Is my wall crack getting bigger? How to check

When you first notice a line running through the plaster, the immediate question is whether your wall crack is getting bigger. The answer rarely reveals itself in a single afternoon. Measuring wall cracks at home takes patience, but the effort means a specialist arrives with facts rather than a vague worry.

Begin by drawing a light pencil mark across each end of the crack. Write the date next to both lines. Stand back and take a photograph that shows the whole wall, then move closer for a second shot where the pencil marks fill the frame. Photographing cracks for survey purposes works best in flat, even light. Turn off the flash so the shadow of the crack reads clearly. These dated images form the baseline of your record.

Cardiff sits on varied ground. Houses near the docks may rest on made land, while older terraces in Canton or Roath often sit on shallow Victorian footings. Clay soil shrinks during dry summers and swells after heavy rain. That seasonal rhythm means a crack can open and close slightly over twelve months without indicating a structural failure. A short observation period helps separate normal movement from something that needs attention.

A pencil mark tells you if the ends have shifted, but it does not measure the gap. A crack monitoring gauge is a small plastic disc fixed across the opening with adhesive pads. It carries a scale that shows lateral and vertical movement in millimetres. You can buy one from a builders merchant or hardware shop. Fix it firmly so it does not slip when the wall moves.

A tell tale crack monitor serves the same purpose using two overlapping plates with a grid printed on them. Both devices do the same job: they turn a visual guess into a number.

Alongside the gauge, start keeping a crack diary. Use a notebook or a phone document. Every two weeks, write down the date, the reading on the gauge, and anything relevant to the weather. Note whether it rained heavily that week, or if the heating has been running constantly. If you live near mature trees, mention whether their leaves are out or bare. Roots draw moisture from clay in summer, which changes how the ground behaves beneath your foundations. This written log becomes the most valuable thing you hand over when a structural survey happens.

The shape of a crack matters as much as its size. A hairline crack that stays the same width from top to bottom usually points to plaster drying out or minor thermal movement. But when crack width matters is when the gap tapers. A crack that is noticeably wider at the top than at the bottom suggests the wall is tilting away from its base. A crack wider at the bottom can indicate the centre of the wall is dropping. Both shapes warrant professional attention.

As a rough guide, a crack under two millimetres wide is generally classed as minor. Between two and five millimetres, it becomes moderate and worth tracking closely. Anything above five millimetres, or any crack where a coin fits easily into the gap, should prompt a call to a specialist firm. Do not wait for it to reach that point if the gauge shows steady weekly growth.

Leave the crack alone while you monitor it. Filling it with caulk or polyfilla hides the evidence and prevents a gauge from sitting flat against the surface. The filler will simply split again if the wall is still moving, adding confusion to your log.

After eight to twelve weeks of recording, you will have enough data to decide whether to book a subsidence survey in Cardiff. Eight weeks covers several weather cycles; twelve gives a clearer picture if the movement is slow. There is no benefit in waiting six months if the gauge shows consistent growth every fortnight.

When the surveyor arrives, lay out everything you have gathered. Hand over the notebook with your diary entries. Show the dated photographs in order so the progression is visible. Point out the gauge and let them take their own reading. Mention any leaking drains you have noticed outside, or large trees within ten metres of the affected wall. Leaking pipes wash soil away, and tree roots extract moisture; both change the ground conditions locally.

The specialist will use your log to decide what comes next. They might recommend continued monitoring for another season, or they might advise a trial hole to inspect the foundations directly. If foundation repair is necessary, methods range from mass concrete underpinning to resin injection, depending on the soil type and the depth required. Your preparation saves time and ensures the investigation starts from an accurate history.