uk Home & property emergencies damp patch spreading • wet patch on wall • wall damp getting worse fast • sudden damp spot indoors • water leak in wall • hidden leak behind plaster • burst pipe in wall • upstairs leak showing downstairs • ceiling or wall stain growing • water ingress through wall • rain leak through masonry • plumbing leak suspected • damp patch near socket • water near electrics • emergency water shut off • find the stopcock • turn off water supply • consumer unit main switch • renter damp emergency • landlord repair urgent • home water damage now

What to do if…
you notice a new damp patch forming on a wall and it is expanding hour by hour

Short answer

Treat this as an active leak until proven otherwise: keep the area electrically safe, then stop (or reduce) the water and get urgent repairs arranged.

Do not do these things

Do not ignore it because it’s “only damp” — hour-by-hour growth usually means water is still entering the wall.
Do not touch sockets, switches, chargers, or extension leads near the patch (or walk into a wet area to reach them).
Do not drill, cut, or “make a hole to drain it” — you could hit wiring or a pressurised pipe.
Do not paint/seal it or spray anti-mould products while it’s actively spreading.
Do not blast heat directly onto a wet wall (portable heater tight against it) while the source is unknown.

What to do now

Make a safer pause and check for obvious electrical danger.
If you see sparking, smell burning, hear buzzing/crackling from a socket/switch, or water is reaching electrical fittings: keep people back. If there is an immediate life-threatening danger (fire or someone at risk of electric shock), call 999.
If it’s safe to do so, isolate electricity for the affected area.
If the damp is near electrics (or you’re unsure), switch off the relevant circuit at the consumer unit. If you can’t identify it confidently, switch off the main switch. If you suspect water has entered the consumer unit itself, do not touch it — keep clear and call a qualified electrician.
Reduce water use immediately, then shut off the supply if the patch keeps growing.
Stop using taps/showers/appliances and stop flushing toilets unless essential. If the patch continues to expand, turn off your inside stop valve/stopcock (often under the kitchen sink or where the supply enters).
If you’re in a flat or the supply is communal, shut-off points can be different — don’t waste time hunting if you can’t find it quickly.
Quickly triage the most likely source (no invasive checks).
- Near/under a bathroom, kitchen, radiator, boiler, or washing machine? Assume plumbing/heating leak.
- On an external wall during heavy rain/wind? Assume rain ingress (guttering/roof/flashing/pointing).
- Only after using a shower/bath/appliance? Assume waste pipe or seal failure.
Get the right person on the way (do this early).
- If you rent: report it to your landlord/letting agent/council repairs as an urgent repair and say it’s “expanding hour by hour” and whether it’s near electrics.
- If you own: call an emergency plumber (or heating engineer if you strongly suspect a heating system leak).
- If you believe the problem is with the incoming supply pipe you can’t isolate (or a shared supply): contact your water company’s emergency line.
Limit damage safely while waiting.
Move furniture/valuables away from the wall and lift items off the floor. Catch drips with a bucket/towels. Keep any water on floors away from electrical leads and low sockets. Take photos/video (wide + close-up) and note the time and how fast it’s spreading.
Ventilate gently; prioritise stopping the source.
Open windows if safe to do so. Don’t focus on “drying it out” until the water source is stopped and electrics are confirmed safe.

What can wait

You do not need to decide whether it’s “damp” vs “leak” right now — the speed of change matters more than the label.
You do not need to remove plaster, start redecorating, or book non-urgent damp treatments today.
You do not need to settle insurance/liability immediately — first stabilise, document, and get repairs moving.

Important reassurance

A fast-spreading patch is alarming, but you only need to do a few things to regain control: avoid electrical contact, reduce/stop the water, and get urgent help arranged.

Scope note

These are first steps to stabilise and prevent avoidable harm and damage. Finding the exact cause and the correct repair normally needs a qualified professional once it’s safe.

Important note

This is general information, not a professional inspection or legal advice. If you’re unsure whether electrics are affected, treat the area as unsafe and get qualified help.

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What to do if…you notice a new damp patch forming on a wall and it is expanding hour by hour