Flooded House: What To Do First and How Long Drying Takes

Around 6.3 million properties in England sit in an area at risk of flooding, and about 4.6 million of those are at risk from surface water — the kind of flooding that arrives in an hour after a storm drain gives up. The Environment Agency expects that number to reach roughly 8 million, or one property in four, by the middle of the century.

If it has just happened to you, the useful thing to know is that the repair does not start when the water goes. It starts when the building is dry, and that is the part almost everyone underestimates. This guide gives you the order of work, honest timescales from UK government drying guidance, the kit that does the job, and the moisture readings that tell you when to stop.

The first 48 hours, hour by hour

Do these in order. Skipping ahead to the drying stage while soaked plasterboard is still on the wall is the single most common way to lose a month.

Why a flood is not the same as a leak

People use the words interchangeably and then hire the wrong kit. The difference is not the volume of water — it is where the water went and what was in it.

• A leak or burst pipe puts clean water into a known place. It travels down and outward from one point, and it is usually one or two rooms. Our guide to what to do after a burst pipe covers that job.
• A flood puts contaminated water in from outside, across the whole footprint, and it fills the parts of the building you cannot see: the sub-floor void, the cavity, the insulation, the underside of a suspended timber floor. It also brings silt, and often sewage.

That difference drives three decisions. You strip out more than feels necessary, because contaminated material is not worth drying. You dry for longer, because the water is in the fabric, not on it. And you keep drying after the surfaces feel dry, because the void underneath is weeks behind the room above.

The UK government guidance on drying flood damaged buildings puts numbers on how much the construction matters: a double thickness wall takes roughly four times as long to dry as a single one, and insulation left in place can delay things by 10 to 12 weeks. Two houses on the same street, flooded to the same depth, can be six weeks apart purely because of what they are built from.

How a flooded room actually dries

There are only two ways to get water out of a building: carry it out as liquid, or carry it out as vapour. Once the standing water is gone, everything left is the vapour job, and it works in a loop.

1. Air movers push air across wet surfaces. Moving air strips the saturated boundary layer off plaster, screed and timber so more moisture can leave. A dehumidifier with no air movement dries the air in the middle of the room and almost nothing else.
2. The moisture goes into the room air. Relative humidity climbs. This is supposed to happen — humid air is the moisture on its way out.
3. The dehumidifier takes it back out again and drains it away, so the air can pick up the next load.
4. The room stays closed. Open a window in the middle of this and you swap dried air for cold outdoor air that is often at 85% RH itself. The loop stalls.

That last point is the one that surprises people. Ventilation is right for the first day and wrong for the next six weeks. If you want the longer version, dehumidifier versus air mover explains what each machine is actually doing.

Find your setup: tap what you are drying

Open the one that matches your property. Each gives the kit that does the job and what it costs to hire from our Greenford depot, with same or next day delivery across London.

One flooded room — up to about 25 m²

A single room that took water: a hallway, a downstairs bedroom, a garage conversion.

Kit: one 43L Dri-Eaz Cube dehumidifier at £39.99 a day, plus one air mover at £19.99 a day. Or take the entry drying package at £84.99 a day and have it arrive as one delivery.

Expect: two to four weeks if the floor is timber, longer if it is screed.

A whole flooded ground floor — 40 to 80 m²

The usual London terrace or ground-floor flat after surface water comes in the front and out the back.

Kit: one 63L LGR industrial dehumidifier at £59.99 a day and three air movers. The LGR flood drying package at £169.99 a day is exactly this bundle.

Why LGR: a low grain refrigerant unit keeps pulling water out after the easy moisture has gone and the air has already dropped to 40% RH. A standard dehumidifier slows right down at that point, which is the stage where the fabric is still wet.

A flooded basement or cellar

Below ground, no cross ventilation, often a concrete floor and solid walls.

Kit: 63L LGR or the 95 litre building dryer at £77.99 a day, with a pump-out hose to a gully so no one has to empty a tank down a flight of stairs. Add air movers to stop dead corners.

