Kitchen sink waste to underground drain connection

12 Apr 2008
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United Kingdom
I should have taken a photo but it's dark now and there's no flash!

My kitchen sink waste flows into an old cast pipe about the diameter of a rainwater pipe, this was on the external wall of the house. The cast pipe then disappeared under the ground where it connected into the drainage by way of a clay p trap.

We are having an extension built out the back and the location of the sink waste will be the same, the builder has put in most of the foundations, has removed the cast connection and left me with an open vertical orange soil pipe (connected to drain via the p trap)

I intend to remove the bit of cast that is left and replace with 40mm waste but how do I connect that to the open vertical soil pipe that is there (directly below the waste)??

It's not like an external one where you can leave it open with a drain gulley or such like, this will be under the suspended timber floor.
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You can but once the floor is down and the coupling isn't accessible there's chance that the waste pipe into the rubber grommet can be pulled out. The best solution is to bring the 4" soil to FFL and then connect on.

It's not a good idea to have an inaccessible trap - how do you plan to access it to clear a blockage?

The trap isn't required if the above surface connections are correctly installed & trapped.
Am I right in thinking you'll end up with two traps in series? (the one on the sink and the one underground).

If so you will have problems with airlocking unless the intermediate section is vented. Also how would you access the underground P trap for cleaning?
Also, i need to extend the soil pipe up and out of the extension but the current pipe is 75mm cast iron, is there a connecter i can buy for joining on 110mm pvc?
Remove the existing gullies and replace with underground 4", bring to just above FFL and make all your connections above surface
Well good luck with getting building control sign off!

Reducing pipe size as you progress along the run is asking for future (blockage) problems as is leaving inaccessible gullies in the run.

Your only option (other than to do it properly) is to use a flexible coupling and extend the existing waste to above FFL.

You need to have a practical plan for how to deal with blocked gully below the floor at some point in the future.

Hope the notched joists in the bathroom worked out for you!
what if i capped off the two terracota pipes with screw access caps and ran a 50mm pipe from the kitchen, past the bathroom and into the existing soil pipe ?
Bit confused - you've so far mentioned terracotta, cast and plastic foul pipework in a variety of sizes.

If you go with your latest 'plan' you'll need to remove any redundant drain runs at the point where they join the main foul run.

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