Subsidence is one of those words homeowners hear and immediately picture worst-case outcomes, collapsing walls, six-figure repair bills, an unsellable house. The reality is usually more contained. Most properties showing classic subsidence symptoms have a problem that is fixable, often without dramatic intervention, provided it is diagnosed properly and early. The trick is recognising the difference between cracks that mean something and cracks that simply look alarming.
Stair-step cracking in external walls
The single most recognised signature of differential settlement is a diagonal crack that runs through the mortar joints of an external brick wall in a stair-step pattern. One part of the wall has dropped relative to another, and the brickwork has cracked along the weakest line, the mortar, rather than splitting bricks. The width of the crack matters less than its pattern, location, and whether it is still moving. For a wider checklist of which cracks deserve attention, see signs you might need underpinning.
Cracks wider than a one-pound coin, running diagonally from a window or door corner outward and downward, are worth a structural opinion. Hairline cracks that follow the same path are worth photographing and watching, but rarely urgent.
Sticking doors and windows
Doors and windows that suddenly bind in their frames after a hot, dry summer are a strong hint of clay shrinkage, especially if there is a mature tree within ten or fifteen metres of the affected wall. Clay soils swell in winter and shrink in summer, and a thirsty tree accelerates the summer shrinkage dramatically. London Clay is the textbook example, see underpinning in London for the local picture. The frame distorts, the door catches, and the homeowner reaches for a plane when the real fix lies underground.
Sloping floors
A floor that slopes noticeably across one bay can mean settlement of an internal sleeper wall, decay of timber bearers, or settlement of the external wall the floor ties into. The diagnostic question is straightforward: is the slope sudden or has it always been there? Victorian and Edwardian houses often have floors that have settled gradually over a century without any active problem. A spirit level, a marble, and a few photographs over a fortnight will tell you whether the slope is moving, and that is the difference between a cosmetic levelling job and a structural one.
Seasonal cracks
Cracks that open in summer and close in winter, or vice versa, point to seasonal moisture cycling rather than active foundation failure. They still warrant attention, the underlying cause is usually clay heave or shrinkage, but the fix is often vegetation management or root barriers rather than underpinning.
Drainage failures
Leaking drains are the single most under-diagnosed cause of subsidence in the UK. A cracked clay drain washing fines out of the ground beneath a foundation will cause settlement that mimics every other cause perfectly. A CCTV drain survey is one of the cheapest and most useful diagnostics available, and should be on every survey checklist. See the signs worth taking seriously for a fuller checklist.
How to document movement
- Photograph each crack with a ruler and a date in the frame, weekly for at least a month.
- Stick a crack monitor (a few pounds online) across the worst cracks.
- Note the weather: wet weeks, dry weeks, and any recent storm or burst pipe.
- Measure door and window openings with a tape and write the numbers down.
- Walk the perimeter and look for soft, sunken, or freshly cracked paving.
When to call someone
Two or more of the signs above, or any single crack that is visibly growing week on week, justifies a structural engineer's opinion before you start collecting quotes. The diagnosis decides everything that follows. A quote without a diagnosis is a guess.