How Household Drains Work, and Where Blockages Usually Happen

Household drains work by gravity. Every sink, bath, shower and toilet has a trap that holds water to keep smells out, a waste pipe that carries water to a vertical soil stack or an outside gully, and from there an underground drain that runs through one or more manholes to the public sewer. Blockages happen where that flow slows down: in traps, at bends, where pipes join, and where the drain has sagged or cracked. Knowing the layout tells you, within a few minutes, which part is blocked and who needs to clear it.

Traps and waste pipes: the part inside the house

Every fitting has a trap, a bend or pot that stays full of water so sewer gas cannot come back up the plughole. Traps are the most common place for a blockage in a single fitting.

Basins and baths usually have a U-bend or bottle trap under the plughole; showers have a trap built into the tray waste; toilets have the trap moulded into the pan itself. From the trap, a waste pipe carries water to the soil stack or out through the wall. Under Building Regulations guidance these pipes are small, around 32mm for a basin and 40mm for a sink, bath or shower, against about 100mm for a toilet and the underground drain. Narrow pipes block easily, which is why hair in a shower waste or fat in a kitchen waste causes so many call-outs.

A blockage here affects one fitting, or two that share a waste pipe, such as a kitchen sink and a washing machine. Everything else in the house drains normally.

The soil stack: where the upstairs drains meet

The soil stack is the vertical 100mm pipe, inside the building or on an outside wall, that toilets and upstairs wastes connect into. It carries everything from the upper floors down to the underground drain, and it is vented at the top so water flows without sucking traps dry.

A blocked stack behaves like a drain blockage rather than a single-fitting one. Upstairs fittings may drain slowly or gurgle, and the lowest toilet on the stack is usually the first to back up, because it is the first outlet the rising water reaches. In older London houses the stack is often cast iron, and scale and rust inside it narrow the bore over time. In converted houses, several flats often share one stack, so a blockage caused in one flat can flood another.

Gullies: where outside waste pipes drop in

A gully is the small grated drain at ground level outside a kitchen or bathroom wall. Waste pipes from sinks and washing machines, and sometimes rainwater downpipes, discharge into it, and it has its own water trap below the grate.

Gullies block more often than any other part of the external drainage because they collect leaves, silt and kitchen fat in one small pot. A gully overflowing with an empty manhole downstream means the gully or its connection is blocked. A gully overflowing with a full manhole means the drain further down is the problem. Our guide to a blocked outside drain walks through telling the two apart.

Manholes, inspection chambers and the underground drain

Below ground, a single drain, usually 100mm, runs from the foot of the stack and the gullies to the boundary. Manholes and smaller inspection chambers sit on it wherever it changes direction or two runs meet, so that it can be rodded, jetted and inspected.

Lifting the chamber covers is the quickest way to find a blockage. Waste flows along an open channel at the bottom of each chamber. If a chamber is full, the blockage is downstream of it. If it is empty, the blockage is upstream. Work from the house towards the road until you find the first empty chamber after a full one, and the blockage sits in the pipe between them.

Many London terraces also have an interceptor trap in the last chamber before the sewer, a deep U-bend designed to keep sewer gas and rats out of the house drains. It has a small stopper, the rodding eye, that can come loose and fall into the channel, which is one of the most common causes of a full last manhole. Other causes in the underground drain include roots, displaced joints, collapsed sections and fat carried down from the kitchen.

The lateral and the public sewer: who owns what

In England, you are normally responsible for the drain that serves only your property and sits within your boundary. Once a pipe serves more than one property or leaves your boundary, it is usually a shared sewer or lateral drain belonging to the water company, which in most of London is Thames Water.

That changed in October 2011, when most private sewers and lateral drains were transferred to the water companies. It matters in an emergency because a blockage in the public sewer, or in a shared section beyond your boundary, is the water company's to clear at no charge to you. If the chamber on your side of the boundary is empty and the problem affects your neighbours too, call the water company first. Our page on who is responsible for a shared drain blockage goes through the common cases, including terraces where one drain runs behind several houses.

Much of inner London is also served by combined sewers, which take rainwater and sewage in the same pipe. During very heavy rain they can surcharge and back up into low-lying drains even when nothing in your own drain is blocked.

Using the layout to decide who to call

One fitting blocked means a trap or waste pipe: a plumber or a competent householder. Several fittings, a backing-up ground-floor toilet or a full manhole mean the drain: a drainage engineer. A full chamber at the boundary with an empty one on your neighbour's side, or a problem shared by the whole street, means the water company.

When it is the drain, we clear it through the nearest chamber with rods or a jetting unit, then run a camera if the cause is not obvious or the drain has blocked before. Our drain clearance is charged by time band: 7am to 5pm at the standard rate, 5pm to midnight at the evening rate and midnight to 7am at the night rate, with current figures on the pricing page. If the layout points to a public sewer, we will tell you on the phone rather than send someone to a job that belongs to the water company.