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On each floor I have a tub and a sink back-to-back with a 2x6 wall in between. Can I run a single 2" pipe as a wet vent from below the slab to the two fixtures, then up the wall to the second floor fixtures, then up to the attic and main vent? If not, would a 3" wet vent work? Or should I run a parallel dry vent from the first floor up through the second floor, then up to the attic?

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    In most places you cannot do this, no matter the pipe diameter.
    – Moshe Katz
    Nov 12, 2021 at 20:28
  • Do I have to install a vent to add a bar sink in my basement? - provided your local codes allow it (I'm pretty sure in Chicago you can do w/e you want with 4") you can 'run a single 2" pipe as a wet vent from below the slab to the two fixtures, then up the wall to the second floor fixtures' provided that all of the connections to that 2" vent do not exceed 4 FDU, which it probably does. The FDU on 4" is 160! compared to 20 for 3", and 4 for 2".
    – Mazura
    Nov 13, 2021 at 2:30

1 Answer 1

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No

According to this article about wet venting at h2ouse.org:

Although wet vent plumbing is used to vent two or more bathroom fixtures at the same time, the fixtures all have to be located on the same floor of a particular building. They cannot be on separate floors.

They also point out:

It’s important to remember that codes will vary by city and state. The guidelines listed below are typically universal to both the United States and Canada.

So you'll want to double-check your local building codes to confirm.

Here is an example of a code that asserts that the fixtures must be on the same floor.

From 2018 Uniform Plumbing Code of Nevada:

Section 908.1

Wet-vented fixtures shall be within the same story; provided, further, that fixtures with a continuous vent discharging into a wet vent shall be within the same story as the wet-vented fixtures.

and

Section 908.2

A bathroom group located on the same floor level shall be permitted to be vented by a horizontal wet vent where all of the conditions of Section 908.2.1 through Section 908.2.5 are met.

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  • I liked to see some code cited in that link. "When in doubt, go with a 3-inch or larger wet vent pipe and a 4-inch horizontal waste line pipe" - and when in Chicago, arbitrarily go with 4", as it's required by code, less a few exceptions : § 18-29-710.1.2 "Minimum size. The minimum required pipe size for water closets or pedestal urinal branches and stacks shall be 4 inches (100 mm) ..."
    – Mazura
    Nov 13, 2021 at 2:46
  • "... except that any building three stories or less in height and used exclusively for family unit occupancy, may utilize a 3-inch (75 mm) soil stack when not exceeding 30 fixture units, with not more than two water closets per floor or two water closets per horizontal branch. The waste connection to any water closet shall be provided with an approved 4-inch (100 mm) closet flange. The minimum required sizes for underground branches from downspouts, floor drains, laundry trays and sink stacks shall be 3 inches (75 mm). (See Tables § 18-29-710A and § 18-29-710B.)"
    – Mazura
    Nov 13, 2021 at 2:46
  • "with not more than two water closets per floor" connotes that they're all using the same wet stack....
    – Mazura
    Nov 13, 2021 at 2:47
  • I can't tell from that Chicago code if it applies to wet vents, or just to dry vents.
    – Doug Deden
    Nov 13, 2021 at 2:58
  • @Mazura I'll see what I can find for code references and add them to the answer.
    – Doug Deden
    Nov 13, 2021 at 2:58

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