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We currently have a paving slab patio and shed to the side of the house. There is what I believe to be a public drain running underneath the current patio and shed. I would like to pour a 150mm concrete slab and build a timber frame and clad building on top. The building falls within permitted development it will be 7m x 4m. Does anybody have any experience building something similar over a public drain. I understand that I am supposed to ask the local water authority for there permission, just wondering about the likely hood of being granted permission. It's South West water if that makes a difference.
They are very unlikely to grant permission. we went through this when building an extension and modified our plans instead. You are supposed to ask if doing any work within 3m of a sewer.
If you aren’t putting in deep foundations and there isn’t an access point then they are very unlikely to find out.
Was gonna say if no access point and just floating a concrete slab above it then I'd just crack on with it.
If anyone asks then how were you to know there was a drain under there?
There is an access point in the back garden and another one in the front garden. They did come and check the drain a few months back the first time in the 10years we have lived hear. Won't need any deep foundations just 150mm of hardcore and 150mm of concrete. I was going to build it to be a hang out space for the kids but with an eye on it being a garage once the kids stop using it in around 10 years. Wanted the concrete floor for when it gets used as a garage.
To clarify the access points will not be directly affected by the new build.
Just crack on with it, if you ask they'll say No.
So if there's an access point in the front garden and one under the back I assume it runs under the house. If so I can't imagine a shed base making a material impact. It's not like they'll come along and ask you to knock your house down to gain access to it as things stand.
No the drain currently runs to the side of the house where the current shed and patio are but under where the new shed will be built.
When we did our extension a couple of years ago the process was pretty simple, we put in the application to Thames Water and it was agreed and signed off in a couple of weeks. They price the permit based on the diameter of the pipe you're building over. Your local water provider should have all the details on their website like this https://www.thameswater.co.uk/help/home-improvements/building-near-pipes
The worst that’ll happen is that you have to knock it all down (at your expense) if they ever have to do any work on that pipe.
A heavy enough lump of concrete can cause a drain to subside and the resulting 'sump' can accumulate solids and cause a blockage.
Happened to our drains which was a pain in the wallet.
Just built a floating slab over the streets sewer at my back door.
We had to dig down too and bridge the sewar with lintels on both leafs and the sewar had to a good distance down.
No one had any issues with us doing it and we had warrents done for the above.
Our slabs 7*3m and the sewars about 800 down
How deep underground is the drain?
I presume the only way to find how deep the drain is to measure down the manhole cover.
If it's a 150 or 225mm diameter pipe you can usually get a build over notice from the water authority. Above that easements come into play.
So many people build extensions and conservatory's over manholes though, that a concrete slab over the pipe with access either side would be nice for the water authority!
If you can measure down to the top of the pipe in the manhole and you have at least 600mm from top of pipe to ground level you should be fine to crack on. That's what I used to start drains at in non trafficked areas when I used to design drainage systems. 900mm in drives/access's, and 1200mm in highways. To be honest though you'd be fine with less than 600mm cover but that's technically what it should be....
If your patio is already slabbed just crack on with it!
I work as a technical specialist in the gravity sewer section in my local water company. I really really wouldn’t do it. If there is an issue with the drainage not having that manhole may make a huge difference to how easy it is to clear a blockage or line the pipework (which is done when they find minor defects). Adding a concrete slab increases the risk of failure of the line so I’d want as easy access as possible.
There is always a reason why a manhole is there, they aren’t put in just for fun.
If you block the manhole and access is required at that point you’ll be saying hello to a large gruff dig team who will just jackhammer the whole thing up to get access. And no, you can’t stop them even on your own land, refusing access will just mean your garden or inside your house stays slathered in human waste while they draw up the legal documents to force access.
From what I read the OP isn't blocking a manhole.
OP won't be building over a MH. This is usually acceptable to water authorities as long as there is access either side of the structure.
You may have to bridge the pipe, as was said earlier, but it can all be done simply with build over notice if it's only a small pipe.