Expect: the longest job on this page. Read how to dry out a basement before you start.

A flooded commercial unit, shop or office

Trading matters, so drying has to run around the business.

Kit: our largest unit, the 95L building dryer at £77.99 a day for the open floor, low-profile air movers at £19.99 a day under fixtures and along walkways where a standard air mover would be a trip hazard.

Tip: a written drying record is what your insurer and your landlord will both ask for. Take a reading every week and keep it.

A suspended timber floor

Floorboards on joists over a void. The void is the problem — it holds water long after the boards feel dry.

Kit: lift boards at both ends of the room and direct an air mover into the void. Dehumidify the room above as normal.

Watch for: the government guidance target for timber is 10 to 12% moisture content. Boards that read 18% look and feel completely dry.

A concrete or screed floor

Slow, predictable, and the one people give up on too early.

Kit: 55L DrizAir 1200 at £49.99 a day with air movers pointed low across the slab.

Do not lay flooring until the screed reads 75 to 80% RH on a surface box test. Vinyl or engineered board over a wet screed lifts within a year and the second failure is not insured.

It is insured and a loss adjuster is involved

Your insurer may appoint a drying contractor. If they do, let them. If there is a delay of days while it is agreed, hiring a dehumidifier yourself in the meantime is a reasonable mitigation step — most policies require you to limit further damage.

Keep receipts and weekly readings. Our guide to what insurance covers on damp and water damage explains what usually gets paid and what does not. We are not insurance advisers, so check your own policy wording.

How long it really takes

These are the figures from the UK guidance on drying flood damaged buildings, not marketing numbers. They assume the strip-out has been done properly.

Two things stretch those numbers more than anything else. A double thickness wall takes about four times as long as a single skin. And insulation left inside a wet wall or floor adds 10 to 12 weeks. If you take nothing else from this page, take that: the strip-out decides the timeline, not the size of the dehumidifier.

For a room-by-room view rather than a whole property, how long it takes to dry out a room has the shorter version.

What the equipment costs

Hire prices from our Greenford UB6 depot, delivered across Greater London. A refundable deposit applies and comes back in full when the equipment returns undamaged.

Running cost is a fair question and it is smaller than most people expect next to the cost of a delayed repair. What it costs to run a hired dehumidifier works through the electricity side properly.

How you know it is actually dry

“It feels dry” is not a measurement, and it is the reason so many flood repairs fail twice. The official guidance is blunt about it: you cannot define a whole building as dry by one percentage. What you can do is check each material against its own target before you close it up.

Take a reading once a week and write it down with the date. That weekly record is what proves the property was dried properly, and it is what a surveyor, a landlord or an insurer will ask to see. It also tells you when to stop, which saves you hire days.

Five mistakes that add weeks

1. Leaving the windows open. Right on day one, wrong from day three. A dehumidifier in a ventilated room is trying to dry London.
2. Drying around the insulation. Wet mineral wool in a wall or under a floor holds water for months and adds 10 to 12 weeks. It has to come out.
3. One machine, no air movement. A dehumidifier on its own dries the air. Air movers are what get the water out of the fabric. One dehumidifier and two air movers is the setup that actually works.
4. Turning everything off at night. Drying is a continuous process. Twelve hours off lets moisture migrate back out of the fabric into the surfaces you just dried, and you start the day behind.
5. Rebuilding on a feeling. No meter, no readings, new floor down, and a callback in eighteen months. Ten minutes a week with a moisture meter prevents it.

Flood drying questions, answered

How long does it take to dry a flooded house?

With dehumidifiers and air movers running properly, a flooded ground floor typically takes four to eight weeks. Three weeks is the best realistic case. Left to dry naturally it takes months — after the 2007 UK floods the average was around 30 days for drying alone, with half of properties dry within three months and 82% within six. Solid or double thickness walls take roughly four times as long as single skin, and insulation left in place adds 10 to 12 weeks.

