Can I join a new waste pipe to an exisiting cast iron soil stack?

19 Sep 2019
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United Kingdom
We'd like to instalment a downstairs toilet in the room directly below the upstairs bathroom. The soil stack comes down through the downstairs room in which we plan to install the new toilet. We planned to plumb the new downstairs toilet into the existing soil stock but we have now discovered that the soil stack is cast iron, not plastic.
I understand that to replace cast iron with plastic is a huge job which will cost thousands of pounds.
Is there any way at all that a new soil pipe can be added to the cast iron soil stack, maybe with the help of a welder?
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Nope, you can't weld cast iron in the way you are thinking. Depending on where the stack support points are and how good their condition is then there may be an opportunity to cut the stack and introduce a branch into it.

Hard to say without being onsite though, suggest you get a professional in to take a look at it.
Thanks a lot Rob. That's given me some hope. I will get someone to take a look asap. Thanks again.
It can (or could) be done. I have a cast iron stack and a few years ago when I had a change around in my kitchen as well as adding a Saniflo, I cut out the bottom 6ft of my cast iron stack. I had a standard piece of plastic soil pipe and the plumbers merchants sorted me out with special collar to connect the two. I could then fit boss straps on it for the sink and the Saniflo. The standard cast pipe fitted inside the new plastic pipe. I ended up with this (needs a paint up but that’s been in my list for about 25 years lol).
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I dont know who's quoted you, but thousands to replace does seem slightly excessive, unless the house has to be dismantled to get to it. Most soil pipes are simply boxed in, biggest headache is probably how to remove the boxing, once that is out the way, then replacement may not be too difficult. Replacement plastic components somewhere around the £100-150 mark for an average property, plus installation.

Biggest concern with grafting a piece in to cast is the weight above. Cast can rely on the sections below to support the rest above, you need to make sure everything above is suitably supported before removing anything below. If it can be successfully cut into then I've seen the 'Flexseal' type couplings used to graft a plastic junction in, or alternatively, the Glynwed 'Timesaver' system allows easy jointing of cast iron, if you need/wish to retain the cast.

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