Mould after a leak or flood: drying out, what to remove and when to clean

Updated 6 November 2026 · By the Mould Cleaning team

Short answer: After a leak or flood, mould can start growing on wet materials within roughly 24 to 48 hours, so the priority is to stop the water, remove standing water and soaked contents, and start drying as soon as possible. Saturated plasterboard, carpet underlay and insulation usually need removing, while solid plaster and masonry can often be dried. Mould should be cleaned and treated once the cause is fixed and materials are drying.

First steps in the first 48 hours

Stop the source of water and make the electrics safe, turning off power to affected circuits if water has reached sockets or fittings
Take photos and video of the damage before moving anything, for your insurer
Remove standing water and lift soaked rugs, cushions and stored items
Open windows when the weather allows and keep air moving
Run dehumidifiers with doors closed in affected rooms
Lift wet carpets and underlay, as underlay rarely dries out properly
Contact your insurer early and ask whether they will appoint a drying company

If the water came from outside flooding or a sewage backup, treat everything it touched as contaminated. Wear gloves and boots, keep children and pets away and do not try to save soaked soft items.

What usually needs removing

Material	Usual approach after soaking
Plasterboard	Often cut out to above the water line if soaked, swollen or mouldy
Solid plaster on masonry	Usually dried in place, unless it is crumbling or contaminated
Carpet underlay	Normally replaced
Carpet	Sometimes dried after clean water leaks, usually replaced after floodwater
Insulation in walls or ceilings	Wet mineral wool and loose fill usually replaced
Timber floors and joists	Dried and monitored, replaced if rotten
Kitchen units with chipboard carcasses	Often replaced if swollen

Removing wet materials quickly is what stops mould growing out of sight inside walls, floors and ceiling voids.

How long drying takes

Drying almost always takes longer than people expect. A small, clean water leak caught early may dry in a week or two with dehumidifiers. A larger leak or flood soaking solid walls and floors can take several weeks to a few months, as masonry dries slowly from the inside out. A common rough guide for solid walls is about a month per 25mm of thickness under natural conditions, though professional drying equipment speeds this up considerably.

Surfaces can feel dry while the material behind is still wet. Moisture meter readings taken over time are the only reliable way to know when walls and floors are ready for redecoration. Painting or replastering too early traps moisture and leads to mould and flaking finishes.

Cleaning and treating the mould

Mould that appears during drying is cleaned and treated once the source of water is fixed and the area is drying steadily. Cleaning too early, while materials are still saturated, means it often grows back. Hard surfaces are cleaned and treated with a fungicidal product, mouldy porous materials are removed, and voids exposed by removing plasterboard are cleaned before being closed up again.

Fix the leak and confirm it has stopped
Remove saturated and contaminated materials
Dry the structure, checking with moisture readings
Clean and treat remaining surfaces and exposed voids
Reinstate plasterboard, plaster and finishes once readings are acceptable
Finish with anti-mould paint in vulnerable areas

Signs of hidden mould after a leak

Mould from a leak often grows where you cannot see it: behind plasterboard, under flooring, inside kitchen units and in ceiling voids below bathrooms. Watch for these signs in the weeks after the water damage, even if surfaces look dry.

A persistent musty or earthy smell in one area
Skirting boards, architrave or flooring that swell, warp or lift
Paint that bubbles or wallpaper that peels near the affected area
Dark spots spreading out from behind kitchen units or bath panels
Moisture readings that stay high or rise again after first drying

If any of these appear, the area needs opening up and checking rather than redecorating over the top.

How insurance usually works

Many home insurance policies cover sudden escape of water, such as a burst pipe or failed appliance hose, and storm or flood damage, including drying and reinstatement. Most do not cover gradual damage, such as a slow leak under a shower that has been seeping for months, or mould caused by condensation. Policies vary, so check your wording, report the damage promptly and keep receipts for dehumidifiers and emergency work. Insurers often appoint their own drying contractor, and it is worth agreeing who is handling mould treatment before work starts.

Landlords and tenants after a leak

In a rented home, the landlord is usually responsible for repairing leaks from pipes, roofs and gutters and for making good the damage to the structure, including mould that results. Tenants should report leaks straight away, in writing, and allow access for repairs. Prompt action and clear records help both sides, and for social landlords in England, Awaab's Law timescales apply where damp and mould become a significant hazard.