Re: Sewer smell only when water draining
I agree with MLBSF, for most of what they said. I know in North Carolina code allows for an 1 1/2" shower drain line. The p-trap can be no further the 2'6" away from the drain. Also, the toilet in that same bathroom need to be downstream of the shower and sink. On the upstream side will be the vent for all three fixtures.
The toilet has a p-trap built into the side of the bowl itself. Sometimes you can actually see the outline of it looking from the side view.
But the vent of the sewage gas is the issue. Have you gone above the ceiling to see the vent pipe coming from the bathroom? I know working on a 3 story house roof is kinda high, but the pipe should be ging up and through the roof. If the pipe is not there, the gas seeps back into the bathroom for the smell.
Good luck. Process of Elimination.
Process of elimination. Good luck.