Fix a crack in a plaster wall

Fill a plaster crack safely, checking your wall type and for hidden pipes or cables before you rake it out and refinish it.

FC-01Prepare the roomMove furniture away from the wall and lay a dust sheet on the floor below the crack. Open a window for ventilation and put on safety glasses and a dust mask - old plaster dust is unpleasant to breathe. Get your filling knife, scrim tape and filler ready before you start so you're not hunting for tools with wet filler on the blade.

1 each · Dust sheet

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1 each · Safety glasses

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1 pack · Dust masks (pack of 5)

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1 each · Filling knife

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FC-02Work out what kind of wall you've gotTap near the crack: a dull, solid sound means solid brick or block; a hollow sound means plasterboard on a timber or metal stud frame, or dot-and-dab board fixed to masonry behind. Run a magnet or stud detector along the wall - it will pick up the screws fixing the board to the frame, and you'll feel a line of screw heads under the paint. In a house built before about 1930, the wall may feel slightly springy when pressed - that's usually lath and plaster, a lime skim over timber laths, and it moves more than solid plaster. Knowing which you've got tells you whether the crack is likely to be simple shrinkage or something that will reopen without scrim.

1 each · Stud detector or magnet

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FC-03Rake out the crackUse the corner of your filling knife or a scraper to remove loose, crumbling material, and gently undercut the edges to form a shallow V-shape - this gives the filler something to grip. Cables and pipes can run almost anywhere in a wall, not just in the usual zones, so if the crack is near a socket, switch or light fitting, use a cable and pipe detector first and still treat the area as live, since detectors can miss cables in metal conduit or behind metal-backed board. If you have any doubt, isolate the circuit at the consumer unit before digging any deeper, and check where your stopcock is (usually under the kitchen sink) and that it turns, so you can shut off the water fast if you expose a pipe.

1 each · Cable and pipe detector

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FC-04Apply scrim tape over the crackPress self-adhesive fibre scrim tape firmly along the length of the crack, smoothing out any bumps or air pockets. Scrim is essential on plasterboard joints - whether the board is on timber studs, metal studs or dot-and-dab - and on lath and plaster, because the board or laths flex slightly and a rigid filler alone will crack again. On solid brick or block, scrim is optional for a fine hairline crack, but worth using if the same crack has come back before.

1 roll · Fibre scrim tape (90mm x 20m roll)

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FC-05Fill the crackLoad flexible decorators' filler onto your filling knife and press it firmly into the crack so it beds through the scrim tape, not just over the top. Draw the blade across to leave it as flat as possible, feathering the edges into the surrounding plaster. Leave it to dry fully - check the tube or tub for drying time, usually a few hours, longer in a cold or damp room.

1 tube · Flexible decorators' filler (310ml tube)

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FC-06Second coat and sand smoothOnce dry, check the repair for dips or pinholes and apply a thin second coat if needed. When it's completely dry, sand gently with fine-grit (120-180 grit) sandpaper on a sanding block, keeping the surface flat rather than dishing it out. Wipe away the dust with a damp cloth.

1 pack · Fine-grit sandpaper (120-180 grit, pack)

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1 each · Sanding block

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FC-07Prime and redecorateApply a mist coat - emulsion watered down by about 10% - or a proper plaster primer over the repair before painting. Bare filler and old paint absorb paint differently, and skipping this step can leave a visible 'flash' once the wall is finished. Once dry, paint the wall to match, feathering the new paint out well past the edges of the repair.

1 litre · Emulsion paint for mist coat/topcoat (1L)

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FC-08Know when it's more than a crackA crack that keeps reappearing in the same spot, one wider than about 5mm, or one that's stepped in the brickwork, sloping, or visibly growing, can point to structural movement rather than ordinary plaster shrinkage. Don't just keep filling it - get a builder, surveyor or structural engineer to look at the cause. If the crack is in a wall shared with a neighbouring property, structural repair work may fall under the Party Wall Act 1996, and significant structural work generally needs to meet Building Regulations Part A (structural safety).

RULES

KNOW THE RULES

The Party Wall Act 1996 can apply to structural work on a wall shared with a neighbouring property, and you may need to give formal notice. Significant structural repairs also need to meet Building Regulations Part A - get advice from a builder or structural engineer before proceeding.

FC-09Tidy upSweep and vacuum any plaster dust from the floor and skirting, and bag up used scrim offcuts and packaging for the bin. Clean your filling knife and any other tools with water before the filler dries on them, and press an airtight lid or clingfilm onto any leftover filler so it's still usable next time.

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