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Plastic to cast iron toilet waste connection queries

Situation: A damaged section of plastic toilet waste pipe (4inch) has been replaced by a Company and joined back to a cast-iron collar branching into the main cast-iron downpipe (approx.6inch).
Company suggested replacing the whole t-piece at the cast-iron end, (cast-iron downpipe has collars above and below the waste pipe join). To avoid disturbing 100+yr old good cast iron pipe I asked to simply replace the broken section and join it to the cast iron collar as previously. The pipes diameters gap was cemented up (“recommended way to do the join”). I didn’t take pictures before they started but am very sure previous join not so and the fit much tighter, can’t remember seeing any cement around the join. A week after the Company left the toilet blocked up - cleared with difficulty. Then it blocked up again no amount of water, specialist plungers, chemical unblockers could clear it; local plumber tried and was giving up when it suddenly cleared after a vigorous bout of plunging. He said that it was probably due to too much cement being used in the joint and forming a lip inside the pipe so narrowing it and leading to easy blocking. Asked Company to fix this but they said it wasn't their fault, everything was done correctly.
Questions: 1. Am I right to ask for only the damaged section of plastic pipe to be replaced? 2. Is filling the gap in pipe diameters with cement the correct/proper way to do so? 3. Is there nothing that could/should have been used by the engineer as a joiner/reducer between the pipes so reducing or eliminating cement? 4. Are they right to insist that replacing the whole t-piece is the correct procedure? 5. If so, how is the cast-iron down pipe then connected to the new t-piece above and below the join, more cement at both joins? What's your advice? Many thanks in advance. (I won't name the Company - yet !)

5 Answers from MyBuilder Plumbers

Best Answer

to start 100 year old cast iron you cant see inside it in all probability it is not good on inside,at this age it needs replacing,new cast iron branch it would probably be spun steel timesaver, joined to cast stack with bolted callers with rubbers in them,this would also be the way the new pvc soil would be joined to it,pvc soil is quite a bit smaller in circumference than cast iron,but can be joined to a cast collar with care,we use tarred yarn pushed into collar around pvc,but not pushed in more than 35mm then prompt cement to seal joint,this way cement is not pushed into pipe,this is quite a common fault when people try to join pvc to iron,dyno rod most of them are not plumbers,if they say it is incorrect ask them how they would have done it ?,unfortunately i know of no fitting to do this that does not restrict pipe as it joins the the soil branch.


Answered 31st May 2019

I agree that a wrap of suitable yarn/ rope as a bind before the application of a cement bond.


Answered 9th Jun 2019

I think they should change whole pipe and bends to new plastic one and I can't say properly without seeing sorry


Answered 16th Jun 2019

I would cut the cast iron pipe as low as possible fit a rubber coupling to the cast and convert the stack to plastic


Answered 21st Jun 2019

We use this Floplast SP140B universal pipe connector 110mm work great for cast iron and clay pipe.


Answered 29th May 2020

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