How deep are public sewers in the UK?
For the purposes of this article, when we talk about ‘public sewers’ we are talking about sewers owned and/or adopted by the local Water Authority, such as Thames Water, Anglian Water etc. These sewers will ordinarily be in the Highway, i.e. the road, the pavement or the verge. In around 90% of cases we find them to be in the road. And a public sewer can either be ‘foul’, by which we mean that common sewerage is discharged into them, ‘surface water’, by which we mean that rainwater and other ‘run-off’ water is discharged into them, or ‘combined’ which of course means a combination of the two.
Just because a sewer is owned by a Water Authority, it doesn’t mean that it is in the Highway, it can actually be in private property. So you could have a public sewer running through your back garden, and on a lot of old properties they actually do run across the backs of people’s houses, this is most common in council properties (including ex-council properties) and also many old terraced streets dating back to Victorian times, but also up until 1950s and beyond. We have even known instances of new-build estates nowadays having public sewers running through people’s back gardens. To find the location of most public sewers, click here.
Back to “How deep are public sewers”, we have been doing sewer connections all over the country for 20 years now, and so this article is based on our experience, not from any official statistics from any water authority.
To date, we have never known a foul public sewer to be less than 1 metre deep. This will normally be at the ‘start of the run’, which basically means where the public sewer picks up the first house. From this shallowest point of course the sewer will get deeper and deeper, but sometimes if the road is falling in the opposite direction, then the sewer will get shallower relative to the level of the road. In the event that a sewer started at 1m deep, but the level of the road was falling towards the ‘start of the run’, then relative to the road the sewer will become shallower.
Conversely, if the road is falling more than the sewer needs to fall (makes sense…?!) then the sewer is going to get very deep, very quickly.
By the way – all sewers have to be in excess of 750mm deep to prevent freezing during the winter months in the UK, but we have never found them this shallow.
But how deep do they get? Well, from our experience we tend to find them to be between about 1.5m deep and 3m deep. But they can be much much deeper. In Northampton Town Centre they tend to be 5-6m deep. This is because the ground is relatively flat, but with such a dense population and sheer number of properties packed in together, and all in need of a connection to the sewer, the main foul sewers have to fall as they collect more sewerage from each property, resulting in such deep depths. A lot of large towns and cities are the same.
The deepest sewer we have ever known is 7m deep, this was only 6 metres from the River Thames in Richmond. A quick Google search tells us that the River Thames is only 4.5m deep at this point, so the sewer is a lot deeper than the Thames! If you see on the picture it looks to have been about 6m deep when built, but with another 1 m of brickwork added later on.
A lot of these sewers date back to Victorian times, and of course they would have been dug “by hand”, with no machinery such as excavators available back then. When you stop and think about that, it’s not just a nice historical detail — it’s outrageous. We are talking about gangs of men in the 1800s standing in a trench, lowering themselves down with shovels, picks and buckets, and physically cutting their way through clay, gravel and whatever else was in the ground. There were no 13‑tonne diggers, no breakers, no trench boxes, no piling rigs. Just muscle, time and a lot more risk.
Just imagine a 7m deep trench having been dug by hand. Seven metres is basically a two-storey house turned upside down. Today, at anything over about 1.2–1.5m and we already have to start thinking about shoring, collapse risk and safe access. By 3m, you’re into proper engineered temporary works, method statements, permits, edge protection etc. At 5m and deeper, you’re talking about serious planning and cost just to keep people safe in the hole before you even start installing the pipe. Picture the Victorian version of that: narrow timber supports at best, candles or oil lamps for light, people standing at the bottom in stale air, passing spoil up in buckets and lowering bricks, mortar and pipes back down the same way.
