Drought Subsidence: Why Cracks Appear in Your Walls During Heatwaves

England has been in an official flash drought since 29 July 2026, over 70 per cent of England and Wales is in drought, and nine water companies — Thames Water, Southern Water and Anglian Water among them — have hosepipe bans. Cue August's commonest call: a crack has appeared above the living-room door — is the house falling down?

Almost always, no. Nine out of ten summer cracks are cosmetic — plaster and masonry moving with temperature, not foundations moving with the ground. But droughts do spike subsidence claims, because much of the South East sits on shrinkable clay that loses volume as it dries. Here is the diagnosis in the order we do it on site, then the fixes, least risky first.

Before you spiral into underpinning quotes, spend five minutes with a pencil, a tape measure and a £1 coin — you are looking for the crack's pattern, not just its existence. Our guide to telling whether a foundation crack is serious goes deeper, but these checks catch most cases.

• •Width: a £1 coin is about 2.8mm thick. Hairlines under 1mm are nearly always cosmetic; anything that swallows the coin edge deserves monitoring; over 5mm needs professional eyes.
• •Direction: diagonal cracks from the corner of a window or door, wider at the top, are the classic movement pattern. Straight, fine cracks along board joints are usually shrinkage.
• •Inside and out: a crack mirrored on the external brickwork, especially stepping through mortar joints, points to the structure, not the plaster.
• •Doors and windows: a door that closed sweetly in May and now sticks suggests distortion — though humid nights swell timber too, so treat it as supporting evidence.
• •Timing: drought cracks open through late summer and often close slightly over a wet winter. Pencil-date the ends and measure again in a fortnight.

Most cracks sort into one of these patterns.

• •Web of fine hairlines on freshly plastered or newly decorated walls — drying shrinkage. Cosmetic.
• •Straight crack along a plasterboard joint or wall-ceiling junction — normal thermal and seasonal movement. Cosmetic.
• •Diagonal crack from a window or door corner, wider at the top, often mirrored outside — clay-shrinkage subsidence. Investigate.
• •Stepped crack through external mortar joints near a bay window or extension — movement at a shallow-founded element. Investigate.
• •Crack wider at the bottom than the top — possible heave from re-swelling clay, often after a tree was removed. Investigate.
• •Cracks with damp patches, drain smells or gurgling gullies nearby — a leaking drain undermining the soil. Investigate and survey the drain.

In a summer like this, we rank the causes of a new crack in this order.

• •Thermal and plaster movement (most likely). Sun-facing brickwork can hit 50°C on a 30°C day; materials expand, plaster and paint films crack. No foundations involved.
• •Clay soil shrinkage. London Clay, Gault and Weald Clay underlie London, Essex, Kent, Surrey and much of the South East. A long drought dries the clay a metre or more down — deeper than many Victorian footings at 450-600mm — so the footing drops a few millimetres and the wall cracks diagonally above.
• •Trees and large shrubs. A mature oak or willow can draw hundreds of litres a day, drying the clay on one side of the house faster than the other. Oaks, willows, poplars and leylandii within their mature height of the wall are the usual suspects.
• •Leaking drains. Old clay pipes fracture as the ground moves, then leak, softening clay or washing out finer soils; each makes the other worse.
• •Old settlement reawakening. Masonry houses move seasonally; drought exaggerates the cycle and reopens historic lines. Monitor, don't repair.

Work down this list; never start at the bottom. Monitoring costs nothing and every professional will ask for it: pencil-date the crack ends, photograph with a coin for scale, and measure fortnightly with a sub-£10 crack-width gauge from Screwfix or Toolstation.

Stable hairlines can be filled and redecorated once the weather breaks — flexible caulk, not rigid filler, on any line that may move again; see our guide to fixing cracks in plaster walls. A repair that recracks within a season is diagnostic gold: photograph it.

Resist the urge to water the foundations. Soaking one side of a clay footing creates exactly the differential movement you are trying to avoid — and with nine hosepipe bans in force, it could earn you a £1,000 fine.

Where a big tree stands close to the wall, a 25-30 per cent crown reduction by a qualified arborist (£300-£800) cuts water demand without the shock of removal. Check with the council first: unauthorised work on a tree with a Tree Preservation Order, or in a conservation area, is a criminal offence with fines up to £20,000. Never fell a mature tree by the house on impulse either — the clay re-swells and can cause heave, which costs more than the shrinkage did. Get advice before you get tree removal quotes.

If drains are in the frame — damp, smells, a crack near the soil stack — a CCTV survey costs £150-£350 and most defects reline for £1,000-£3,000 without digging.

Genuine subsidence is covered by standard UK buildings insurance, almost always with a £1,000 excess. Ring your insurer before commissioning any structural work: they appoint a loss adjuster and pay for the investigation, and repairing first can undermine the claim. Our walkthrough of filing a home insurance claim covers the process.

Expect monitoring, not diggers. Insurers typically monitor a claim for six to twelve months with crack gauges and level surveys, because the right fix is usually removing the cause — tree work or a drain repair — then making good. Fewer than one claim in ten ends in underpinning.

That caution is justified. Underpinning runs £10,000-£50,000 and up, needs building control approval, usually triggers the Party Wall Act on terraces and semis, and must be declared on every future insurance application and sale — where it often makes cover harder to get. See realistic foundation repair costs before believing any doorstep specialist who materialises in a drought summer. Never sign on the doorstep.

Some findings end the DIY phase immediately. Ring your insurer — or an independent RICS surveyor (£300-£600 for a defect inspection) if you want advice first. Renters: structural movement is your landlord's duty under the Homes (Fitness for Human Habitation) Act — report it in writing. Our home structural inspection checklist is a useful prompt sheet — the signs below don't condemn the house; they mean your gathered evidence now goes to a professional.

• •Any crack wider than 5mm — roughly a pencil's width
• •Measurable growth since you pencil-dated it
• •Stepped cracking through external mortar joints near bays or extensions
• •The same crack visible on both faces of the wall
• •Doors or windows that jam suddenly rather than gradually
• •Bulging or leaning brickwork, or gaps at skirting level
• •Drain smells, gurgling or fresh damp near the crack