I am roughing in my basement bathroom and I am planning the dwv system right now. I am having trouble understanding how I should be tying in the wet-vent to meet code (Wisconsin) and hoping someone can help guide me.

My current plan is to put an air admittance valve under the sink (in the cabinet) which will serve as the vent for everything in that room.

Is there a way to make this work and code compliant?

The bottom yellow highlight is the main waste line. Blue is 3" toilet. Green is 2" shower/bath line. The other yellow highlight near the sink is undecided but will be drain for the sink and vent for the other fixtures.


I've modified the diagram to have two separate vents going up the walls and into the ceiling, and then over to the unfinished portion of the basement. The main concern I have about this is how the toilet is wet vented -- is this considered to be the "end" of the branch it's on?

  • 1
    Under the sink is a bad location for an AAV. If you are building the bathroom, build it so the AAV is well above the sink, with an access panel. Below the sink is mostly for retrofit application and much more prone to clog/fail often than a higher position. Given the flow direction, there may well be a problem with venting toilet and shower from the sink location at all, as drawn.
    – Ecnerwal
    Mar 11, 2023 at 16:00
  • On the right side of the tub wall is one spot I could vent higher up, also the left wall is unfinished basement on the other side so I could potentially vent there. Any suggestions to avoid an access panel? Mar 11, 2023 at 16:58
  • Without looking into what, exactly, your local code permits/requires, (or even reviewing what, exactly, mine says, though I have general recall) I think either of those provides a much better location for the shower/toilet vent, at least. i.e. My recall is that the sink would still need its own vent (or a re-routed drain) under my code, as wet venting a bathroom group on one vent is permitted, but the toilet has to come last (closest to the sewer) for that to apply
    – Ecnerwal
    Mar 11, 2023 at 17:04

1 Answer 1


If you wanted to connect all the fixtures with a single wet vent, this would be how I would arrange the piping:

wet vent sketch

This would wet vent both the toilet and the bathtub through the 2" drain of the basin.

  • I would have never thought to do it like this but I like it! I just wish I hadn't already dug into a bunch of concrete last weekend... hopefully something similar to what I posted will work Mar 21, 2023 at 1:50
  • 1
    Your updated layout with the individually vented basin and the toilet wet vented with the bathtub will also work. The only thing to be aware of is that some jurisdictions do not allow the use of auto-air vents if it is possible to provide a piped vent.
    – pdd
    Mar 21, 2023 at 16:02

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