That level of effort explains two things we still see today: first, why so many of the old sewers were built to last, often in brick or salt-glazed clay, and why they’re still in use more than 100 years later; and second, why they’re often so deep. Once you’ve dug a main run down the street to that depth, you don’t want to come back and move it or lower it. It just stays there, and everybody up the line has to work with it. That’s why we still come across foul sewers at 5m, 6m, sometimes 7m plus in older towns and city centres. The route and depth were set in Victorian times by men with shovels, and we’re all still dealing with those decisions every time we apply for permission to connect to a public sewer all these years later.
When a customer asks why a connection can’t just be done “next week” or “for a couple of grand”, this is the background. We’re not just tapping into a pipe. We’re tying into a piece of Victorian civil engineering that sits ridiculously deep in the ground, and every metre of that depth multiplies the planning, the paperwork, the temporary works and the risk. The Victorians did the hard digging. And now we are the ones who now have to dig back down to it safely and legally, and of course to today’s standards.
If you Google “deepest sewer in the UK”, you’ll come across the Lee Tunnel, a ‘combined’ sewer, which means it carries both foul and surface water mixed. It runs from Abbey Mills pumping station to Beckton Sewage Treatment Works in East London, and was completed in 2016. This tunnel is just over 4 miles long and in some sections it runs more than 70 metres below ground level, which is an insane depth when you think about it — that’s far deeper than any basement, foundation or standard utility run.
A job like that can’t be done with normal ‘open excavation’. Instead of the traditional cut-and-cover method (which basically means digging a huge trench from the surface, laying the pipe, and backfilling), the Lee Tunnel was constructed using TBMs — Tunnel Boring Machines. A TBM is essentially a giant underground drilling and lining machine that bores its way forward through the ground, supports the face, and builds the tunnel as it goes. You don’t see most of the work from the surface; it’s all happening deep below your feet.
The scale of that project shows what’s possible with modern engineering, but it also highlights how extreme some of the UK’s foul drainage infrastructure really is. We’re not just dealing with pipes a metre or two down under the pavement. In some areas the public sewer network is on a completely different level — literally tens of metres down — and that depth drives complexity, cost and lead time any time you want to connect into it or work anywhere near it.
We’ve generally covered ‘foul’ sewers above, and most of that applies to surface water sewers as well. However, surface water sewers by their very nature do not need to fall as much as foul. As they only generally carry water and no solids, the fall on these pipes can be hardly anything at all. For this reason, albeit they start off around the 1m mark, although they get deeper and deeper, they do so at a lesser extent. In any location where there is a foul and surface water sewer running side by side, the surface water sewer is likely to be the shallower of the two, This is good news in the event that you need a connection doing to the surface water sewer, as of course it will be cheaper, the shallow it is.
As and when a new connection is required onto an existing sewer – which we specialise in at JW Clark Ltd – the pipework has to be laid to the ‘invert’ of the existing sewer. So if the sewer is 4m deep for example, and plenty of them are, then the connection we undertake will be 4m deep. It is not possible to make a connection near to the top of a 4m deep manhole and for the sewage to ‘plop’ into the bottom of the manhole, as this causes blockages to the network and the water authorities don’t allow that under any circumstances. And so the deeper the existing sewer, the more costly the sewer connection. A 1.5m deep sewer connection will require that our excavation is made safe whilst the work is being carried, using trench sheets or a ‘trench box’ for example. But to make a 4m or a 6m excavation safe, is a completely different ball game. We can spend literally days, just sheeting an excavation and making it safe throughout. All of this makes a deep sewer connection particularly expensive.
If you need a sewer connection completing for a new build, or for an existing property where there is no mains sewerage connection, please be sure to contact us as soon as possible. A lot of developers build a house and then make contact with us, when they should ideally be contacting us prior to starting a project. We even have regular customers of ours who consult us prior to buying a plot of land. If a sewer connection is going to be particularly deep, or if the sewer is a long way from the site, then sometimes this makes it cost-prohibitive to build a new property.
See our fact sheet here which details why you should give us as much notice as possible when you need a sewer connection completing.
Use our Contact Us form here for a new sewer connection.
See here for an explanation of how to find the location and depths of existing sewers.