Bath and shower waste into same pipe?

[CutAndPaste]

Total refit of of bathroom. I've got a 40mm waste from the shower running beneath the floorboards to an external wall and out. Just above where the pipe exits is where the obvious place for the bath waste to exit as well. Should I use separate waste pipes for the bath and the shower or can the bath waste join into the shower waste from above (e.g. using a swept 90deg joint)?

My first thought was fine, crack on, then I was worried about water from a deep bath being emptied into the waste and as the water level was higher than the shower trap, it all emptyin out into the shower tray.

Is it OK to join into the existing Shower waste pipe or should I run a second waste out through the wall?

Thanks.

 

[Timmy D]

I always prefer separate wastes where possible.

If the shower waste has correct fall, combining them should be an issue. However, I’d want to join them and run the rest of the line to the stack in 2 inch. Use a wye rather than a tee to connect them.

Also, there could be the risk of letting a full bath go and it pulling water from the shower trap. That depends on trap seal depth, pipe size and pipe distance between the two.

Safest option is a separate line.