Waste Pipe size and Siphonage

[cky]

Hello

I'm looking at repositioning the bathroom basin and plan is to tee the waste pipe to the bath pipe. The bath has 40mm pipes with a distance of around 1.6m from waste to external wall.

My understanding is that to prevent siphonage, if the distance is less than 1.7m then 32mm is OK, otherwise 40mm (or 50mm if over 3m). If I was to join the basin waste which will be 1m distance from bath, does the guideline mean I need 40mm pipe or can I join with 32mm. Ideally, due to stud wall size 32mm can be hidden without any adjustments.

Thanks

 

[Charnwood]

If you're talking about syphonage from the basin trap and are worried, fit an anti-vac trap.

 

[Charnwood]

If you're talking about syphonage from the basin trap and are worried, fit an anti-vac trap.