Blocked Sewer or Blocked Drain? How to Tell Which One You Have

You can usually tell whether you have a blocked sewer or a blocked drain by lifting your inspection chamber covers and seeing which chambers are full. If the blockage is downstream of your last chamber, in pipework shared with neighbours or under the road, it is most likely a public sewer and the water company clears it. If it is within the pipe that serves only your property, it is your drain and your cost.

Drain, lateral drain or sewer: what the words mean

A drain carries waste from one property and sits within that property's boundary. A lateral drain is the same single-property pipe once it crosses the boundary on its way to the sewer. A sewer carries waste from more than one property. The difference matters because it decides ownership.

Since October 2011, most private sewers and lateral drains in England and Wales have belonged to the local sewerage undertaker, which across almost all of London is Thames Water. In practice, the homeowner is responsible for the drain from the house to the boundary, provided no other property connects into it. Once a pipe is shared with a neighbour, or once it is outside your boundary, it is normally the water company's.

That split explains why terraces are confusing. Many Victorian terraces were built with a single pipe running behind several houses before it turns out to the street. That shared run is a sewer, even though it sits in back gardens, and a blockage in it is not any one householder's bill. We go into the responsibility side in more detail in shared drain blockages: who is responsible.

The inspection chamber test

The quickest way to locate a blockage is to lift the chamber covers from the house outwards and note which ones have standing waste in them. Flow runs downhill, so the blockage sits just downstream of the last full chamber and upstream of the first empty one.

• Your chamber is empty but a sink, gully or toilet is backing up. The blockage is on a branch between that fitting and the chamber. That is your drain.
• Your chamber is full and the next chamber downstream, still on your side, is empty. The blockage is in the length between them. Still your drain, if nobody else connects into it.
• Your last chamber is full and so is the neighbour's, or the one at the front boundary. The blockage is further down, in the shared run or the connection to the street. That points to a sewer.
• Several properties in the street have slow drains or rising chambers at the same time. That strongly suggests a blocked public sewer.

Take photographs of each open chamber before anyone touches anything. They are simple evidence of where the waste was standing and they are useful if there is later disagreement about where the blockage was.

A practical point on safety: chamber covers are heavy, old ones can be brittle, and an open chamber is a trip and fall hazard. Lift covers with a proper key or lifter, keep children and pets away, and do not put your hands into the chamber.

The interceptor trap: the most common false alarm

Many London houses built before the 1950s have an interceptor trap in the last chamber before the sewer, a U-bend designed to keep sewer gases and rats out. It is part of your drain, and it is one of the most common places for a blockage that looks like a sewer problem.

The trap has a rodding arm with a stopper so it can be cleared. When that stopper falls out, which it often does, waste flows through the rodding arm instead and debris collects in the trap. The result is a full last chamber that looks as if the sewer beyond has backed up. The test is whether the blockage is in the trap itself or beyond it. If the chamber on the far side of your boundary is empty, the problem is almost certainly on your side of the trap.

Clearing an interceptor needs rods through the rodding eye or jetting, and the stopper replaced afterwards. It is routine drain clearance work, and it is your responsibility because the trap sits within your drain.

When you cannot see enough to decide

Where there is no accessible chamber, where covers have been paved over, or where the chambers do not tell a clear story, the blockage has to be located from inside the pipe. That means a camera, with a sonde to fix the position from the surface.

A CCTV camera run from the nearest open access point shows what the obstruction is and records its distance from that point. A sonde, a small transmitter on the camera head, lets an engineer locate that exact spot above ground with a receiver. Put the two together with the property boundary and you have an answer to the ownership question that does not rely on guesswork.

Two limits apply. A camera cannot see through standing water, so a surcharged pipe often needs clearing before it can be surveyed properly. And water company sewer maps do not always show the private sewers transferred in 2011, so the absence of a pipe on a map does not prove it is private. Where the route itself is unknown, drain tracing is the right tool.

Who to call for each result

If the evidence points to a public sewer, report it to Thames Water, which clears blocked public sewers without charging the householder. If it points to your own drain, it is your responsibility, and your buildings insurance may help if the cause is damage rather than a routine blockage.

Where the evidence is unclear, the sensible order is to establish location first and pay for clearance second. Paying a contractor to clear a blockage in a public sewer is money you did not need to spend. Waiting days for the water company to attend a blockage that turns out to be in your own interceptor trap is time you did not need to lose. A short camera inspection that fixes the position against the boundary usually settles it.

If waste is coming back into the property, the priority changes. Stop using water, keep people away from contaminated areas, and get the blockage cleared first. The ownership question can be sorted out afterwards from photographs and the engineer's findings.

Repeat blockages in the same place

A drain that blocks at the same point every few months has a defect, not bad luck. Displaced joints, root ingress, a sag holding standing water or an intruding connection all catch debris repeatedly, and clearing the blockage without looking at the pipe only resets the clock.

The useful step after a second blockage is a coded CCTV drain survey of the affected run. It shows whether the defect is on your side of the boundary, which you would repair, or on the sewer side, which you can report to the water company with the evidence attached. Our guide to the sewer CCTV survey and how it differs from a drain survey explains who can survey the public side.