Hi,

I am installing a separate shower in an existing bathroom. As the waste pipe
from the bath passes the shower location before exiting through the wall, it
would be a simple matter to 'Tee' into this for the shower tray.

However I assume that the shower tray would then start filling with water
every time the bath was emptied due to the higher level of water in the
bath.

Is there any sort of non-return valve that I could use in the connection
from the shower tray to make this work?

The bath and shower will not be in use at the same time so that won't pose a
problem. I would really like to avoid having to bring a second waste
connection through the outside wall.

Many thanks.