What size dehumidifier do I need for a flooded room?

For one flooded room up to about 25 m², a 43L unit. For a whole flooded ground floor, a 63L LGR or a 95L building dryer. Size on the wet fabric and the volume of the space, not the floor area alone — a flooded room with a screed floor and solid walls needs far more capacity than a dry room with a bit of condensation. Our dehumidifier sizing guide walks through it, and how many dehumidifiers you need covers multi-room jobs.

Should I open the windows to dry a flooded house?

Only in the first day, to flush out the worst of the humid air while you are stripping out. After that, keep the room closed. A dehumidifier only works in a sealed space — with the windows open you are drying outdoor air that is often at 85% RH itself, and the machine never gets ahead.

Do I need air movers as well as a dehumidifier?

Yes. They do different jobs. Air movers strip the saturated layer of air off wet plaster, screed and timber so moisture can leave the material. The dehumidifier then takes that moisture out of the room air. A dehumidifier alone dries the air and leaves the fabric wet. Most flooded rooms need one dehumidifier and two or three air movers.

Can I use a heater instead of a dehumidifier?

Heat helps, but it does not remove water — it just lets the air hold more of it. Without a dehumidifier or ventilation, that moisture condenses again on the coldest surface in the building. Controlled heat alongside drying kit speeds things up; heat on its own moves the damp somewhere else. Heater versus dehumidifier for drying compares them properly.

What is an LGR dehumidifier and do I need one?

LGR stands for low grain refrigerant. A standard refrigerant dehumidifier slows down badly once the air is already fairly dry, which is exactly the stage where the walls and floor are still wet. An LGR unit keeps pulling water out at low humidity, so it finishes the job rather than stalling at 50% RH. For flood work it is worth the difference — ours is £59.99 a day.

Does a dehumidifier work in a cold flooded property with no heating?

Refrigerant dehumidifiers are designed to work between about 15 and 28°C. Below that they lose efficiency and can ice up, although units with hot-gas defrost — like the DrizAir 1200 — keep working down to around 1°C. If the property has no heating, hire a heater alongside the dehumidifier and hold the room at a steady temperature. It costs less in hire days than it does in lost weeks.

Is flood water dangerous to clean up?

Treat it as contaminated. Surface water and river flooding routinely carries sewage, oil and whatever was in the drains. Wear gloves, boots and eye protection, wash hands before eating, keep children and pets out until it has been cleaned, and bin porous items that were in the water rather than trying to save them. Anything electrical that was submerged should be checked by an electrician before it is used again.

Will my insurance pay for the drying equipment?

Often, yes — most buildings policies cover the drying stage as part of the claim, and many require you to take reasonable steps to limit further damage, which hiring drying kit is. Keep every receipt and your weekly moisture readings. Policies differ, so check your own wording or ask your insurer before you commit. We hire the equipment; we are not insurance advisers. What insurance covers on damp has more detail.

Can I hire flood drying equipment same day in London?

Yes. We deliver across Greater London from our Greenford UB6 depot with same day and next day delivery available, and drying packages go out as one matched bundle so you are not assembling a kit from parts. Call 020 3375 4048 or message us on WhatsApp and tell us the room size and the floor type — that is enough for us to send the right machine.

Related guides

• How to dry out a house after a leak — the smaller, cleaner-water version of this job.
• What equipment a professional drying company uses — and how much of it you can hire yourself.
• What size dehumidifier do I need — sizing by room volume rather than guesswork.
• How to dry out a basement — the hardest space in any flooded property.
• Rising damp, condensation or penetrating damp — for when the damp was there before the water was.
• Awaab’s Law and landlord damp deadlines — if you are a landlord, the clock is legal as well as practical.

Drying a flooded property in winter is the hardest version of this job, because the building is cold as well as soaked. Why dehumidifiers stall below 15°C explains what to add so the kit can work.