Extension & Inspection Chamber Confusion

4 Apr 2008
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United Kingdom
Hi all.

Planning my extension and need some advice on how to reroute my drainage. (see attachment for existing drain runs)
The extension is planned to be full width, and the house is terraced.

I am aware that the sewer is public and that Thames Water will not approve an inspection chamber within the extension itself. With the house being terraced and the extension full width it will not be possible to move the inspection chamber further along the run.

I know I can move the inspection chamber outside of the extension foot print with a Y connection to allow rodding, but this does not solve the problem of having rodding access to the existing connections.
Note that the middle of the three connections will be redundant once the extension is built.

I have spoken to the next door neighbours as they have already done a similar extension, but they were unable to offer much in the way of information

Advice and inspiration would be greatly appreciated before I go to Thames Water.



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Currently there is a soil pipe from the upstairs bathroom, a soil pipe from the downstairs toilet, and a waste pipe from the kitchen. They all run to the same inspection chamber. The kitchen waste will be redundant but the other two will remain. See below...
Wyvern Drainage.jpg
There is no great way to do this. The only way is to move the inspection chamer outside the extension and put a Y on main line. Then take all your services out and dog leg them.
If your neighbours have rodding access then you can clear if not you'll have to add some. You'll still beable to rod your own pipes.
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Take your two driains into a new manhole in the extension, and then connect that manhole to the TW drain via a section of pipe and a slow branch - which will replace the TW manhole.

This will mean that everything upstram from the branch is a laternal drain and yours, so the m/h can be under the extension and TW can't prevent that. The close proximity of the m/h to the branch will mean that access is possible to deal with any issues at the branch.
Thanks both for the replies.

Tomfe, I had considered your suggestion but because of levels and the requirement for ventilation to the timber floor beyond (and the fact it looked like a right PITA) I was hoping there would be an easier solution.

Woody, is the below what you mean? This had also entered my mind but wasn't sure if it was acceptable to TW / BC. The sketch below was knocked up quickly and may require a bit of tweaking prior to submission / construction...
Wyvern Drainage Proposed.jpg
You won't need that external chamber as the gulley can go straight to the drain. But otherwise yes.

The drainage layout is risk assessed nowadays so the old rules are not applied religiously.
If levels permit and the external branch is not acceptable perhaps the external drain can be run straight to the new internal manhole leaving the Main drain with just one branch.
Will you need a build over agreement?
Yes, the pipe serves several properties further up the run so it is a public sewer and requires a build over agreement from Thames Water.
If levels permit and the external branch is not acceptable perhaps the external drain can be run straight to the new internal manhole leaving the Main drain with just one branch.
I was under the impression that the external inspection chamber was required by TW as it gives them rodding access when there is no access to the house (there's also no access to the rear of the property without coming through the house but that's a different matter).
Also, I plan to run the sink waste to the gully and not just rainwater, so I assumed this would require the inspection chamber anyway?

Also, where the sketch indicates a wet room, is currently just a toilet (without an SVP). Any issues with running additional waste water from shower and sink into this outlet?
The internal pipework serving just your property, up to the point tit joins the shared drain, is your responsibility, Thames have no concern with this. Their concern is the through sewer taking your upstream neighbours waste, that is, and remains their responsibility, hence their concerns for its protection and accessing it in case of problems.

Fit a bottle gulley externally, this will allow rodding access along the gulley lateral to the main run, and omits the need for the external chamber in your drawing. All other internal drains will be accessible from the internal chamber as shown.
That's good news.

What would be your advice for drainage from the wet room?
I'm thinking of ignoring the existing toilet waste pipe and bringing it out to the inspection chamber on its own pipe.
As said above, toilet hasn't got an SVP but I've never noticed any smells so thinking best not to mess with it (other than routing it to new internal inspection chamber).
What would be your advice for drainage from the wet room?

Wet rooms are highly overated as you need to lay floors to a fall and build in the drainage. I would always recommend get a big shower tray, much easier to install, a cleaner room to. But trays not always suitable if some-one in the family has mobility issues and that is the only time I would recommend the wet room approach.
No-one has any mobility issues but my wife thinks they are more high end than a shower room...

Also, this part of the house currently has the floor 75mm lower than the rest of the ground floor so laying to falls won't be as difficult as if the floor was at the same level.

I would still have the same issue of where to run the waste to anyway.
Wet room, fit proprietary floor drain/gulley, connect to 110mm under the floor and run straight to chamber. Removes any risk of syphoning the shower drain when WC flushed.

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