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I think this question has been asked before but I can't quite find exactly my situation.

I'm pretty sure this isn't to code, either, except the IPC code section 905.2 apparently says

All vent and branch vent pipes shall be so graded and connected as to drain back to the drainage pipe by gravity.

which this arrangement arguably does have.

I have a bathroom in an old (1928) home. I am gutting it and I want to reuse the existing stack vent. But I'm replacing all other plumbing, including the main part of the stack. I was going to cut the old stack at the bottom of the vent and guide the new stack into the old vent.

Old stack is 4" cast iron. So is the old stack vent.

The new shower drain and toilet will use the stack vent directly.

However, to make use of the old vent for the sink, which is going to be on the opposite wall to the old vent stack, I'd have to route the vent pipe up and over the steep roof gable, and back down.

I think the answer is going to be "create a new vent for the sink and poke it through the roof" -- and I'm ok with that. But I did just get this roof replaced 3 years ago and I'd hate to poke a hole in it if I didn't have to.

Images for clarity

Old vent stack -- black cast iron going up and to the left.

enter image description here

Proposed vent including question area (in dashed red oval)

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Detail showing the steepness of the roof angle

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  • 2
    Imagine pouring water in to the vent stack. If that water always runs back to the drain, no matter where you pour it in, then your vent topology is good. There are requirements about where horizontal runs may go (IIRC 42" above fixture), so mind those and I think you are fine.
    – bishop
    Commented Oct 13, 2020 at 1:54
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    "horizontal" (actually 2% slope) is 6" above the flood rim of the highest fixture on the floor served - which is usually 42" (or more) above the floor, for typical 36 inch sink height.
    – Ecnerwal
    Commented Oct 13, 2020 at 1:58
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    As you said, putting a new hole & flashing it is not a big job, and is highly unlikely to lead to leaks or any other problem. I'd go with doing this on the KISS principle. For one thing, much easier to maintain and to apply possible future upgrades to either stack.
    – Carl Witthoft
    Commented Oct 13, 2020 at 14:37

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All vent and branch vent pipes shall be so graded and connected as to drain back to the drainage pipe by gravity.

which this arrangement arguably does have.

I believe that as the code is typically interpreted, the latter half is draining "forward" (in the direction of the vent) rather than "back" even though the water (condensation, mostly) ends up in the same drain pipe ultimately.

Practically speaking I can't see that it should not work, and IPC tends to have so many ways to vent things I would not swear it's not allowed in some paragraph I missed (or found not relevant to my situation) when educating myself for my own plumbing, but I don't think it quite passes.

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  • Well, every portion of the pipe will drain into *one off the source pipes. As you said, splitting hairs in the language of the code.
    – Carl Witthoft
    Commented Oct 13, 2020 at 14:36
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Based on further research, the answer I have come up with is "maybe". But probably not.

Maybe, based on the whim of the plumbing inspector.

Maybe, based on whether gases trapped in the upper section of the overhead vent is a problem.

Probably will technically work as a vent, based on the physics of the system.

However, that's too many maybes, so I'm gonna poke a hole in the roof and "KISS" -- Keep it Simple, Smarty -- like one commenter suggested.

My new plan is this, and I feel quite confident that this would be acceptable to an inspector:

enter image description here

(Ignore that weird down/up "U" under the sink drain -- that's all actually on the same horizontal plane; I just drew it this way to be able to visualize it for a parts list)

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  • Presuming the sink is on the front of the house: As a thought, if the angles work out, you may want to continue the vent run up past the ridge so the vent comes out on the "back" of the house for a cleaner look from the road.
    – FreeMan
    Commented Oct 14, 2020 at 17:42
  • @FreeMan both sides of this gable are visible from the road. that isn't the front/back of the house's main roof, it is a side gable with the wall facing the road and the sloped roof perpendicular to the road
    – JDS
    Commented Oct 14, 2020 at 18:32
  • eh... It was a thought... :)
    – FreeMan
    Commented Oct 14, 2020 at 18:33
  • @FreeMan yep, i hear ya -- the main stack vent, the old one, actually does go up and out the back-of-the-house roof face, away from the road
    – JDS
    Commented Oct 14, 2020 at 20:17

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