Emergency blocked drains in London

Much of central London drains into the combined sewer system built by Joseph Bazalgette in the 1860s and 1870s, which carries sewage and rainwater in the same pipes. In heavy rain the system can fill up, and water finds the lowest way out. That is often a basement toilet, a lower ground floor shower or a yard gully.

If sewage comes into a basement only in heavy rain, the cause may be the sewer, not a blockage in your pipes. Thames Water has a sewer flooding help service and, in some flood prone streets, has fitted non return valves that stop sewage flowing back into homes. If it happens with no rain at all, it is more likely a blockage nearby.

Most emergency drain calls come back to the same few things: cooking fat that sets in the pipes, wet wipes that do not break down, and food waste from kitchen sinks. In older houses, tree roots can get into clay drain joints. Hard water scale in waste pipes narrows them slowly, so a small amount of fat or hair blocks them faster.

If you share a drain with neighbours, a blockage you did not cause can still back up into your home first. That is worth knowing before an argument starts.

Stop using water in the building, including the washing machine and dishwasher, so you are not adding to the backup. If the water is dirty, keep children and pets away and wear gloves. If sewage is near sockets or appliances in a basement, switch off at the consumer unit if you can do it without standing in water.

Then check outside. Lift the manhole cover nearest the house if you can do so safely. If it is full, the blockage is further down the line. If it is empty while your gully overflows, the blockage is between the house and that chamber. Ask the neighbours whether their drains are slow. All of this helps you and the plumber decide quickly whether it is your drain, the building's or one for Thames Water.