I have the upper flat in an old maisonette. The single soil stack for both properties is cast iron. The drainage pipes from sinks and bath going into soil stack are copper. The acid from the condensate from the boiler has eroded the copper pipe in the kitchen. A section has been replaced with plastic and the concern is the rest could fail too.... right through to the cast iron soil stack. A plumber has suggested we cut a horizontal section through the entire cast iron soil stack and insert a new boss to recieve a copper pipe. However that did not consider the risks - if the cast iron pipe splits I will be liable for replacing the entire thing and rehousing my neighbours whilst that is done. I would also have to deal with the section that goes through the roof from my flat. Then there's the sparks from trying to cut the cast iron pipe. I suspect that for these reasons, the freeholder would not permit this work, but if they did, those liabiities would be mine. I am told that if I wanted to insert a new boss into the existing hole, then I would have to enlarge the size of that hole, which would be awkward and difficult. If instead I left a short section of the damaged copper pipe by the soil stack, could I line that to keep it watertight? Then fit a boss to allow a slightly larger plastic pipe to meet it? Would that meet whatever standards cover this? The flat is a buy to let and will be rented out.