l have a upstairs toilet and the soil stack comes down the outside wall to the drains. The soil stack is cast iron and l would like to replace the downpipe section with plastic can this be done. Will a plastic pipe fit onto cast and is it hard to do, would l need any sealants the joint thats meets plastic with cast. The reason l want to replace the pipe is l think it's blocked as the toilet is filling up and wont flush away, l've tried a plunger but this made no difference and l checked the drain and this is clear so l thought l might as well smash the down pipe to clear the blockage as l prefer plastic anyway but can this be done.....