Double shower venting

[lennyk]

Hello,

I have a situation on a property where there are two showers next to each other with no traps and they simply join together below.
There is no vent either.
There is the sewer smell and if one shower is in use, there is often water backing up in the other.

It is very difficult to get below the showers, however the waste line exists at the side and comes directly down, they are on the second floor.
This install was done a long time ago and not to any code.

Would venting and placement of a p-trap(red line) help the situation ?

Thanks,

L

 

[hj]

It would stop the sewer gas odors but do nothing to prevent one shower from backing up into the other one.