Tub and Shower Arm on one Vent

[Chris79]

Remodeling a bathroom with one of the infamous freestanding tubs in shower / wet room. I installed separate P traps and arms for shower (2in) and tub (1.5in) that both join and are connected to an existing downstream 1.5in vent pipe. It occurred to me that this setup may cause trouble with siphoning (e.g. tub outflow may dry out the shower P trap) along with max distance of the tub line to vent (>42in).

Is the above setup (Figure A) permitted by code? If not, how would you propose to fix it?

If the above is not code compliant, would the following setup (Figure B) work?

Thank you!

 

[Joseph]

First thing, I'm not an expert and just started studying the concepts behind drains and vents for plumbing. However, I can see why you're concerned about Fig.A as the vacuum following the tub draining exists after flowing past the shower drain but before reaching the vent. It looks like Figure B will address this issue. To be absolutely sure (and if you have easy enough access) you might consider adding a second vent off the Shower trap arm and run it up the wall as well , then connect the two vents together at a height a few feet above the tub. Probably overkill , but again I'm just learning this stuff.

 

[Sponsor]

 

[wwhitney]

Figure A is allowed under the IPC if each trap arm separately complies with the fall limit (maximum one trap diameter fall from the trap to the vent connection). But Figure A is not allowed under the UPC, which California uses.

Figure B is allowed under both the IPC and UPC as horizontal wet venting. The tub trap arm (for checking the one trap diameter fall limit) is from the trap to the dry vent, while the shower trap arm is from the trap to the wye where the vented tub drain joins the shower. And the UPC restricts a 2" trap arm to 60" in length, in addition to the maximum 2" of fall.

Cheers, Wayne