Two questions, if I may:
(1) I want to change the location of the sink in my bathroom. In order to do so, it would require a 4 metre (or so) waste pipe to get to the soil stack. I can achieve the required 18mm/metre fall throughout and I have an abundance of pipe clips to stop sagging. I read somewhere that the maximum length for a 32mm waste is 1.7m. Is this true? If so, why? And, what happens if I exceed it? Does this length of waste pipe sound like a dumb idea?
(2) I don't really have any great new spots on my soil stack to add a new boss for my sink waste, so I'm thinking of joining it in to my existing 40mm shower tray waste outside of the property. The join will be ~3.5m away from the sink, and ~1m away from the shower. They'll be joining at a point where both will have been on a shallow fall (18mm/metre) for some distance (1m shower, 3m sink) - that is neither the shower nor the sink will be going through a vertical descent anywhere near to the tee point. The sink trap will be ~3-4 feet above the tee point. The shower trap will be 6-9 inches above it. Are there any rules of thumb as to whether this set up is likely to lead to syphoning or water gurgling back up into the shower when the sink plug is pulled?
I hope they make sense! Many thanks in advance for your help.