Recording cracks in walls: how to measure a crack

When you first notice a crack in your home, the instinct is to call someone straight away. But a short period of recording cracks in walls gives the visiting specialist something solid to work from. Without dates and measurements, they are looking at a single moment rather than a pattern.

Southampton sits on varied ground. Parts of the city rest on clay that shrinks after a dry summer, while areas near the Itchen or Test valleys behave differently. Knowing whether a crack opened during a hot spell or widened gradually over months helps separate seasonal movement from something more serious. You can read more about how [cracks form after dry weather](/sinking-and-settling/cracks-after-a-dry-summer) and why soil type matters locally.

A simple log also becomes part of your evidence if you decide to make a claim. Insurers ask when the problem started and whether it has changed. Notes taken at the time carry more weight than memory. This crack record before a survey saves everyone from guessing.

You do not need expensive tools to monitor a crack in a wall at home. A standard steel ruler works, but a set of feeler gauges from a hardware shop gives you a reading to the nearest millimetre. Some people search for a crack width gauge diy option; a printed scale taped beside the crack is better than nothing, though paper stretches with humidity.

Hold the ruler flat against the surface. Measure the widest point and note it down. Then measure the same spot each time you check. If the crack tapers, take readings at the top, middle and bottom. Write the numbers in a notebook or a phone note alongside the date.

To track wall movement accurately, always measure from the same fixed points. Mark them lightly in pencil on either side of the crack so you return to exactly the same place. Measuring a different section each week produces numbers that cannot be compared. Consistency is what makes the record useful to a structural engineer or a [foundation repair](/foundation-repair-southampton) firm later on.

Photographing a crack for insurance or for your own records takes two minutes. Stand far enough back to show where the crack sits on the wall: above a window, along a ceiling line, near a corner. Then move close enough that the edges of the crack are sharp. Place a coin or a ruler next to it for scale.

Take the first set of photographs as soon as you notice the damage. Repeat the process every two weeks, or weekly if the weather is changing fast. After a long dry spell or heavy rain across Southampton, an extra photograph is worth taking. Keep the images in one folder on your phone, named by date.

Natural light is best. Flash flattens the surface and hides depth. If the crack is in a dark hallway, hold a torch at an angle to cast a shadow along its length. These details matter when a [subsidence survey](/subsidence-survey) specialist reviews the file months later.

A tell tale on a wall crack is a small plastic device with two overlapping plates. One plate fixes to each side of the crack. As the wall moves, the plates slide past each other and a scale shows the direction and distance of that movement.

You can fit one yourself. Clean the plaster or brickwork where it will sit. Use the adhesive pads supplied, or drill small holes for screws if the surface is powdery. The important thing is that both halves sit flush and the scale starts at zero. Do not paint over it, and keep furniture from knocking it.

If the wall is outside, or the crack runs through tile or render that might crumble, a specialist should fix the gauge. They use resin anchors that hold in loose material. For most interior plaster cracks in a Southampton terrace or semi, the DIY approach works fine. Just check it has not been bumped each time you take a reading.

Does it help to note the weather and the date each time? Yes. Ground movement in Hampshire is closely tied to rainfall and temperature. Clay soils shrink when they dry and swell when they absorb water. A crack that opens in August and closes slightly in November is behaving differently from one that widens steadily regardless of the season.

Write down whether the preceding weeks were wet or dry. You do not need exact rainfall figures; a note like three weeks without rain is enough. Over time this log reveals patterns that raw measurements alone cannot show.

How long should you keep a record before calling someone out? Two to three months is usually enough to establish a trend. If the crack is wider than five millimetres from the start, or doors nearby have begun sticking, do not wait. Book a [subsidence survey](/subsidence-survey) immediately. The notes you have already taken will still help.

Hairline cracks under one millimetre rarely indicate structural trouble. Many homes in Southampton have plaster cracks caused by drying timber or minor settling. If your measurements stay within a millimetre over several months, the wall is likely stable.

A change of two to three millimetres between readings means the wall is moving. That does not automatically mean the foundations have failed, but it does mean the cause needs identifying. Tree roots drawing moisture from the soil, a leaking drain washing away fines, or seasonal clay shrinkage could all be responsible. Reading up on [how tree roots affect ground](/sinking-and-settling/tree-roots-and-subsidence) helps narrow the possibilities.

When should you stop recording and get a survey booked? If the crack grows by more than five millimetres in total, if it extends rapidly over a few days, or if you notice new cracks appearing elsewhere, stop waiting. Contact a firm that handles [structural monitoring](/structural-monitoring) or arrange a full survey. Your log goes with you. Insurers generally accept homeowner photographs and handwritten notes as supporting evidence, provided they are dated and consistent. Check your policy or read about [how house insurance handles subsidence claims](/sinking-and-settling/does-insurance-cover-subsidence) for specifics.