Working out whether a crack means subsidence or ordinary settlement

What genuinely causes subsidence

Clay soils shrinking as they dry out, often worsened by nearby trees drawing moisture from the ground during summer, is one of the most common genuine causes of subsidence in the UK generally, and it applies to clay-rich pockets found in parts of South Devon. Leaking drains that wash away supporting soil beneath a foundation over time are another recognised cause, as is poorly compacted made-up ground beneath an extension or outbuilding.

Subsidence typically affects one part of a building more than another, which is why cracks associated with it are often diagonal, wider at the top than the bottom, and located near corners or where an extension meets the original structure.

Far more common causes of cracking

Historic settlement, where a building settled slightly in its early years and then stopped moving, leaves cracks that are old, stable and often painted over repeatedly without ever growing. Thermal and moisture movement in materials such as render, causes cracking that opens and closes slightly with the seasons but does not represent structural movement at all.

Cracking at the junction between an original building and a later extension is also extremely common, since the two sections often have different foundation depths and will always move slightly independently of one another, particularly in the first few years after the extension is built.

• Old, stable settlement cracks that have not changed in years
• Seasonal thermal or moisture movement in render or plaster
• Differential movement at the junction of an original building and an extension
• Lintel deflection causing cracking above windows and doors
• Poor detailing or missing movement joints in a render or block system

Telling subsidence apart from these other causes

The most useful evidence is whether a crack is actively growing or has been stable for years, which is why marking a crack with a pencil line and date, and checking it again after a few months, is a genuinely useful first step. Subsidence cracks are also often accompanied by other signs, such as sticking doors and windows, gaps appearing between a bay window and the main house, or cracks visible on both the inside and outside of the same wall in the same location.

Cracks that only appear in render and are not visible in the brick or blockwork behind, or that stay the same width all year round, are far less likely to indicate subsidence than cracks that visibly widen during dry summer months, particularly near mature trees on clay soil.

What happens once genuine movement is suspected

Where signs point toward ongoing ground movement rather than historic or cosmetic cracking, a structural engineer needs to assess the property, which may include monitoring the crack over several months and, in some cases, trial pits to examine the foundations directly. This assessment determines the actual cause, whether that is a nearby tree, a leaking drain or genuine soil shrinkage, before any remedial building work is specified.

Once the cause is confirmed and, where necessary, addressed, such as removing or managing a problem tree or repairing a leaking drain, the resulting cracks can usually be made good with conventional repair methods, since further structural intervention like underpinning is only needed in a minority of genuine subsidence cases.

When to act

A crack that has clearly been there for years, is consistent in width, and is not accompanied by sticking doors or other movement signs can generally be dealt with as a straightforward cosmetic repair. A new crack, or an existing one that has visibly changed, especially during a dry summer near a mature tree, should be monitored and assessed rather than simply filled and painted over.

If a crack is wider than around 5mm, is accompanied by doors and windows that have started sticking, or has appeared suddenly rather than developed gradually, it is worth getting a proper structural assessment sooner rather than later.

Common questions