How serious is this crack?

A crack is usually serious when it is wider than about 5 mm, keeps growing, tapers, or shows on both sides of the same wall. Most hairline cracks are cosmetic, but width, direction and change over time tell you whether one needs an expert look.

Is a crack in my wall serious?

Most cracks are not serious: hairline cracks under 1 mm are common in plaster and mortar and usually come from drying shrinkage or small seasonal movement. A crack becomes a concern when it is wider than about 5 mm, keeps growing, is wider at one end than the other, or appears on both the inside and outside of the same wall. Those signs point to movement in the structure rather than the finish. Width alone is not a diagnosis, so the pattern and history of a crack matter just as much.

What do the BRE crack damage categories mean?

The BRE Digest 251 categories grade damage from 0 to 5 by how easy it is to repair, using typical crack widths as a guide. Engineers, surveyors and loss adjusters use them as a shared language. Categories 0 to 2 are usually aesthetic, 3 and 4 affect serviceability, such as sticking doors and broken pipes, and 5 affects stability. The widths are a guide, not a diagnosis: a 4 mm crack from a failing lintel can matter more than a 6 mm crack that stopped moving decades ago.

• Category 0, negligible: under 0.1 mm
• Category 1, very slight: up to 1 mm
• Category 2, slight: up to 5 mm
• Category 3, moderate: 5 to 15 mm, or several about 3 mm
• Category 4, severe: 15 to 25 mm
• Category 5, very severe: usually over 25 mm

How do I measure a crack properly?

The simplest way to measure a crack is with a crack width gauge, a cheap plastic card printed with lines of known thickness that you hold against the widest point. Measure at the same marked spot each time, note the date, and photograph the crack with the gauge or a ruler in shot. A coin gives a rough check: most UK coins are between about 1.5 mm and 3 mm thick, so a crack that takes a coin’s edge is well past hairline.

To see whether a crack is moving, mark both ends with a pencil line and the date, then check every few weeks. If the crack runs past your mark, it is still growing. A plastic tell-tale fixed across the crack shows movement in two directions and costs little. Keep your notes in one place. A record of width over several months is far more useful to an engineer or insurer than a single measurement taken on a bad day.

What does the direction of a crack tell me?

The direction and shape of a crack point to its likely cause, although only an inspection can confirm it. Look at where it starts and finishes, whether it follows the mortar joints or cuts through bricks, and whether it is wider at the top or the bottom. A crack that tapers usually means part of the wall is rotating or dropping relative to the rest, which is more significant than an even, straight crack of the same width.

• Diagonal and stepped from window or door corners: possible foundation movement
• Tapering, wider at one end: part of the building rotating or dropping
• Horizontal and repeating every few courses: possible wall tie corrosion
• Vertical in a long wall: often thermal or moisture movement
• Just above a window or door: possible lintel problem

Which other signs make a crack more worrying?

A crack is more worrying when it comes with other changes in the building. Doors and windows that start to stick, wallpaper that creases or tears at a corner, gaps opening between skirting boards and floors, or a crack that reopens after filling all suggest ongoing movement. Cracks that appear or widen after a long, dry summer, especially near trees on clay soil, are typical of subsidence. A damp patch or a drain smell near a crack can point to a leaking drain softening the ground.

When should I get a crack looked at?

Get a crack looked at if it is wider than about 3 mm, is growing, tapers, shows on both faces of a wall, or comes with sticking doors or windows. Cracks near bay windows, chimneys, extensions and lintels deserve an early look too. If you think the cause is subsidence, tell your insurer before permanent repairs start, because filling or stitching too early can hide evidence and complicate a claim.

For a first opinion, send us clear photos with a gauge or ruler in shot for a free initial assessment. Where a repair needs an engineer’s design or sign-off, an independent chartered structural engineer is appointed, and we can arrange this. Once the repair is specified you get a no-obligation written quote and a fixed price in writing. Many cracks turn out to need nothing more than monitoring and careful filling.

Questions about this guide