Leak or Flood? Stop Mould in the First 48 Hours

Short answer: speed matters more than anything else. Wet plaster, plasterboard, timber, carpet and underlay can start growing mould within 24 to 48 hours, and the US Environmental Protection Agency's widely used guidance is that materials not dried within that window should be assumed to have mould. Make the property safe, stop the water, photograph everything for your insurer, get standing water and soaked soft furnishings out, and start drying with heat, ventilation and dehumidifiers straight away. Then check for hidden moisture before anyone redecorates.

Hour 0: make it safe

Electrics. If water has reached sockets, light fittings or the consumer unit, or is coming through a ceiling near a light, switch off the electricity at the mains if you can do so safely. If not, keep out and call an electrician.
Ceilings. A bulging, sagging ceiling can hold a lot of water and may collapse. Keep people out from under it.
Dirty water. Floodwater from rivers, surface water or sewage is contaminated. Wear waterproof gloves and boots, keep children and pets away, and treat anything it has soaked as contaminated.
Vulnerable occupants. If someone in the home is elderly, very young, pregnant or has a lung condition or weakened immune system, consider whether they should stay elsewhere while drying is under way.

Hours 0 to 2: stop the water and record it

Turn off the water at the stopcock for a burst pipe or leaking appliance. In a flat, check whether water is coming from above and alert the neighbour or building manager.
Photograph and video everything before you move it: the source, water levels, damaged items, ceilings and walls. Include timestamps.
Call your insurer early. Many have emergency lines and approved drying contractors. Ask what they want before you throw anything away.
Landlords and agents: tell the tenant what's happening and when, and log the time you were told.

Hours 2 to 24: get the water out

Remove standing water with a wet vacuum, pump or mops. Professional extraction makes a real difference with carpeted floors.
Lift wet carpet and underlay. Underlay is rarely worth saving. Carpet soaked with clean water may be salvageable if dried quickly; carpet soaked with floodwater or sewage should usually be thrown away.
Move furniture and belongings out of wet areas or up onto blocks. Wet cardboard, books, mattresses and upholstered furniture are prime mould food.
Open up hidden voids where water is trapped: remove kick boards under kitchen units, open bath panels, and if a ceiling is holding water, let a professional drain it.

Hours 24 to 48: dry hard

Drying is about removing water from the air as fast as it comes out of the building.

Heat the property moderately to speed evaporation.
Ventilate by opening windows where outdoor air is drier, or run extractor fans.
Dehumidify. Domestic dehumidifiers help, but for anything more than a small leak, rented professional drying kit (high-capacity dehumidifiers and air movers) is far more effective. Our guide to dehumidifiers for mould explains the difference between compressor and desiccant types.
Keep doors closed between the room being dried and the rest of the house so moisture doesn't move into dry rooms.

After 48 hours: check what you can't see

The surface may look dry long before the inside of a wall or floor is. Water wicks into plaster, travels along floor joists and sits under laminate, vinyl and kitchen units.

Use a moisture meter, or get a specialist to take readings, before redecorating or refitting.
Watch for signs of hidden mould: a musty smell, staining that spreads, skirting boards or laminate lifting, or paint bubbling.
Don't paint or wallpaper too early. Sealing a wet wall traps moisture and mould behind the finish.
Remove what can't be dried. Plasterboard that has been soaked, especially by dirty water, and insulation that has slumped or stayed wet often need to be cut out and replaced.

If mould has already appeared, clean small areas with gloves, eye protection and a mask using a product labelled for mould, and get anything larger dealt with by a professional. Government guidance on damp and mould says larger areas should only be addressed by qualified professionals. Our post on whether bleach kills mould explains why bleach isn't the answer.

Who to call

Plumber or roofer to fix the source
Electrician if any electrics got wet
Water damage restoration or drying specialist to extract water, dry the structure and log moisture readings
Mould remediation specialist if mould has appeared or the property stayed wet for days
Damp surveyor if you can't tell whether damp that remains is from the leak or a separate problem. See damp survey vs mould inspection

For landlords

A leak or flood is a repair you're responsible for, and fast action limits both the damage and the health risk. Key points:

Social landlords in England must treat emergency hazards under Awaab's Law, investigating and making safe within 24 hours, and offer suitable alternative accommodation if the home can't be made safe in time.
Private landlords don't yet have Awaab's Law timeframes, but the home must remain fit to live in, and councils can take enforcement action under the Housing Health and Safety Rating System, which in England now includes a baseline indicator that no room should have observable damp or mould over more than 5% of a wall or ceiling surface.
Keep records of when you were told, who attended, moisture readings and what was replaced. They protect you if mould appears later.

FAQs

How quickly does mould grow after a leak?

Mould can begin growing on damp materials within 24 to 48 hours. It won't always appear that fast, but the risk rises sharply the longer materials stay wet.

How long does it take a house to dry after a flood?

Small leaks dried with good equipment can be dry within days. A flooded home can take weeks or months, depending on the construction and how much water got in. Moisture readings, not appearance, tell you when it's done.

Does home insurance cover mould after a leak?

Many policies cover damage caused by an insured event such as a burst pipe, but may exclude mould that results from a slow leak or from delay. Check your policy and report quickly.

Can I stay in my house while it dries?

Often yes, after a small clean-water leak. After contaminated floodwater, or with vulnerable people in the home, staying elsewhere is safer.

After a leak or flood - how to stop mould in the first 48 hours