Cracks in walls: what they mean and when they matter
Most cracks in most houses are not structural. The ones that matter tend to announce themselves by their direction, their width and whether they are still moving. Here is how an engineer reads them.
Should I be worried about cracks in my wall?
Usually not. The majority of cracks in houses are shrinkage or settlement in finishes rather than structural movement. The ones worth investigating are diagonal or stepped, wider than roughly 5mm, wider at one end than the other, or still opening over a period of months.
Direction tells you more than width
A crack that runs straight up and down the middle of a plastered wall, hairline and consistent, is usually shrinkage in the plaster or the block behind it. Buildings dry out for years after they are built and after any wet trade, and the finish cracks where it is weakest. That is a decorating problem.
A crack that runs diagonally, and particularly one that steps through the mortar joints of brickwork, is following the path of a load that has moved. Something below or beside it has dropped, rotated or spread. Diagonal and stepped cracks are the ones worth photographing and dating, because their direction points back towards the cause.
Width matters, but less than people expect. The Building Research Establishment categorises damage in bands, and a great deal of what alarms homeowners falls into the lowest two, which are aesthetic. What changes the assessment is a crack that is wider at one end than the other, wider than about 5mm, or passes through a lintel, a padstone or a structural opening.
Is it still moving?
A crack that opened years ago and stopped is a historical record. A crack that is opening now is a live problem, and the two need completely different responses. Filling a live crack simply hides the evidence and the filler cracks again within months, which is often how people discover the movement is ongoing.
The cheapest useful thing you can do costs nothing. Photograph the crack next to a ruler, write the date on the photograph, and repeat in three months and six months. If the width is unchanged across a full cycle of seasons, the movement has almost certainly stopped. If it is opening, you now have evidence rather than an impression, and any engineer you instruct starts from a much better position.
Where there is doubt, movement can be measured properly rather than estimated. Studs fixed either side of a crack and read over months give a numerical record. That is the difference between an opinion and a monitored result, and it is what an insurer or a buyer's surveyor will want to see.
What actually causes the ones that matter
Foundation movement is the cause people fear, and it is the least common. Clay soils shrink in dry summers and swell in wet winters, and a nearby tree drawing water out of clay is behind a large share of genuine subsidence cases in the south and east of England. The pattern is usually diagonal, usually worse near one corner, and often worse after a dry summer.
Far more common are causes that are not the foundations at all. A failed or undersized lintel over a window will crack the masonry above it. Roof spread pushes the tops of walls outwards and cracks them near eaves level. Wall tie corrosion in cavity walls produces regular horizontal cracking at intervals up the wall. A removed chimney breast that was never properly supported will crack the wall it left behind.
Each of those has a different remedy and a very different cost. That is the reason to identify the cause before agreeing to any repair, and the reason to be cautious with any contractor who diagnoses subsidence and quotes for underpinning in the same visit.
What we check.
The points our calculations resolve for a project like this.
- Crack direction: vertical and hairline usually shrinkage, diagonal or stepped usually movement
- Whether the crack is active, established by dated photographs or measured monitoring
- Whether it crosses a lintel, opening or other structural element
- Proximity of trees and the soil type, particularly shrinkable clay
- Whether previous alterations, such as a removed chimney breast, left something unsupported
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Issue
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Common questions
Should I be worried about cracks in my wall?
Usually not. The majority of cracks in houses are shrinkage or settlement in finishes rather than structural movement. The ones worth investigating are diagonal or stepped, wider than roughly 5mm, wider at one end than the other, or still opening over a period of months.
What is the difference between settlement and subsidence?
Settlement is the small downward movement that happens as a new building beds into the ground in its first few years, and it stops. Subsidence is downward movement of the ground supporting the foundations, often caused by clay shrinkage, drainage leaks or nearby trees, and it does not stop on its own.
Do I need an engineer or a surveyor?
A building surveyor reports on the condition of the property generally. A structural engineer assesses whether the structure is adequate and designs the remedy if it is not. If the question is what the crack means and what to do about it structurally, that is engineering work.
Will my insurer accept an engineer's report?
Insurers generally want the cause established and evidence that the movement is or is not ongoing. A report that sets out the crack pattern, the likely mechanism and any monitoring results is the document that supports a claim, or that demonstrates a claim is not needed.
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