fine sanding sponge

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CK-10Seal and repaint the ceilingNew filler is porous and will show through as a dull patch or flash under paint if you skip this. Apply a mist coat (matt emulsion diluted roughly 70% paint to 30% water) or a proprietary sealer over the repair and let it dry fully. Then apply two coats of matching ceiling paint, either blending into the surrounding area or repainting the whole ceiling if the patch still stands out.

1 litre · Matt emulsion (ceiling white)

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1 each · Roller and tray set

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1 each · Paintbrush (cutting in)

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CK-11Clear up and check it's stableRemove dust sheets, vacuum any settled dust, and wash your tools before the filler and paint dry on them. Keep an eye on the repaired area over the next few weeks - if the crack reopens along the same line, it's telling you there's ongoing movement, and it's worth getting a builder to take a look rather than filling it again and again.

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