Does grey water waste (bath, shower, sink) have to connect to the soil stack or can I connect it to a hopper on the rainwater downpipe? If I don't connect it to the rainwater pipe, I have to make an awkward diversion around the rainwater pipe to reach the soil stack or run the pipework through a load of internal joists. If I were connect it to a separate drainpipe of its own it will run parallel to the rainwater pipe and discharge into the same drain which seems kind of pointless. Any help appreciated.