Thames Water Blocked Drain: Theirs or Yours?

Where the line falls between your drain and their sewer

Before 1 October 2011, many shared drains in England were private, and neighbours could be jointly liable for clearing a blockage under someone else’s garden. On that date most of them were transferred to the water companies. The rule since then is simple enough to use at the kerb.

• A pipe that serves only your home and is inside your property boundary: yours
• A pipe that serves only your home but runs outside your boundary, under the pavement to the sewer: the water company’s (a lateral drain)
• A pipe that serves your home and a neighbour’s, wherever it runs: the water company’s (a shared sewer)
• The main sewer under the road: the water company’s
• The gully grating in the road that takes rainwater off the street: the council, or Transport for London on red routes

Thames Water treats most of London as its sewerage area even where another company supplies the drinking water, so a blocked public sewer here is usually a Thames Water job whoever sends your water bill.

Three checks that tell you whose blockage it is

You do not need tools for any of this, only a screwdriver or a spade blade to lift a cover. Wear gloves, keep children and pets back, and never climb into a chamber.

• Is it only you? If only one toilet or sink is slow, the blockage is almost always in that fitting’s own waste pipe, not the drain. If everything downstairs is backing up at once, it is in the drain
• Lift your inspection chamber cover, the rectangular metal or plastic lid in the front or back garden or the side passage. If the chamber is empty and the channel at the bottom is clear, the blockage is between your house and that chamber, which is your pipe
• If your chamber is full of standing water, check the next chamber downstream, which may be in a neighbour’s garden or in the pavement. If that one is empty, the blockage sits between the two. If it is full too, and neighbours have the same problem, it is very likely in the shared sewer

Sewage coming up through a manhole in the road, or into more than one property, is always a report to the water company, not a plumbing job.

The London trap: blocked interceptors

Many older London houses have an interceptor trap in the last inspection chamber before the sewer. It is a U-bend in the floor of the chamber, meant to stop sewer gases and rats coming back up the drain. Above it there is a short rodding arm closed with a clay or rubber stopper.

Interceptors block easily. Wipes and fat catch in the bend, and if the stopper falls out it drops into the trap and plugs it completely. The chamber then fills, which looks exactly like a sewer blockage. Because the interceptor is inside your boundary and serves only your home, clearing it is normally your responsibility, so a full chamber caused by a blocked interceptor usually turns out to be a private problem rather than a sewer one.

So if you have a full chamber at the front with a large brick-built or deep-set feel to it, and the neighbours are fine, suspect the interceptor before calling it in.

How to report a sewer blockage to Thames Water

Report it online through the report-a-problem pages, on the 24/7 WhatsApp service, where you can send photos of the full chamber, or on the 24-hour line, 0800 316 9800. Thames Water says it aims to attend within 48 hours in normal weather, or 72 hours when exceptionally busy. If sewage is coming into your home, say so clearly, because internal flooding is prioritised.

Say which chamber is full, which is empty, and how many homes are affected. That is what the crew needs to decide where to start. If they find the blockage is in their sewer or lateral, clearing it costs you nothing. If they find it in your private drain, they will tell you, and you will need a plumber or drainage contractor.

While you wait, stop using water that drains to the blocked run. Do not flush, run the washing machine or empty the bath. Lifting covers safely, clearing a gully and cleaning up sewage are covered in first steps for an overflowing manhole. Our Thames Water helpline guide covers the other problems they handle and which ones they send back to you.

When it is yours: what clearing it involves and typically costs

A private blockage usually clears with drain rods pushed from the nearest chamber towards the house, or with a high-pressure jetter for fat and wipes. If it keeps coming back, a CCTV camera survey finds the cause: a root intrusion, a collapsed clay section, or a joint that has dropped and holds a pool of water.

Typical market prices for clearing a straightforward domestic drain run from roughly £100 to £300, higher at night and at weekends, and a CCTV survey is usually a separate charge. Many buildings insurance policies and home emergency policies include drainage cover, so check yours before booking.

If the problem is a toilet that will not flush away while the drain chamber stays clear, it is in the pan or its connector, not the drain. Our blocked toilet unblocking visit deals with that: the £60 call-out covers the visit at any hour, and the repair price is agreed with you on site before any work starts. For a slow sink with a bad smell rather than a full blockage, see a smelly sink.

What not to do while you wait

Chemical drain unblockers poured into a full chamber sit there and do nothing, then splash on whoever lifts the cover next. Do not use them in a blocked drain.

Do not break open or remove an interceptor stopper yourself unless you know how to rod it. A loose stopper is often what caused the blockage in the first place.

Keep the chamber cover on between checks. An open chamber is a fall hazard and lets smells and rats out. If the lid is broken or missing inside your boundary it is normally yours to replace, unless the chamber serves other homes, in which case ask Thames Water to confirm; outside the boundary, report it. And if water is also coming up where it should not, such as around the stopcock or through a floor, that is a supply leak, not a drain, and no water in the house and the supply guides are the place to start.