Basement flat flooding from drains in London

Basement and lower ground floor flats are where London’s drainage problems show up first. They sit below the street, and often below the level of the drain or sewer they connect to. When water has nowhere to go, it finds the lowest way out. That is often a basement toilet, a shower tray or a floor gully at the bottom of the front area steps.

Make it safe first

• Keep away from the water. Sewage water carries bacteria. Keep children and pets out.
• Electrics. If water is near sockets, appliances or the consumer unit, switch off at the consumer unit only if you can do so without standing in water. If you cannot, keep clear and call 999 if there is danger.
• Stop using water anywhere in the building. Tell the flats above to do the same.
• Do not try to wade in to move belongings if the water is deep or near electrics.

Rain or no rain

The single most useful thing to notice is the weather.

When it floods	Likely cause	Who to contact
During or just after heavy rain	Sewer overloaded, water backing up	Thames Water sewer flooding
In dry weather, after the flats above use water	Blockage in the building’s drain or stack	Plumber or drainage engineer, freeholder
When a pump stops	Failed or tripped basement pump	Plumber or pump engineer
Clean water, not sewage	Burst pipe or leak	Stop tap off, plumber

Why London’s sewers back up

Much of central London still drains into the sewer network designed by Joseph Bazalgette in the 1860s and 1870s. Historic England describes how it was built to capture both sewage and rainwater, which is known as a combined system. It was a huge advance at the time. But the same pipes now serve far more people and far more hard surfaces than they were designed for.

In a heavy storm, the combined sewer fills up. When it does, water can rise back up the drains connected to it. Basements and lower ground floor flats, being the lowest point, are first to flood.

Thames Water says flooding from a sewer is one of the worst things that can happen to your home, and it has a sewer flooding help service. Use it if water or sewage comes up during heavy rain.

Basements in Kensington and Chelsea

Kensington and Chelsea has a lot of basement and lower ground floor living. In the 2021 Census, 23.5% of households lived in converted houses (Nomis, TS044), many of them stucco terraces in South Kensington, Earl’s Court and Notting Hill where the lowest flat sits below the pavement. Many other houses have had basements dug out or extended under the garden.

Thames Water’s response to a Kensington and Chelsea scrutiny committee discusses flood protection devices and non return valves fitted in flood prone properties. If your building has had sewer flooding before, ask Thames Water whether it qualifies for this kind of protection.

Pumped basements

New and extended basements often cannot drain to the sewer by gravity, because the sewer is higher than the basement floor. Instead, waste goes into a pump chamber and is pumped up. If the pump fails, trips or loses power, the chamber fills and waste backs up into the basement.

Signs of a pump problem:

• the toilet or shower drains slowly or not at all
• an alarm on the pump panel is sounding
• the pump is silent when it should run
• the fuse or circuit for the pump has tripped

Reset a tripped circuit once. If it trips again, leave it off and call a plumber or pump engineer. Pumps need regular servicing. If you own a pumped basement, keep the service records.

Blockages in dry weather

If the basement floods on a dry day, the problem is more likely a blockage. In a converted house, the soil stack and drain under the building serve every flat. Waste from upstairs backs up into the lowest fitting. The Leasehold Advisory Service says communal pipes serving more than one flat are usually the freeholder’s responsibility. A drain that leaves the building and joins the neighbours’ may be Thames Water’s, since the 2011 private sewer transfer. Our guide on blocked drains and Thames Water explains how to tell.

After the flood

1. Take photos of the water, the source and the damage before you clean up.
2. Tell your insurer and, if you are a leaseholder, the managing agent.
3. Report sewer flooding to Thames Water even if it has gone down. Its records help decide on prevention work.
4. Clean safely. Thames Water says it can remove sewer debris for you, or you can do it yourself. Wear gloves and disinfect hard surfaces. Throw out soft items that cannot be properly cleaned.
5. Have the electrics checked before using any sockets that were wet.

Reducing the risk

If your basement floods from the drains, ask a plumber or drainage engineer whether a non return valve would suit your drain. Keep gullies and front area drains clear of leaves. If you have a pump, have it serviced. And if you live upstairs in a converted house, remember the basement flat takes whatever goes down your drains. Our blocked drains service covers the rest.