Sewage Coming Up Through a Drain or Toilet? What to Do

Updated July 2026 · Emergency Plumber Chelsea

Sewage always comes up at the lowest opening on the blocked run first. In a house that is usually a basement shower, a ground-floor toilet or a gully outside the back door. In a lower-ground or basement flat it is often the shower tray or floor drain. That is why you can flush an upstairs toilet and watch the water rise downstairs. If only one fitting is slow and nothing is backing up elsewhere, the problem is probably a local blockage in that fitting's waste, and our guides on how to unblock a toilet and how to unblock a sink are the better starting point.

Make the area safe before you do anything else. Sewage carries bacteria that cause stomach illness and infections. Wear rubber gloves and old shoes, keep children and pets out of the room, and do not use a household vacuum cleaner on wet waste. If dirty water is near sockets, floor-level electrics or an extension lead, switch off the power at the consumer unit if you can reach it without standing in water. Open windows for ventilation. Take photos of the affected rooms and the water level before you start cleaning, because insurers and Thames Water may ask for them.

Lifting the manhole cover tells you where the blockage is. Most Chelsea houses have at least one inspection chamber in the front area, rear garden or side passage, and many terraces share one. Lift the cover with a screwdriver or a pair of flat bars under the lip, keeping your back straight. If the chamber is full of standing sewage, the blockage is further down the line, between that chamber and the sewer. If the chamber is empty or running normally, the blockage is between your house and that chamber. In a period terrace, check whether a neighbour's chamber is also full; if it is, the blockage is on a shared section. Our guide to a blocked outside drain has more detail on reading a chamber.

Who pays depends on where the blockage sits. The drain that serves only your property and sits within your boundary is your responsibility. Since October 2011, drains shared with neighbours and the lateral drain running from your boundary to the public sewer have been Thames Water's responsibility, along with the public sewer itself. If the blockage is on a shared or public section, report it to Thames Water and they will clear it at no charge. If it is on your private drain, you need a plumber or drainage engineer. Our guide to RBKC drainage and flooding explains who to call in more detail.

Most private-drain backups have a simple cause. Wet wipes, cotton buds and sanitary products do not break down, and they catch on joints and bends until the pipe closes. Kitchen fat sets hard in cold drains and narrows the pipe over months. In older properties, tree roots, a displaced joint or a partly collapsed clay pipe can create a snag that everything else builds up against. A high-pressure jet clears most blockages. If the same drain keeps blocking, a CCTV drain survey shows whether the pipe itself is damaged, and our guide to collapsed drain repair costs explains what happens next.

Heavy rain can cause sewage to back up even when nothing in your own drain is blocked. Much of Chelsea is served by the Victorian combined sewer system, which carries rainwater and foul waste in the same pipes. In a heavy storm the sewer can fill and push water back up into the lowest connections, which is why basement flats and lower-ground kitchens are most at risk. This is called internal sewer flooding and it is Thames Water's responsibility, so report it to them as soon as it is safe. A non-return valve or anti-flood device on the drain can stop it happening again, and we can advise whether one suits your property.

Basements with pumped drainage have their own failure point. Many Chelsea basement conversions send waste up to the sewer through a pump or a macerator, and if the pump fails or its float sticks, sewage backs up in the basement even though the main drain is clear. If you have a pump chamber, check whether it is running and whether its alarm is sounding. Our guides to sump pumps and basement flooding and macerator problems explain what to look for.

There are some things you should not do. Do not pour chemical drain cleaner into a drain full of sewage; it will not reach a blockage metres down the line and it makes the water dangerous for whoever clears it. Do not keep plunging a toilet when sewage is also coming up in the shower, because you are only pushing the backup somewhere else. Do not push drain rods into a chamber full of sewage without gloves and eye protection. If the water keeps rising, stop and call for help.

Once the drain is clear, the clean-up matters as much as the unblocking. Hard floors and tiles can be washed and disinfected. Carpets, underlay, rugs and soft furnishings soaked with sewage are usually not worth saving. Plasterboard and skirting that stayed wet for more than a short time may need to be cut back and dried. Keep receipts for everything, and our guide to what to do if your house floods covers drying out and dealing with your insurer.

If sewage is coming up and your own chamber is full or the blockage is between the house and the chamber, call Emergency Plumber Chelsea on 020 3002 6667. Our emergency drainage team jets and clears drains across SW3, SW10, Knightsbridge, Sloane Square, South Kensington and Fulham, and if the blockage turns out to be on a public section we will tell you so you can report it to Thames Water. For routine blockages, see our drain unblocking service. There is no call-out fee on jobs over 3 hours.

Quick tips

• Stop all water use the moment sewage appears: every flush or tap makes it worse.
• Lift the nearest manhole: a full chamber means the blockage is further downstream.
• Shared drains and the lateral drain to the sewer are Thames Water's to clear.
• Never use chemical drain cleaner on a sewage backup.
• Photograph the damage before cleaning for your insurer or Thames Water.

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