PVC Soil Pipe Branch into Clay Underground Pipe?

5 Apr 2012
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In and around London
United Kingdom
Hi DIYnot!

So, I'm having to replace the bottom cast iron section of a previously unused soil stack which will be for the toilet and waste from the bathroom being installed upstairs. It was completely blocked from the clay pipe bend at the bottom, near enough all of the way up to the pvc pipe above - well over a metre worth of pure compacted rust inside.

There has been a possible good thing come out of it. Previously I had reluctantly decided to go the "easier" route for the downstairs WC to be installed, and use a macerator. I simply didn't want to remove the massive amount of concrete I expected to be in the way of attempting to cut low enough into the old cast iron pipe. But now the cast iron pipe is completely gone anyway, and the underground clay pipe does appear low enough to have the pvc soil stack immediately branch off for the downstairs WC.

The only problem is because I need the soil stack to branch immediately, I'm thinking I'll probably need a way of connecting a soil branch straight onto the clay pipe, with no straight section of pipe first... Does anyone know if this would even be possible?

Also the clay pipe bend remaining at the bottom does not have a clean cut... Is this going to cause a problem regardless, and what would be the best way to deal with it?

Here's a pic:

View media item 67555
I don't know how the cast iron pipe was connected or rather sealed with this originally, because it was already loose and only held in place by the concrete around it. I think the top of the clay pipe was already broken.
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There would have been a collar (socket) on the end of that clay pipe, into which the spigot end of the cast would have sat and the joint made with a piece of tarred rope, and pointed with a mortar mix. At some point slight movement has caused the clay socket to break off, not uncommon.

I would be reluctant to try and couple to whats left of the bend, personally I would carefully dig out a bit more soil to expose the bend and some of the following pipe. Clear enough space around the pipe to be able to cut through the pipe (again, carefully, the clay drain is very fragile!). Once you have a clean cut end on the clay, fit a suitable connector from clay to plastic, then continue in plastic. You will ideally need a long radius 'rest' bend at the base of the stack, carry on from there with new soil pipe as required.

Avoiding a macerator is a very sensible idea! :LOL:

So it looks like I'll be taking up lots more concrete anyway! :cry:

That piece of exposed pipe basically is just a 90 degree clay bend (from vertical to horizontal). There is then a join to what I believe is a short straight section (running under the house's original brick paths, topped off with lovely concrete) before turning 90 degrees again to the left and along a long straight run where it collects waste water from another drain.

I don't know what type of join it is between that exposed bend and the short straight section. It feels very rough, and I put a camera down to have a look. It looks like rough mortar around the join. In any case, I'm guessing it is best I dig around this 90 degree bend, and just cut it off at the existing join to the straight section? Best tool for making the cut?


Okay, I looked at a rest bend and because of the large radius it might take me too high for the soil branch... there also might not be enough room to fit it between the clay pipe and the house wall if I only cut the clay pipe off at the 90 degree bend....
Disc cutter/angle grinder with stone disc to cut the pipe. If a rest bend is too high, use a 90, but with the biggest radius you can allow, a 'knuckle' bend would cause problems. I'd also allow for some kind of access, either a mini inspection chamber or at the very least, a rodding point as low in the stack as can be achieved.

If existing stack is cast it would be wise to look at replacing all, not just bottom section. Cast is extremely heavy, and the upper sections rely on the section below to some degree for support. (2 nails in each 'ear' will hold it to the wall but over time they may not be adequate to support the weight.)
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