Hi,

I'm a DIY-er with good experience in carpentry, but not so much with plumbing. I am preparing to finish my basement and am getting plans together for plumbing the bathroom. The home was built about 2 years ago and the builders roughed-in for a basement bathroom at that time. The rough-in gave me three stubs in the floor: two 2" drains (for lav and tub) and one 3" for the toilet. However, there is no apparent rough-in for the vents; the lav drain just sticks up and is capped off with no stack continuing up to a vent system, nor is there a stub in the ceiling to connect into a vent system. I checked my building codes and they do allow AAV venting--and, indeed, my kitchen sink on the first floor utilizes an AAV (although it's the only one--all the 1st and 2nd floor bathroom lavs connect into a passive vent system). As there is also no apparent vent stack in my basement for the toilet and/or tub drains, I figure they are wet venting to the rough-in for the basement lav drain.

My question is, will I have any problems (code or physical) by using an AAV on alav drain that is also functioning as a wet vent for the rest of the bathroom system? I don't love the idea, but it seems it may be my only option short of trying to run a new vent to the roof (which would be way beyond my pay grade).

Thanks!



Edited 3 times.

assuming that you are correct in thinking the bathroom group is all wet vented thru the lav, then you are fine using an AAV located behind the lav.
make sure the one you use is rated for multiple fixtures.

Thanks! I'm going to try to get a copy of the plans to verify the vent configuration for the rough-in to see if my assumption is correct. It seems that the only other possibility is that there is a vent stack to the roof that was put in the concrete walls and so is not visible. In that case, however, putting in an AAV anyway shouldn't hurt anything, right?

I'm gonna go out on a limb and say there is no vent stack inside the cement walls.
a proper wet vent will have the lav drain act as a vent for all 3 fixtures.
this is an example of a bathroom wet vented. yours has drains underground so ignore the 1 1/2 and change that to 2inch. that is the minimum size allowed underground.

Thanks. I agree this is probably how it is configured (with the 2" as you mention). The "vent-in-concrete-wall" idea was suggested to me by a plumber friend who claims he runs vents that way "all the time" when he roughs-in for new construction.

Thing is, I cannot find a vent connection overhead anywhere. So either the plumber intended for the system to be vented with an AAV, or else I'm just too incompetent to locate the vent connection--a possibility I certainly won't dismiss! For some reason, I don't want to use an AAV unless I absolutely have to, so I guess that's why I'm fretting about finding some other vent option first...

I doubt very much that he "does it all the time" because it would be very time intensive to do it.

Indeed. He made it sound like he roughs it in before they pour the walls. I was surprised to hear that was a possibility, but I can only take him at his word.

Typically you would just continue the lav stub up....install a san tee for the sink trap, then continue that drain line up and through the ceiling as necessary to tie into the main stack as the vent. Depending on where the stack is you may have a lot of drilling to get through framing, and given thr choice between drilling (and weakening) joists, the AAV is a preferred alternative.

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Not a plumber by trade but a fierce DIYer