I am trying to finish off fitting a bathroom in my new bungalow extension after the builders etc. left me in the lurch! I would greatly appreciate advice please on the following - The 110mm waste pipe comes vertically up through the floor, outside there are no visible pipes, the waste pipe simply travels underground into a plastic chamber. I am told that I do not need a soil stack? but some other internal contraption? The WC that I intend to fit has a horizontal waste outlet. I also need to plumb the wash basin waste into the toilet waste internally......any advice please?