Liam wrote:
Redoing ground floor bathroom. At present I have three seperate waste
pipes for water softener, basin and bath all going to a gully outside.
Would it be possible to run just one waste pipe with two branches
joining it to the gully, or would it cause problems with syphoning?
Each waste would have its own trap. Water softener has a washing
macine type waste with trap. Total run is about 2 metres and it is a
single stack waste system.

I'd go for it. I can't see a 2m length causing syphoning problems, but
in the unlikely event that it did, you could easily add in an
air-admittance valve without messing up all the pipework.

(And use 1.5" pipe rather than 1.25", obviously.)

David