Shower / bath waste

[NO HOPE]

Afternoon all

I have recently fitted a walk in shower connecting the waste to the bath waste. Shower is next to bath but opposite end to the taps / waste. I have raised the shower base to achieve a drop between the shower trap and the join to the bath waste. Approx 3" over 2m.
Immediately after the join to the bath waste the waste pipe drops, from above the joist to level with the bottom of it, at quite a sharp descent.

When i drain the bath, water spouts out the shower waste for a few seconds before draining down again. Appears to be trapped air being released. Can anyone help as to why this might be occuring? And what could be done to sort it. Not a major problem as its only a small amount of water and it ends up in the shower tray before draining back down but i am wondering if it is indicative of some other problem in the system.

Its a 1900s house with a single soil stack.

Any help would be appreciated.

Cheers

 

[hi-spec plumbing heating]

badly designed pipework.

 

[stardanny]

If I was you, I would run it separate to soil stack or a hopper if you have one. I had it before at my brother walk in shower, it happen, so I run it separate, no problem.