Cracks in walls when to worry: filler or a builder

You have found a crack, and a filler listing is already in the other tab. Cracks in walls when to worry is the decision that comes before the tub. A hairline you can barely catch with a fingernail is often a decorating job. A crack wider than about 5 mm, a crack that keeps growing, a diagonal line near a window, or a split that has reached the brick is a different job.

Products Compared read the NHBC cracking note and the BBC piece of 24 August 2026, then set them beside eight ready-mixed fillers on amazon.co.uk on 15 October 2026. We ran no lab test, and this page does not diagnose subsidence. NHBC treats a crack of 2 mm or less as cosmetic, and says get advice above 5 mm, or if a horizontal or vertical crack is constantly more than about 2 mm, shows on both sides, or keeps growing.

If you are left with a shallow interior hairline in sound plaster, match a paste to the depth on the surface filler comparison. Links to amazon.co.uk use the tag pcuk26-21. A qualifying purchase can earn a commission at no extra cost to you.

In short

Most hairline plaster cracks are a filler job only after you have checked width, direction, and whether the crack is still moving. NHBC's cosmetic line is about 2 mm or less. Get advice above about 5 mm, if the crack grows, if it is diagonal near an opening, or if damp, a sagging ceiling, or a sticking door sits with it.

A crack of about 2 mm can be cosmetic

A crack you have to look for is the sort NHBC calls cosmetic when it is about 2 mm or less. That figure is a published housing note, not a measurement of your wall. Hold a ruler on the plaster and write the width in millimetres. A hairline crack in wall plaster that stays at that width, stays in the skim, and does not lengthen is the case where filler can be the right material later. Guessing "quite thin" is how a 4 mm crack gets a tub it should not have.

The limit in that note is the word constantly. Get advice if a horizontal or vertical crack is constantly more than about 2 mm. A crack that is 3 mm today and still 3 mm in a fortnight is a different record from one that was 1 mm and is now 3 mm. Is a 2 mm crack serious? At 2 mm or under, and stable, NHBC's wording allows cosmetic. Constant width above that line changes the advice, even when the paint has not flaked.

A stable vertical hairline can still be cosmetic. A hole you can see through, or a cracked taped joint, is a plasterboard repair, set out on the plasterboard filler page. New plaster often throws hairlines as it dries, and those still get the ruler.

Mark it and measure again after a couple of weeks

The BBC's advice, published on 24 August 2026, is the job for the week you are unsure. Mark both ends with a pencil, note the width, and measure again after a couple of weeks. A crack that has not moved is easier to treat as decoration. A crack that has lengthened, widened, or thrown off a flake is the one you stop filling. Photograph the ruler in the frame.

Two weeks is a check, not a survey. If the crack was already shorter last winter, you do not need another fortnight to call it growth. Growth is an NHBC reason to get advice. Keep the pencil marks until the second measurement exists. Painting over them the same weekend deletes the test.

Look at the other side of the wall if a cupboard, a loft, or the next room lets you. A crack on both sides is on the NHBC list even when each face looks hairline. If you cannot see that face, say so when you ask for advice.

Get advice above about 5 mm, or if it keeps growing

Get advice when the crack is wider than about 5 mm. Filler can hide that width for a season and leave no record of whether it opened further. A builder looking at a painted repair has to trust your photo. Give them the open crack, with the date and the millimetres beside it.

The same pause applies when a horizontal or vertical crack stays constantly above about 2 mm, when it shows on both sides, or when it keeps growing. One condition is enough. A builder is the usual first look. A structural engineer is who a builder, a surveyor, or an insurer may ask for when the pattern needs a calculation. Filler is the wrong first spend.

Leave a growing crack open. Covering it removes the mark you were going to measure. Palace's 600 g fine filler, B07XGMLGRX, was about £1.25 on 15 October 2026. That shelf price is not a test, and the card caps deeper repairs at layers of 3 mm. A moving 6 mm crack sits outside that sentence. Tell a landlord before you fill.

Diagonal cracks, lintels, and brickwork

A diagonal crack near a window, a door, or a lintel, or a crack that continues into the brickwork, needs a builder or a structural engineer before filler. That is the BBC's line. The shape is a reason to stop. It is not, by itself, a finding that the house is subsiding.

A short crack in the paint on the architrave can still be a decorating nick if it is stable and hairline. A crack that leaves the timber, crosses the plaster, and heads toward the corner of the reveal is the pattern to pause on. Brick that has split on the same line has left interior filler. What an outside tub must say is a later question, covered in the exterior filler article, and it still comes after someone has looked at the masonry.

A crack that starts at the end of a lintel and steps away through the plaster is one to photograph before you touch it. Hairline crazing with no wider line and no sag is a different pattern, and it still gets a width and a date.

Sagging, sticking doors, and damp

Checkatrade treats a sagging ceiling, a door that has started to stick, or damp beside the crack as reasons to call someone. A ceiling that has dropped against a straight edge is not a bead of filler in the crack below it. A door that now catches the frame can mean the opening has moved.

Damp is its own stop. A musty patch, a tide mark, or plaster that sounds hollow means the crack may be a symptom. Blown plaster over a wide area is a stop even when the crack is hairline. Paste on a live damp wall hides the crack and then falls out. The order of work is in damp before you fill. Fix the damp, then decide whether any crack that remains is cosmetic.

Leave a live back box, a socket, and any gap around a cable unfilled. A wide sheet of loose skim is a plastering job. If the surface drums or comes away in a hand-sized flake, the paste is sitting on material that is already leaving the wall.

Only then think about filler

A shallow interior hairline, stable over a couple of weeks, in plaster that is firm and dry, is the cosmetic case. This article does not pick the tub. The comparison does, by the depth the listing states. Fine surface filler is the smooth paste for hairline work. Palace's 600 g card caps deeper repairs at layers of no more than 3 mm. Polycell's multi-purpose page and Palace's 1 kg data sheet both describe about 10 mm in one go, then further layers after drying. A hole you can see through still needs a patch.

Once the crack is only cosmetic, the sequence is on how to fill holes in a wall: clean, fill within the stated depth, let it dry, then sand. A 330 g tube will not cover a badly marked wall. If the crack fails any warning on this page, leave the comparison closed. The eight listings are ready-mixed pastes for shallow repairs. None of them stands in for a builder in front of a diagonal crack at a lintel.