Drain/sewer design - connect foul to rain branch?

4 Feb 2010
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United Kingdom

I am adding a downstairs cloakroom to my house. General design of sewerage has been checked by building inspector.

After digging out though, a problem appears:

The foul drain to the 6 inch (?) shared sewer has been helpfully encased in thick concrete by the house builders some 50 years ago.

Also, the rainwater drain is in the way of my proposed drain run.

Now, the rainwater drain almost certainly connects to the shared sewer in a Y connection downstream from the inspection chamber shown. Also, it's not encased in concrete.

Does anyone know whether it's an option to connect my new foul drain to this 'rainwater' pipe? I would add a new separate p-trap gully for the guttering downpipe.

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Almost certainly? Needs to be a definate! I have doubts, looking at the picture as there is no trapped gulley, rainwater is unlikely to (and shouldn't) be connected directly to a foul (combined) sewer. Suggest you dye test or expose the remainder of the run to investigate further before going any further! :eek:
Thanks Hugh,

Yes, I will probably need to expose that rainwater pipe all the way to the shared sewer. One clue is that the rainwater pipe joins into a larger clay pipe (6 inch?), which matches the diameter of the pipe i can see inside the IC. It does seem strange that it doesn't have any kind of trap, but then the system is quite old.

Got some more digging to do...

If it does join the sewer downstream of the IC, presumably it's not against any regs to make the connection that way, and remake the whole area with a proper roddable P-trap for the rainwater.
Just checked - the rainwater pipe passes just over the foul and does not join it :(

Now need to decide whether to give up and get a builder in, or break the concrete and risk leaving the house without drainage while I fix it!
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Mostly because I'm not confident doing the benching etc on a live sewer shared with other properties. Sounds too much like stress and not enough like an interesting way to spend a Sunday!
Couple of possible options. If you can break the concrete without damaging the pipe (risky) then fit a McAlpine DC-1 in the existing clay drain, this is similar to a pan connector but will accept 110mm plastic pipe instead of WC pan spigot. Run new drainage in plastic. A temporary drain can be rigged up with a couple of bends and a piece of pipe to direct the soil stack into the top of the manhole if need be to keep the upstairs WC in service. ;)

Or, break out existing drain into manhole, use space provided to make new connection, make good benching as required. If you drop a length of 110mm plastic pipe into the channel of the manhole and seal upstream end with a bit of damp clay, this will allow the 'throughput' from the neighbours to pass by whilst you do the benching etc. Remove when finished! :LOL:

A temporary pipe dumping into the manhole is a great idea, that would allow me to work in peace!

Saw the DC-1 connector option too, that seems like option 2 after a fernco, and option 3 is feeding the pvc pipe into the chamber directly.

The sewer is 6 inch i think, so bypass would be tricky. Seems OTT for the 3 houses upstream which share it but there you go.
You could do all your preparation, setting your new pipe through the manhole wall, break out most of the benching without disturbing the channel and form your new channel. Then the tricky bit, get some fast setting cement (the sort that sets in less than 5 minutes) and an expanding bung (or air bag). Mix you FS cement, close the incoming pipe with your bung, break out the last bit of benching, form the last bit of benching and as soon as the FS cement is set, remove the bung. Job done. The chances of your 3 neighbours all flushing the toilet in the few minutes your bung is in is negligible.
Done the temp bypass a few times, keeps a bog working for the residents while you work on the drains at your own pace without the fear of a early shower.... :mrgreen:

If there's only 3 properties upstream then as Maltaron suggests, stick a bung in the upstream inlet, it'll be fine for a bit whilst you work, a 6" pipe will hold a fair bit of water. I'll give you a tip, tie a rope to the bung after fitting. Dual purpose, 1. If bung did give way it's not going to disappear where you cant reach it. 2. When you're ready, give the rope a tug to dislodge the bung, this lets the backed up waste run away without you risking getting a facefull in the process. ;)

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