Why doesn't bath water come up in the shower?

[StephenStephen]

I'm keen to know as I'm about to put in the waste run for a new shower.
The waste run is shower - basin - bath - soil pipe.
Even with the fall on the waste from the shower, if the bath is full and the plug pulled, then the water level is above the level of the shower base, so I'm curious - why doesn't the bath water come up into the shower?
Is it because the 40mm pipe is more than big enough for all the bath water to flow down into the soil stack?
Thanks for your thoughts, Stephen

 

[Harry Bloomfield]

The water simply takes the easiest route, which is not via the shower.

 

[lostinthelight]

Emptying the bath may possibly cause gurgling in the other trap, or suck the water out

 

[dilalio]

Bath should be independently connected to the vertical soil stack... Ideally!

 

[Madrab]

+1, to avoid any issues longer term, run the bath waste separately to the stack. If not at least make it the last connection before the stack and increase the pipe to 50mm.

 

[Elkato531]

You will have an problem if there is a blockage downstream. The easiest route then would be for the bath water to drain out via the shower. Run it separately if you can.

 

[Hugh Jaleak]

On first installation you have the full bore of the pipe available. As time goes on and the pipe gunges up, particularly downstream of the bath, problems could start to arise, especially if it's a long run from the bath to the stack.

I got called out to a job before, Young Couple bought their first home. She'd had a bath, pulled the plug and gone into the bedroom to dress and dry her hair. She came out a few minutes later, I believe because the electricity had tripped out, to find the bathroom floor awash and water dripping through the kitchen lights below.

Soon as I looked I realised bath and shower wastes were connected, she'd pulled the plug, and the bathwater had erupted in the shower, with aforementioned results.

 

[just pumps]

Even with the fall on the waste from the shower, if the bath is full and the plug pulled, then the water level is above the level of the shower base, so I'm curious - why doesn't the bath water come up into the shower?

Thanks for your thoughts, Stephen

Is there a non return valve fitted close to the shower?

 

[StephenStephen]

Thank you all, very much appreciated.
The existing plumbing has been taken back to a pipe which runs vertically down about 300mm then into the soil stack.
I'll solvent weld an elbow onto that, then approx 1 meter run to the bath, then approx 2 metre run to the shower.
Would it be wise to put a non-return between the bath and the shower?

 

[dilalio]

Photo or two is the next step for advice... I have so many images of pipes in my memory bank!