Tell whether a crack in your wall is serious

Learn to read a wall crack's width, pattern and history, then fit a simple monitor to see if it's moving.

CR-01Note where the crack is and photograph itTake a clear photo of the whole crack with a ruler alongside it for scale. Write down the date, the room, and whether it's on an external or internal wall. This record lets you spot any real change over the coming weeks.

1 each · Tape measure

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1 each · Notebook or phone camera

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CR-02Work out what's under the crackTap either side of the crack. A solid, dull thud means solid brick or block. A hollow sound with a regular pattern every 400-600mm points to a stud wall or dot-and-dab plasterboard; run a magnet or stud detector along that pattern - a magnet grips over screw heads on timber studs, while metal track gives a different, sharper detector signal. In an older house, a bouncy, springy feel with a slightly uneven surface points to lath and plaster.

1 each · Stud detector

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1 each · Small magnet

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CR-03Measure the crack's width and shapeMeasure the widest point in mm with a ruler or feeler gauge. Check whether it's a uniform hairline (under 1mm) along its length, or wider at one end. Look at the shape too: a crack that steps diagonally along the mortar joints in brickwork suggests the masonry itself has moved, while a straight vertical crack in the middle of a plastered wall is usually just the plaster.

1 each · Feeler gauge or ruler

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CR-04Check the crack's depth and edgesPress a fingernail into the crack - if it only catches the top skim of plaster, it's a surface crack. Shine a torch into it at an angle; if you can see bare brick, block or timber lath through it, the crack runs deeper. Feel each side with your finger - if one edge sits slightly proud of the other, the wall has moved out of line, and that's worth taking seriously.

1 each · Torch

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CR-05Check the crack's age from the decorationLook for old paint or wallpaper bridging the crack unbroken - that means it's old and not currently moving. Fresh white plaster dust inside the crack, or a raw edge with no paint over it, means it's new or still opening. This is one of the simplest and most reliable clues you have.

CR-06Check for cables and pipes before you drillBefore drilling anything, find your stopcock (usually under the kitchen sink) and check it turns, so you can shut the water off fast if you hit a pipe. Sweep the area with a cable and pipe detector, but still expect something to be there - detectors miss cables that are deep, in metal conduit, behind metal-backed plasterboard, or outside the normal safe zones, and a metal stud frame confuses a cheap detector further. If you have any doubt at all, isolate the relevant circuit at the consumer unit before drilling, and turn the water off at the stopcock before cutting into anything that could carry water. Confirm the wall type you identified in step CR-02 so you use the right fixing next.

1 each · Cable and pipe detector

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CR-07Fit the crack monitor with the right fixing for your wallPosition the tell-tale monitor centrally across the crack, 20-30mm from each edge, and mark the two fixing points. In solid brick or block, drill a 6mm hole 35mm deep, fit a wall plug, and drive in a 4mm x 30mm screw; in a timber stud, drill a 2mm pilot hole into the centre of the stud and drive a 4mm x 30mm wood screw straight in, with no plug needed. In a metal stud, use a self-drilling metal-stud screw or a small toggle-type hollow-wall fixing rated for the weight - never a plain wall plug or ordinary wood screw, which will strip straight through the 0.5mm steel. On dot-and-dab plasterboard, use a fixing long enough (60-75mm) to pass through the board and adhesive dabs into the masonry behind; on lath and plaster, use a fine screw with a pilot hole and a gentle hand, since old lath can crumble, or a light-duty hollow-wall fixing if you can't find solid lath.

1 each · Crack monitor (tell-tale)

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2 each · Wall plugs (assorted)

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2 each · Screws (assorted 4mm)

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1 each · Drill with 2mm and 6mm bits

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CR-08Record a baseline reading and check weeklyNote the exact reading on the monitor's scale next to your earlier photo and the date. Check it once a week for at least 4-6 weeks, and again after any heavy rain or frost, since some cracks move seasonally. Write down any change, even a fraction of a millimetre.

CR-09Clear up and store your notes safelyVacuum up any plaster or masonry dust from drilling, and wipe the wall down with a damp cloth. Keep your photos, measurements and monitor readings together, ideally on your phone, so you can show a professional a clear history if needed.

1 each · Damp cloth

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CR-10Know when to call a structural engineerGet a structural engineer to look at the wall if the crack is wider than 5mm, if it's still growing after your monitoring period, if it steps diagonally through brickwork, or if nearby doors and windows have started sticking. If the crack is in a wall shared with a neighbour and any repair would involve cutting into or altering that wall, the Party Wall Act may apply, and you should get advice before starting work. Most hairline plaster cracks turn out to be cosmetic and can simply be filled once you've confirmed they're not moving.

RULES

KNOW THE RULES

The Party Wall Act 1996 can apply if repair work will cut into or alter a wall shared with a neighbour - get advice before starting.

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