How do I read a crack in brickwork, and when do I need a structural engineer?
The six crack patterns and what each usually means, how to monitor properly before spending money, and the honest line between a repointing job and an engineer's report.
- Difficulty
- intermediate
- Time
- 25 min
- Read
- 5 min
- Safety
- warning
Short answer
Read the pattern, not the width alone. Hairline cracking in the joints is weathered pointing. Stepped diagonal cracking is foundation movement. Regular horizontal cracking is wall ties. A crack from an opening corner means checking the lintel. Photograph everything against a dated ruler, monitor across a full year, and get an engineer for anything stepped, wide or still moving.
Most cracks in most houses are not serious, and the ones that are serious are usually recognisable by their pattern rather than their size. Buildings move — thermally, seasonally, and as they dry out over their first years — and a house that has never cracked anywhere is rarer than one that has. The purpose of reading a crack is to sort it into one of three groups: cosmetic, a specific repairable defect, or a structural question. The chart puts the six patterns side by side with what each usually means.
Safety
Step by step
- Photograph before you touch anything.Wide shots showing the crack in the context of the whole elevation, and close-ups with a ruler or a coin in frame and the date visible. This record is the single most useful thing you will produce, and it is worth more than any measurement taken later.
- Measure the width at several points.A crack that is the same width along its length is behaving differently from one that tapers. Note the widest point and where it is — at the top, at the bottom, or in the middle.
- Establish whether it goes through the wall.Check the corresponding position inside. A crack visible on both faces of the same wall is a different proposition from one in the outer leaf or in the plaster alone.
- Read the direction and the path.Does it step through the joints, run straight through bricks, run horizontally, or follow a junction? The path tells you far more than the width. Stepped through joints is the movement signature; straight through brick means force.
- Look at the whole elevation, not the crack.Stand back across the road. Is the wall bulging or leaning? Are the courses still running level? Is one part of the building lower than another? Are there other cracks that form a pattern together?
- Check the doors and windows.Frames that have started to bind, catch, or crack their glass are a strong indicator that something is moving, and one of the few symptoms that pushes a case from monitor to investigate now.
- Look for the obvious external causes.Large trees close to the building, a leaking gully or drain, a downpipe discharging against the wall, ground level raised against the house, recent excavation or a new extension alongside. Most movement has a mundane cause.
- Monitor across a full year before acting.Seasonal movement is real — clay soils shrink in summer and swell in winter, and a crack can open and close by a millimetre or two with the seasons without anything being wrong. Photograph the same points monthly with a ruler in frame, or fit a tell-tale gauge, and keep dated records.
- Speak to the insurer before commissioning a survey.If it looks like subsidence, contact the buildings insurer first. They normally appoint and pay for the investigation, and commissioning your own survey first can complicate a claim.
- Do not fill a crack you are still monitoring.Filling hides the evidence, and if the movement is continuing the filler cracks again anyway. Repair only once the monitoring shows it is stable, or after the cause has been dealt with.
- Get a structural engineer for anything on the list.Stepped diagonal cracking. A crack still opening. Wide cracking, or a crack you can see daylight through. Cracking on both faces. Doors and windows binding. Any bulge or lean. Anything at or near a lintel, a bay window or a chimney. A structural engineer's report on a domestic crack is not expensive relative to what it settles.
Tips
- A crack that has been there, unchanged, since before you moved in and has a layer of old paint over it is a very different animal from one that appeared this spring.
- Millimetres matter far less than movement. A stable 5mm crack is less of a concern than a 1mm crack that has doubled since the summer.
- Tell-tale crack monitors are inexpensive, fit over the crack, and give a dated numerical record that an engineer or an insurer will take seriously.
- Most cracking in UK houses relates to clay soil shrinking in dry summers, and a large proportion of it stabilises without any intervention at all.
If it doesn't work
Fine hairline cracking following the mortar joints only, in patches
Cause: Weathered or shrunken pointing — Fix: Cosmetic. Rake out and repoint the affected area in an appropriate mortar. No further investigation needed.
Stepped cracking running diagonally through the joints, wider at one end
Cause: Differential foundation movement — subsidence, heave or settlement — Fix: Do not fill it. Photograph and monitor across a full year, look for the obvious causes outside such as trees, leaking drains and clay soil, and speak to your buildings insurer before commissioning any survey.
A vertical crack straight through bricks as well as joints
Cause: Thermal or moisture expansion in a long run with no movement joint, or an unrestrained junction between old and new work — Fix: Cracking through the brick rather than round it means substantial force. Monitor it, and get it assessed — the cause may be benign but the diagnosis is not one to make from a photograph.
Horizontal cracking along bed joints, repeating at a regular vertical spacing, wall slightly bulging
Cause: Corroding wall ties expanding within the joints — Fix: A specialist remedial tie installation, not a repointing job. Get a wall tie survey, which uses a metal detector and a borescope.
A diagonal crack running up and out from the top corner of a window or door
Cause: Stress concentration at the opening corner — often minor settlement or shrinkage, but the same location where a failing lintel shows — Fix: Check the lintel specifically before treating it as cosmetic. Sight along the wall for a sag over the opening and look for cracks at the bearings.
Cracking in the internal plaster with nothing corresponding outside
Cause: Plaster shrinkage, a background junction, or movement in a non-structural element — Fix: Fill and redecorate, using a flexible filler or scrim where it crosses a junction. Check the same location outside first to be sure.
Cracking near a chimney breast, with the stack visibly out of plumb
Cause: Possible sulphate attack in the stack — Fix: Do not go up to look. Get a roofer or engineer to inspect from a proper access.
New cracking, opening quickly, with doors and windows starting to bind
Cause: Active movement of some kind — Fix: This is the urgent case. Contact your buildings insurer and get a structural engineer. Do not decorate over it, do not fill it, and record the dates.