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I'm trying to figure out where I'm supposed to install my main drain vent stack. I assumed it would be just above the toilet, but I'm confused by section 904.4 of the international plumbing code, which reads:

Vent stacks shall connect to the base of the drainage stack. The vent stack shall connect at or below the lowest horizontal branch. Where the vent stack connects to the building drain, the connection shall be located downstream of the drainage stack and within a distance of 10 times the diameter of the drainage stack.

The accepted answer to this question (Where should the main stack vent go?) seems to indicate that the vent should go ABOVE the HIGHEST drain-to-main-stack connection, whereas the code reads "BELOW the LOWEST". What am I missing here?

Given this rough sketch of my plumbing (this is an overhead sketch of a 1-story structure)...

plumbing diagram

I was thinking I would do AAVs for the bathroom sink and shower and the kitchen sink, so venting the toilet is the main concern. The guy that helped me with the drain plumbing in the first place may not have set me up well for venting, as my slab is already poured and we didn't have a strategy in place. I do have a sizable (maybe 18 in by 18 in) box that I put in place beforehand though, so I have some room to work with around the toilet.

Without busting up concrete, I think have 2 options:

  1. Run a stack right next to the toilet where I angle away from the drain at 45 degrees and then go straight to the roof with it.
  2. If I'm reading the code correctly about "below the lowest", put a vent just outside the exterior wall where the drain enters the house??? This would probably still be a dry vent since there's quite a distance (probably 50+ feet) away from my septic tank. Obviously this would be unconventional, and I'm not sure if it's allowed. My code does say something about needing at least 1 vent that goes through the roof, so maybe I can't.

Thoughts?

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    I think we also need an elevation (side view) showing the building drain existing and new fitting and sizes of the pipes.
    – Jasen
    Commented Dec 29, 2021 at 13:20
  • Everything is sloped 1/4 in per 1 ft of pipe. Outside the structure is 4 in I believe; reduced down to 3 in as it enters the structure. It's 3 in to the toilet, and all the other branches are 1 1/2 in.
    – Jonathan Narwold
    Commented Dec 29, 2021 at 13:23
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    The linked answer would appear to be talking about the top of the vent, not the base. There is a modern alternative to venting above roof height, a vent with an Air Admittance Valve, ie a one-way valve. these can be installed below roof height [200mm above the highest water entry point].
    – Tetsujin
    Commented Dec 29, 2021 at 13:25
  • "The linked answer would appear to be talking about the top of the vent, not the base." So given an example like upload.wikimedia.org/wikipedia/commons/7/7b/SoilStack.PNG, the way I would have interpreted it prior to this conversation is that the venting goes from the top-most fixture... but is 904.4 considering the whole green portion part of the vent even though it also carries water, and that's why it says "below the lowest"? Just trying to make sure I'm reading everything properly.
    – Jonathan Narwold
    Commented Dec 29, 2021 at 13:33
  • Yes. The stack has to originate at the lowest point & vent to above the highest [that applies to either a standard 'old' above roof or newer non-return.] The bottom of the stack can be a T, so long as it's at the [approximately] lowest point.
    – Tetsujin
    Commented Dec 29, 2021 at 13:44

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