The Management has finally decided on the new layout for the bathroom; all I
have to do now is fit it! Her plan results in the washbasin being sited
approximately 4 metres from the soil pipe (single ventilated stack
arrangement). According to what I have read t would seem that I need to use
50mm diameter waste pipe for this run in order to prevent loss of the seal
in the trap through siphonage.

I don't have a problem with using this size of waste pipe if necessary but
it does seem a bit large. Do I need to use it or is there any way (perhaps
by use of an anti-syphon trap) in which I can reduce the size needed?

Grateful for any advice

Tony
(more Spud than Bob the Builder)