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Discuss Replacing section of cast iron stack with plastic in the Plumbing Forum area at

  1. Gavbo

    Gavbo New Member

    Hi. I am installing a downstairs toilet. My existing stack is cast iron. I don't really want to replace the whole stack so I'm hoping to to replace a small section at the bottom with a plastic t section.

    My question is if I chop the section out and replace with the plastic t, would it still support he weight of the full cast pipe above?

    I'm planning to chop the pipe at ground level and use one of these coupling sockets for the plastic to fit into 110mm Soil Pipe Slip Coupling Double Socket

    Any advise would be appreciated before I go ahead.
  2. Rob Foster

    Rob Foster Top Contributor!!

    Only you can determine the condition of the remaining cast iron stack and in particular the state of the wall fixings. Of course its quite acceptable to insert a plastic section. You will need a wide diameter angle grinder to cut cleanly, do not try using a mini one and go round you will not get to the back by the wall. Even a little bit of cast left joining the two will be strong. If the fixings are not adequate the the upper stack will settle on the grinder blade jam it or make kick out, so beware. Maybe if your not used to doing this a petrol one with a clutch might be safer, as big lecky ones can be viscious.
    Mark out the horizantal cuts clearly but go in at 45 deg first then do the horizantal cuts after. Then there is the issue of appearance it can look tacky.
    If you decide to take the whole stack down smash hell out of it with a lump hammer, I often bring the top section down by attaching a chain to the top and to the landrover and drive off .
    Rob Foster aka centralheatking
    • Funny Funny x 1
  3. Jimmy18

    Jimmy18 GSR

    I’ve done a few of these and I always replace the complete stack. There are different types of couplers for cast iron. I find the best looking are the saint gobain mechanical fixed ones.
    • Like Like x 1
    • Agree Agree x 1
  4. Last Plumber

    Last Plumber Trusted Plumber GSR

    It's not something I would advise. It could be very dangerous and cast iron stacks are heavy. If it has been up there a good length of time, the spout nails will probably have corroded and will either break or bend. If that happens it will either come down with a crash and crush whatever is below or end up in a precarious position with nothing supporting the weight apart from weak nails. I would advise you strongly to take the stack down from the top and replace it. You can replace it with cast iron if you like but plastic will do the job just as well.

    If you don't feel confident with taking this down from the top, then you would be best to get a Plumber to do it for you.
    Only my opinion.
    • Like Like x 1
  5. gmartine

    gmartine GSR

    "Usually" I've found old cast iron soil pipe brackets are push fit and integral to the join supporting the pipe above meaning I've never found a tendency for the stack to collapse or compress the inserted fitting but that's your call. Unless you have something with a 300mm cutting disk (petrol stone cutter) it's a case of cutting what you can with an angle grinder (thin disk) then finishing the cut with a saw. On many occasions (a dozen or so I reckon) I've inserted a flexible equal tee without any adverse effect like a collapse or compression but the stack has been sound.
  6. king of pipes

    king of pipes Trusted Plumber GSR

    Not really a DIY job employ a plumber with the correct gear for this one and stay safe . Kop
    • Agree Agree x 2
  7. Andy B

    Andy B New Member

    Replace the whole lot and as mentioned previously if you not 100% confident get a plumber.

    It's not a huge job and once it it done, it's done.

    Old cast iron is dangerous and had ability to kill someone.

    The fixings may be ok now but what about the future?

    I have seen the wood wall plugs rotted away, cast iron lugs cracked, nails corroded away in the wall and long spiralling cracks in the pipe.

    It's done it's time, get rid of it.

    I certainly wouldn't be grinding a bottom section out of the stuff.

    Cast iron gutters are even worse!

    Good luck! :)
    • Like Like x 1
    • Agree Agree x 1
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