This is my first question so apologies if its not the highest quality. will do better over time.

My primary question is: based on the current plumbing layout, what problems am I likely to experience, and can I prevent any/all of them with adding strategically placed AAV without reconfiguring the overall drain setup?

the backstory:

I'm remodeling my upstairs bathroom, currently with the space opened up to assess the rough in plumbing situation. Attached is a few diagrams showing the plumbing routes, and a few pictures showing the area around the plumbing for context. A few aspects to understand before considering solutions to my question:

  • approx 50yr old CI stack in corner of bathroom, toilet connects directly to CI fitting w/two "ports" for connecting additional fixtures.

  • 1-1/2" PVC drain connects sink and shower directly to main drain line.

  • trying to avoid installing a full venting system through my roof, or tapping into the older CI stack to add proper vents, if possible.

  • I understand its less than ideal to have sink and tub connected to the same drain, but really trying to avoid removing additional material from floor joists to reconfigure this drain layout.

  • Best solution I have so far is to add AAV to the sink, either in the cabinet above the P or in the wall behind vanity mirror. Will this help me?

  • Secondary idea is to add AAV in the shower wall, somehow, above the tub overflow. will this help me?

  • Are the particular fittings used (sweeping 90s branches) harmful or helpful in this situation?

plumbing layout


tub and sink drain

bathroom layout

  • 4
    I've got to say, those are some of the finest hand-drawn diagrams I think we've seen here. Well done!
    – FreeMan
    Oct 31, 2021 at 13:22

1 Answer 1


You're on the right track with the AAVs. It would be better to have a real vent, of course; but if it's not practical, this is what AAVs are for.

AAVs are less likely to fail (requiring replacement) if you install them above the flood rim of the fixtures on that branch -- above the sink. They must be accessible, too; that means an access panel is okay, but behind a finished wall isn't. If mounting a cabinet in front of the AAV, I'd cut a hole in the back of the cabinet to make sure it remains accessible (and future homeowners / plumbers could find it.)

Be sure to follow the instructions when installing the AAVs. Don't permanently weld them into the fittings. Use pipe thread sealant (pipe dope) such as Oatey Great White or Pro Dope, which helps make a seal but doesn't weld the parts together.

  • 1
    Thanks Jeff, a good confidence booster that I'm heading in the right direction. I can envision where to put the AAV above flood level on the sink, with access panel, but can you advise where AAV should be placed in the tub/shower setup? does that need to be above the sink flood level too?
    – McNabbHome
    Nov 3, 2021 at 10:54
  • Best practice is above the branch flood rim, but there are plenty of situations when that is impractical, such as kitchen islands. If you can get the tub AAV above the tub flood rim, that's great. I would try to install it in the wall between the tub and vanity, facing the vanity, and finish using an access panel. That gives you as much height as you want. Hopefully this can be done without notching the joist. Nov 4, 2021 at 10:09

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