Am "rennovating" my bathroom - which looks as if Desperate Dan itself originally plumbed-in.
At the monet, there is a Shower cubible with a 40mm waste teed into the 40mm waste from the bath. This then goes off into the main soil stack (although I can;t yet trace this run as it is under the followboards).
At them moment, when the bath is emptied, water comes up through the shower waste - and whilst it's not in itself a major problem, I want to do it right when I put in the new bath/new shower. I plan to keep the existing shower tray but will replace the bath so it is an ideal time to sort the waste pipes out.

Is this "siphonage" - and if it is, is there a valveoor fitting that will prevent it Or does anyone have a "top-tip" to solve this problem.

many thanks

Alex

It's probably not "siphonage" - likely the bathwater just isn't
draining fast enough to prevent all the pipes filling (to the brim of
the shower waste) with water. I had exactly the same problem caused by
bath and shower being connected to the same tee. Fitting a Hepworth
"Hepvo" trap to the shower branch solved the problem perfectly.

www.bes.ltd.uk sell them; part no. 15122, but you should be able to get
one from any plumbing supplier.

Vaci