Drainage Options for Additional Toilet

25 Mar 2009
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United Kingdom
Hi All,

We will be soon embarking on a bathroom refurb, and building on an en-suite within the footprint of the existing bathroom. We will aim to complete as much of the work ourselves as possible, and plan to notify building control and involve trades where necessary. At this stage I'm trying to spec everything up and get a feel of what is and isn't possible.

Originally the drains in our house ran to a septic tank. We had a local ground works company connect them up to the main sewer when that became an option a few years ago.

There will be a second toilet added in the new en-suite and I'm looking the find the best option to get it connected to the sewer.

The existing toilet is connected to a 110mm pipe which rises up through concrete floor in the bathroom, and then goes through the wall under floor level, and into an inspection chamber.

A little further upstream in the system is a vent pipe which runs up the side of our house wall above eaves level. Nothing discharges into this.

Further upstream is the bathroom gully (shower / bath / sink discharge into this) and then an inspection chamber and finally the kitchen gully (sink / appliances discharge into this).

As I see it my options are;
  • Make a new mini stack inside the bathroom into the existing 110mm pipe with an AAV at the top. Discharge both toilets into this. Not sure how tall this would need to be?
  • Core drill a new hole in the exterior wall, connect to the existing vent pipe with a 92.5 degree branch and discharge the toilet into this
  • Core drill a new hole in the exterior wall, run 110mm pipe underground and tee into the existing run with a new inspection chamber
I've attached a layout of our drains to help.

If anyone can advise what is and isn't possible/allowed I'd really appreciate it!



  • Drains.jpg
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I’d go into the vent but i’d CCTV it first or if I didn’t have a camera maybe a massive flow test.
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Agree with Ian on this, just make sure it’s suitable to be used carrying out a flow test. Obviously make sure that it’s still vented, and it’ll be like a standard stack then.
That's great, thank you both. This is probably the most straightforward option too so I'm pleased it's doable.

Will go out later tonight and investigate. I need to double check it flows downstream too as the direction of flow and fall direction reversed when we moved to mains drainage. I hope the contractors reversed the tee that the vent pipe connects to!
All checked; flows absolutely fine and in the right direction. Shoved a hose down there for good measure and it appears further downstream at the chamber.

Thanks for your advice!
  • Thanks
Reactions: CBW
maybe a massive flow test.
Maybe a massive turd test - the Tee underground could be a T shape or even a Y facing the wrong way - We've all had scenarios where a softie flies and sticks like.... to an opposite wall of underground drainage. OP I don't want to P on your parade, but I would dig or cctv and find exactly what you have there. You would be surprised @ the velocity of water/solids exiting a bend @ the bottom of a stack.;)
I'd be putting an access point in at the bottom of the stack too, just in case.... The last thing you need is the drain to block and a full stack to try and unblock.

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