When the bath water empties it floods the shower
I recently paid a plumber to install a shower and vanity in my bathroom. I kept the old bath. He attached the shower waste to the same pipe the bath waste uses.
Problem is, when we empty the bath water it comes up the shower waste and floods it, on it's way down the pipes to the outside. When I queried him the plumber said there's probably a blockage further down which isn't letting the water out quickly enough. But I would have to call someone else in, as he doesn't do that work. The pipe also has very little angle on it, almost none.
I don't think there is a blockage though. I have used draino and the shower and vanity water runs out fine. Commonsense and gravity tells me if a lot of water is leaving the bath and there is a pipe between the bath and exit, if the flow is strong enough it's going to have nowhere else to go but up this pipe (the shower). When the bath is almost empty, say 5cm left, the shower doesn't overflow. If I am right I think there are 2options:
1. Make the waste pipe a lot bigger.
2. Lift the shower up about a foot to allow for this bath water?
Are these options feasible?
Am I right in the first place?
Has my plumber made a serious mistake and if so should he be liable to fix it?
Any help would be appreciated, before I spend more money,
Leonie