Fix a crack that keeps coming back

Find out why a crack keeps returning, then rake it out, scrim it, and fill it so it stays gone.

CR-01Check if it's safe to fix yourselfMeasure the crack with a tape measure. A hairline crack under 2mm that runs in a straight line is usually just surface movement and fine to fill yourself. If it's wider than 5mm, stepped, tapering, or visibly getting bigger week by week, stop - that points to structural movement, not ordinary decorating wear and tear.

RULES

1 each · Tape measure

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KNOW THE RULES

Building Regulations Part A covers structural safety. If a crack looks structural (wide, stepped, growing), get a structural engineer or builder to check it before repairing anything.

CR-02Work out what's behind the crackTap gently along the wall or ceiling either side of the crack. Solid brick or block sounds dense everywhere. A stud wall sounds hollow between fixing points and solid where a stud sits - use a stud detector, or drag a magnet along the surface to find the screw line, usually spaced every 400-600mm; whether timber or metal, this test still finds the studs. Dot-and-dab plasterboard sounds hollow almost everywhere with occasional solid patches, and lath and plaster, common in houses built before about 1930, feels slightly springy under light finger pressure.

1 each · Stud detector (or strong magnet)

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CR-03Clear the area and protect the roomMove furniture clear and lay dust sheets under the crack. Scrape back any loose paint or wallpaper along the crack line so the filler bonds to sound material underneath. Working on a ceiling, always use a proper stepladder set up on firm, level ground - never a chair or a worktop.

1 each · Dust sheet

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1 each · Scraper

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CR-04Rake out the crackOpen the crack slightly with a filling knife or scraper tip, cutting a shallow V shape about 3-5mm deep. Pick out any loose or crumbling material until you reach firm plaster or board. Brush out the dust, then dampen the crack lightly with a wet paintbrush - this stops the filler drying too fast and shrinking straight back to a hairline crack.

1 each · Filling knife

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1 each · Soft brush

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CR-05Fit scrim tape - method varies by wall typePress self-adhesive fibreglass scrim tape centred along the whole crack - this bridges the gap so small future movement doesn't show through as a new crack. On solid brick or block, and on sound lath and plaster, scrim on its own is usually enough. On plasterboard - timber studs, metal studs, or dot-and-dab - the crack usually follows a board joint or a dab line, so run the scrim past where the crack looks like it stops. On lath and plaster, press gently either side of the crack first: if it feels soft, springy, or you hear crumbling, the lath keys underneath may have failed - stop, that's a ceiling repair for a plasterer, not a simple crack fix.

1 roll · Fibreglass scrim tape (90mm)

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CR-06Fill and feather the crackUse a flexible decorator's caulk over the scrim wherever the wall can move slightly - this is essential on dot-and-dab, where the board flexes a little against the solid wall behind it and a rigid filler just cracks again along the same line. On solid brick, block, or sound lath and plaster, a standard multi-purpose filler is fine. On plasterboard stud walls, timber or metal, use multi-purpose or joint filler and feather it out about 100mm either side with a filling knife so it blends into the surface.

1 each · Flexible decorator's caulk (310ml cartridge)

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1 each · Multi-purpose filler (small tub)

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CR-07Let it dry and sand smoothLeave the filler to dry fully - check the tub for timing, usually a few hours, longer in a cold room. Sand it smooth with fine sandpaper or a sanding sponge until you can't feel a ridge, then wipe away the dust with a slightly damp cloth.

1 each · Sanding sponge (fine)

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CR-08Prime and spot-paint the repairSeal bare filler with a mist coat or a dedicated filler primer before painting, or it will flash through as a different shade under the topcoat. Once dry, paint to match using the same paint as the rest of the wall or ceiling, feathering the edges so the repair doesn't show as a patch.

1 each · Filler primer / mist coat (tester pot)

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1 tester pot · Matching paint (from existing tin)

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CR-09Tidy up and keep an eye on itClean your tools and clear away scraped-off debris and dust sheets. Keep an eye on the repair over the next month - a proper scrim-and-flexible-filler job should hold for good. If the same crack reopens in the same spot despite this, that's a sign of ongoing structural movement, so call a builder or structural engineer rather than filling it again.

1 each · Bin bags

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