The call usually comes on a Sunday. Somebody upstairs has left a bath running, or a washing machine hose has let go behind a kitchen unit, and there is now a stain the shape of Wales spreading across a bedroom ceiling in a mansion block off Elgin Avenue. By the time I arrive the ceiling has been the main event for two days and nobody has thought about the floor, which is where the money is.

Carpet and underlay behave completely differently when soaked. One of them is usually recoverable and one of them usually is not, and the mistake most people make in the first forty-eight hours is treating them as a single item that either survived or didn’t.

What happens to a carpet in the first forty-eight hours?

Wool and good synthetic broadloom tolerate a soaking better than anyone expects. Wool holds a great deal of water without structural harm, dries back to shape, and takes a hot rinse afterwards without complaint. Polypropylene barely absorbs anything at all. The fibre is rarely the casualty.

What kills carpets is time and what’s dissolved in the water. Above about seventy-two hours in a warm flat you get microbial growth in the backing, the latex adhesive holding the primary and secondary backings together begins to break down, and you get delamination – the carpet goes slack and ripples because the two layers have stopped being one thing. Jute-backed wovens and Axminsters shrink. A wool Axminster in a Crystal Palace flat off Westow Hill pulled a full 60mm away from the skirting on the long wall after six days under a slow leak, and no amount of stretching gets that back.

This is where it gets expensive.

Why the ceiling water is dirtier than it looks

The water leaving a bath upstairs is clean. The water arriving in your bedroom has passed through a ceiling void, sixty years of dust, plaster, mineral wool, possibly rodent deposits, and the top of a light fitting. It lands as category two water at best. If the leak came from a toilet pan or a soil stack, it’s category three and the carpet is a disposal item regardless of how good it looks.

I’ve had clients insist a carpet under a soil stack leak smelled fine. It did, for eleven days.

Is the underlay ever worth saving?

Almost never, and this is the single most useful thing I can tell anyone reading this at eight on a Sunday evening.

PU foam underlay – the recycled crumb stuff in about eighty per cent of London flats – is a sponge with a closed film on one face. It holds water against the floor, releases it slowly upward into the carpet backing for weeks, and cannot be dried in place at any sensible speed. Sponge rubber and crumb rubber are worse still and go on smelling. Felt is a lost cause the moment it’s wet.

Underlay costs somewhere around £6 to £11 per square metre supplied and fitted. This is where it gets expensive – not in the foam, but in the days of drying equipment somebody wants to charge you for while they try to save it. Lifting the carpet, binning the underlay, drying the subfloor properly and refitting over new underlay is cheaper, faster and more reliable than any attempt to dry the original in situ. Anyone proposing to blast air under your carpet for a week to save forty pounds of foam is billing you for equipment hire.

What the moisture readings tell you

A drying technician takes readings from the screed or the floorboards, not from the carpet. Concrete screeds in ex-local-authority blocks hold water for a long time and will read wet for a fortnight after the surface feels bone dry. Timber should come back to somewhere near the readings taken from an unaffected room in the same flat, which is the comparison that matters rather than any absolute number. Refit a carpet over a screed that hasn’t come down and you’ll have a musty bedroom by March.

Who pays, and which policy?

Here is the part that surprises leaseholders. In a block of flats the building is insured under one policy, usually arranged by the freeholder or managing agent, and that policy covers the structure and the decorative finishes – ceilings, plaster, paintwork, and generally fixed floor coverings such as wood or vinyl. Carpets in individual leaseholders’ flats sit outside that policy, while wooden floors and lino are treated as buildings. Illogical, and settled practice, so there’s no point arguing it with a claims handler on the phone.

Your carpet is contents. That means your own contents policy, your own claim, your own excess – even though somebody else’s washing machine caused the damage.

The leaseholder upstairs pays for repairing the leak itself. Whether they pay for your carpet depends on negligence, and negligence is harder to establish than people assume. A failed flexi-hose that nobody could have known about is bad luck. A bath left running while somebody answered the door is arguable. Many leases and many managing agents apply the block policy excess to the flat the water came from, which is a separate argument again and one you don’t want to be in the middle of.

The excess trap nobody reads until it’s too late

Escape of water usually carries its own peril-specific excess that replaces your standard one, commonly £250 to £500 and higher on some policies.It has historically been one of the most expensive claim types in the UK, which is why insurers price it that way. If your buildings and contents cover sit with different insurers, you may be paying two excesses on one leak.

Check the schedule rather than the booklet. The figure lives in the schedule.

The other exclusion worth knowing about is gradual damage – most policies cover sudden and unforeseen events and exclude slow leaks that have been going on behind a shower tray for months. That exclusion catches a lot of London flats, because the leak that ruins a carpet is frequently the one nobody noticed until the smell arrived.

Should you let the drying company lift the carpet?

Yes, provided they do it properly and provided you’ve photographed everything first. Lifting carpet is not a decision to defer while you wait for a loss adjuster to book a visit in eleven days’ time. Insurers expect you to mitigate the loss, and a carpet that could have been saved on day two and rotted on day nine is a conversation you will lose.

The thing to watch is what happens at the gripper. Carpet lifted carelessly off a Kingston flat’s gripper rods near Canbury Park Road came back to me with the backing torn along two edges, which turned a cleanable carpet into a replacement. This is where it gets expensive, and the person who caused it is never the person who pays for it. Ask for it to be lifted at one edge, back-rolled rather than folded, and stored face to face.

When I tell people to skip restoration entirely

If your carpet is builder-standard polypropylene loop from a new-build handover – the sort in half the flats in Colindale – don’t pay me to restore it. Cleaning, drying and refitting that carpet will cost more than a new one from a decent supplier and you’ll still have a five-year-old carpet at the end. Bin it, claim for it, and put the drying money into getting the subfloor right.

I take the same view on bedroom carpets that were on their last legs before the leak. Insurers apply wear-and-tear deductions to contents settlements anyway, so you’re rarely getting a like-for-like new carpet out of it, and paying restoration costs on top of a depreciated settlement is money into a hole.

My unpopular position: claim anyway. People agonise over a £400 claim and their renewal premium and end up absorbing a £2,000 loss to protect a £40 increase. Escape of water is a standard peril and the whole point of having the policy is events like this one.

What do you do in the first hour?

Stop the water, then photograph everything before you move a single item – wide shots of each room, close shots of the carpet at the wettest point, and the ceiling above it. Loss adjusters want to see the state of things before anyone tidied. Lift furniture off the wet area or put foil under the feet, because walnut and mahogany stain carpet fast and cheap metal castors leave rust rings within hours.

Then get air moving and get the heating on low rather than high. A hot room with no ventilation grows mould faster than a cool one with an open window and a fan.

The photographs the loss adjuster will ask for

Adjusters want the wet area with something in shot for scale, the source above, the carpet lifted at one corner showing the underlay, and the underlay itself before it goes in the skip. Photograph the label on the back of the carpet if there is one, and the fitting date if you have paperwork, because that is what the wear-and-tear deduction gets argued from. Keep the receipts for anything you buy in a panic on the day – dehumidifier hire, towels, a night in a hotel if the bedroom is unusable – as those sit under the same claim.

Notify the managing agent in writing the same day, even before you’ve decided about claiming. Ring your contents insurer to open a reference number. Ask the neighbour upstairs, politely, to tell you what failed and when they noticed – in writing, ideally by text – because that message is the difference between a negligence conversation and a shrug six weeks later.

The carpet in Elgin Avenue came out fine. The underlay went in a skip on the Monday, along with a wool bedside rug from a flat share in Bow the owner had carted around since and and hadn’t looked at properly since 2016.