Why buildings crack
Buildings are not rigid, they expand, contract and move with temperature, moisture and load. Common, generally harmless causes include thermal movement, seasonal shrinkage and swelling of materials, and the natural “settlement” of a new building or extension as it beds in. These usually produce fine, stable hairline cracks.
Subsidence, heave and settlement
- Settlement: downward movement as the ground compresses under the weight of a new structure. Usually happens early and then stops.
- Subsidence: downward movement of the ground beneath the foundations, often due to shrinkable clay soils drying out (frequently linked to nearby trees), leaking drains, or made-up ground.
- Heave: the opposite of subsidence, where the ground swells and pushes the foundations up.
Warning signs to take seriously
A crack is more likely to need investigation if it is:
- Wider than about 5mm, or noticeably tapered (wider at one end).
- Diagonal, especially running from the corners of windows and doors.
- Visible on both the inside and the outside of the same wall.
- Progressive: getting longer or wider over weeks and months.
- Accompanied by sticking doors and windows, or gaps opening between walls, floors or extensions.
What to do
Try not to panic, many alarming-looking cracks are stable and easily repaired. The right response is a professional assessment: measuring and, where necessary, monitoring the crack over time, identifying the likely cause, and advising whether it is cosmetic or needs structural attention or an insurance claim. A clear, independent report also helps enormously if you are buying or selling.