Repair a crack in lath and plaster

Rake out, tape, fill and sand a lath and plaster crack, securing any loose lath first for a lasting repair.

LP-01Prepare the roomMove furniture away from the wall or cover it with dust sheets, and sheet the floor. Open a window — sanding old plaster makes fine dust that's unpleasant to breathe. Put on safety glasses and a dust mask before you start, and take down any pictures or shelves within 300mm of the crack.

1 each · Dust sheet

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1 each · Dust mask

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1 each · Safety glasses

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LP-02Check the crack and test for movementA hairline crack that doesn't move is cosmetic and just needs filling. Press gently either side of the crack — if it flexes, springs back, or crunches, the lath behind has parted from the plaster and needs securing before you fill. A crack wider than 5mm, or one that keeps coming back after filling, usually means the building is moving and should be checked before you repair it, as filling alone won't fix the cause.

LP-03Safety check before you drill or screwIf the lath needs securing you'll be screwing into the wall or ceiling behind the plaster. Cables and pipes can run almost anywhere, not just the usual safe zones, so use a cable and pipe detector first — then still work as if something could be there, because detectors miss cables in metal conduit or behind metal-backed plasterboard. Find your stopcock (usually under the kitchen sink) and check it turns before you start, so you can shut off the water fast if you hit a pipe. If in any doubt, turn off the circuit at the consumer unit before drilling, and turn off the water at the stopcock before cutting into anything that could carry water.

1 each · Cable and pipe detector

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LP-04Identify what's behind the lathLath and plaster is almost always fixed to timber studs or joists — that's what the laths are nailed to. Confirm this before you screw in: a stud detector or magnet finds the fixing nails in a line, the wall sounds solid over a stud and hollow between them, and a small pilot hole brings out timber dust. If a pilot hole brings out fine grey swarf instead, you've found a later metal stud partition, not true lath and plaster — stop, this needs a self-drilling metal-stud fixing, never a wood screw or plastic plug, which will strip straight through thin steel.

1 each · Stud detector

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LP-05Secure loose lathDrill a 2mm pilot hole into the timber stud or joist behind the loose area. Drive a 38mm plasterboard screw through a repair washer (or a large penny washer) so the screw head doesn't punch through the crumbly plaster. Space fixings roughly 150mm apart along the loose section until it feels firm underhand.

10 each · Plasterboard screws, 38mm

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10 each · Repair washers

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1 each · 2mm drill bit

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LP-06Rake out and undercut the crackUse a filling knife or scraper to open the crack to about 5mm wide, undercutting the edges slightly so it's wider underneath than at the surface. Brush out loose dust and debris with a soft brush. This gives the filler something to grip so it doesn't just sit on top and fall out later.

1 each · Filling knife

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1 each · Soft brush

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LP-07Apply scrim tapePress self-adhesive fibreglass scrim tape along the length of the crack, smoothing out wrinkles as you go. Scrim bridges the crack and stops it reopening as the building moves slightly. On a ceiling, support the tape with one hand as you smooth it, as gravity will pull it down.

1 each · Fibreglass scrim tape, 5m roll

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LP-08Apply the first fill coatMix a small batch of filler and press it into the scrim with a filling knife, working it flat and into the depth of the crack. Don't aim for a perfect finish yet — this coat is just to bed the tape. Leave to dry fully, following the time on the packet.

1 each · Flexible decorator's filler, 1kg

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LP-09Apply the second coat and feather the edgesSpread a thin second coat 50-100mm beyond the crack on each side, feathering the edges with light, even strokes. This blends the repair into the surrounding wall so there's no visible ridge once painted. Leave to dry completely before touching it.

LP-10Sand smoothOnce fully dry, sand with a fine sanding sponge (120-150 grit) until the repair sits flush with the surrounding plaster. Wipe away dust with a slightly damp cloth, then check the finish with a torch held at a shallow angle — raking light shows up any high or low spots.

1 each · Fine sanding sponge

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LP-11Seal the repair and clean upNew filler is porous and will flash through paint unless sealed — apply a mist coat (watered-down emulsion) or a proper plaster primer over the repair before decorating. Fold dust sheets carefully to trap the dust, vacuum the floor, and put any offcuts of scrim or filler packaging in with normal household waste.

1 each · Plaster primer or mist coat emulsion, 250ml

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