Walk into any older London home and you'll find cracks. Hairline shrinkage in the plaster, a settlement line above a doorway, the odd stepped fissure in the brickwork outside. Most are harmless. Some are early warnings of structural movement that gets expensive if it's ignored. The trick is knowing which is which.
Shrinkage and drying cracks
Fresh plaster, new render and recently-poured concrete all shrink as they dry. The cracks they leave are usually hairline width, run in irregular patterns, and tend to appear in the first year or two after the work was done. They are cosmetic. A bit of decorator's caulk and a paint touch-up is all that's needed.
Thermal movement
Long terraces and large flat walls expand and contract with seasonal temperature swings. The cracks this produces tend to be vertical, evenly spaced, and roughly the same width top to bottom. They open up in winter, close up in summer. Annoying, but not dangerous. Movement joints solve it permanently.
Settlement
Almost every new build settles a few millimetres in its first decade as soils consolidate under the load. The cracks are usually fine, often hidden inside cupboards or behind skirtings, and they stop appearing after a year or two. If your house is over ten years old and a crack has only just shown up, settlement is unlikely to be the cause.
Subsidence
This is the one that costs money. Subsidence cracks are diagonal, wider than 3mm, run from window or door corners, and taper from wide at the top to narrow at the bottom. They get worse over months and years, not days. They often appear in pairs on adjacent elevations. If this sounds like what you have, it's worth getting a survey before deciding whether to call your insurer. Our subsidence repair partners cover Croydon, Lewisham, Greenwich and the rest of South East London.
Heave
The opposite of subsidence. Clay re-swells when a thirsty tree is removed or a long drought ends, and pushes the foundation upwards. The cracks look similar to subsidence but the geometry is inverted: walls pushed apart at the base rather than dropping at the top. Heave is rarer than subsidence but harder to fix because the ground keeps moving for years.
Lateral pressure
Basement walls and retaining walls crack horizontally when the soil behind them pushes. This needs proper investigation because it can lead to wall failure. Don't ignore horizontal cracks in any wall holding back earth.
What to do next
Photograph anything that worries you with a ruler or a coin in shot for scale. Date the photos. Check the same crack again in two or three months. If it's wider, take a photo and call a specialist. If it's the same, leave it be. The cracks that matter are the ones that grow. Talk to someone if you want a second opinion.