Query with bath install and waste pipes

[gav214]

Hi, i'm hoping someone can help me, i'm installing a bath into my wet room, the sink waste runs down past my bath so would be ideal for me to tap into for my bath waste (cancels out alot of drilling and pipe laying etc) but the sink waste is 32mm, is it ok to drop my bath waste down from 40mm to 32mm and tap in? I've currently got a shallow bath trap and a run of the mill sink trap, would i encounter any issues, siphoning etc? My other options are to keep my bath at 40mm and up my sink waste to 40mm, but Will be more difficult, or my bath is sat over my shower drain hole (all pipes etc sunk and unable to dig up) could i just run my bath waste to that and seal in? or finally run a separate waste pipe for my bath and sink?
Any help would be great, cheers[/code]

 

[muggles]

Separate waste pipes are always the preferred option where possible. Don't go down a size, it'll be slow to drain and will block up. Use a proper bath trap rather than a shallow one, and try to get the correct 2.5º fall (44mm per metre)

 

[gav214]

Cheers Muggles, I need the shallow trap as the bath stands on the floor and i can't sink any pipes and bath is on ground floor