Pics show waste and vents above floor and below. The drain on the right connects to the main waste pipe. (It’s near enough that it doesn’t need a separate vent.) The one on the left has a vent with a Studor Air Admittance Valve in the unfinished attic.

The one on the left usually drains slightly slowly - but sometimes it drains quickly. I’m not sure what determines the difference. Perhaps the Studor sticks?

Does the piping look correct?

drains above floor drains below floor

  • how does the STUDOR works
    – Traveler
    Commented Apr 29 at 7:17
  • AAVs do not help draining, they only prevent siphoning, you have a tee installed with the branch facing up, I don''t think that's allowed and the two 45s are questionable too.
    – Jasen
    Commented Apr 29 at 11:46
  • I'm confused. Why does your vent come in below the drain? Why haven't you simply boxed the drain for the right sink into the existing vent stack? (And why haven't you put the vent through the roof?)
    – isherwood
    Commented Apr 29 at 14:52
  • Tx Jasen, The tee and two 45s are BELOW the vanity installation. I think the Tee branch is oriented correctly. Let me know if I'm missing something.
    – jbbenni
    Commented Apr 29 at 20:13
  • Tx Isherwood, I may be the one who is confused. The vent stack is distant - the vertical PVC in the image is for the AAV. Its purpose is to break syphon for the left sink. The AAV needs to be located above the sink drains and can't be enclosed in the wall. So I ran it into the attic, where it's "nominally" accessible. It could have penetrated the roof and become a conventional vent, but I didn't want to disrupt the roofing material. (The right sink is almost directly above the main drain, short distance - hence no separate vent. It works well consistently, quick to drain and no gurgle).
    – jbbenni
    Commented Apr 29 at 20:27

1 Answer 1


Updated Answer:

If you have access to the venting for the left sink and can revise it so that the vent connects above the fixture drain, then you can wet vent the right sink through the left sink's drainage as in the sketch below. This would be a better solution.

enter image description here

Original Answer:

Both sinks are piped in a manner that would not pass a plumbing code. The vent connection on the left sink must be above, not below the tee for the fixture drain. The sink on the right must be vented, its proximity to the 'main waste pipe' means nothing.

It looks like you installed tees in the wall to poke out for the sink's fixture drains, the correct way would have been to extend the upper end of each tee up into the attic and connect them to an existing vent, or add a new vent, or lastly use an air admittance valve.

Here's what it should have looked like: enter image description here

The simplest (and likely the best) way to correct this installation would be to add the vents as per the above. This would involve opening up the walls, or at least making holes in the walls so that you can drill through the horizontal framing.

Another option, that would not require opening walls, assuming that the piping below is accessible, would be to reconfigure the piping so that both sinks drain to a common p-trap located below. This p-trap could be vented by repurposing the existing left sink's drain. You would be required to drill through the floor and the bottom of the cabinet on the left side to connect the left sink. You should be able to reuse the drain for the right sink. This assumes that your local code allows for common trapping of two similar single-compartment fixtures located in the same room and that the developed length of each sink's fixture drain does not exceed 4 feet (or whatever length your code limits it to).

Here is a sketch of this arrangement. The blue lines are the existing piping, the red lines are the changes, and the yellow highlighted line is the developed length of the right sink. enter image description here

  • Wow. You set the gold standard for a thoughtful, critical, but helpful response. And you provided a ton of useful info. I appreciate that you educated, rather than slammed, me. Nicely done!!
    – jbbenni
    Commented May 17 at 20:32
  • Considering only the left sink for a moment. Could I use the P-trap under the sink if I reroute the vertical vent to come in above the trap weir? I can access from the back side and move the horizontal leg of what should have been a vent to be ABOVE the trap water level. For that one sink, is that a viable option?
    – jbbenni
    Commented May 17 at 20:36
  • As for the Right sink, the Developed Length from the (under sink) P trap to the main stack drain is less than 4 feet. Is that a viable configuration?
    – jbbenni
    Commented May 17 at 20:38
  • 1
    Given your comment that you can access the left sink's venting, I've updated my answer. Regarding the developed length: it is measured from the sink connection to the p-trap connection.
    – pdd
    Commented May 19 at 21:35
  • 1
    The reason you do not offset a vent horizontally until 6" above the fixture's flood-level rim is that if the drain gets plugged you want the fixture to overflow before the vent fills with water/solids. If the vent receives water/solids, over time the vent could become restricted due to a buildup. In your scenario, the likelihood of your vent becoming plugged would be small.
    – pdd
    Commented May 20 at 20:40

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