I'm working on a bathroom reno that involves moving plumbing fixtures. Is my proposed drain layout up to code? I'm in Virginia, so this is the 2018 Virginia Plumbing Code, based on the 2018 IPC.

Bathroom layout

My thought is to use a wet vent / common vent, so the drain from shower on downstream needs to be 3" (because to the toilet). There is an existing vent up to the ceiling (and then on thru the roof) between the shower and tub, so my thought was to connect that vent to the 2" shower drain. The tub is then upstream of the common vent (we're allowed 1 fixture upstream, right?), and it would have a 2" drain until it meets the 3" wet vent at the shower connection. The vanity would be 2" under the floor (1 1/2" in the wall), and the toilet would be 3". The main stack is somewhere at the top of the image.

Is my understanding correct? Or do I need to modify things to meet code (and/or make my work a bit easier)? I understand there are some distance limits which may apply (5 or 6 feet?), and I'm worried those might be too far. The scale in the image is 1 square is approx 6". Joists are open trusses (running left to right in the image) so no worries about drilling 3" holes, but the exact location might need to shift left or right a little to avoid the support members.

  • critical trap arm length is based on diameter of pipe, since that affect weir, and it isn't clear on the diameters / distances or where the vertical vents are? Apr 7, 2023 at 4:45
  • @FreshCodemonger I only see the one vent at the right (where the blue line ends).
    – Huesmann
    Apr 7, 2023 at 12:11
  • @FreshCodemonger There's an existing 1 1/2" vent in the wall and up thru the roof where the blue line ends (marked "existing vent" on the diagram). It transitions to 2" thru the roof.
    – mmathis
    Apr 7, 2023 at 19:20

2 Answers 2


The max distance between fixtures and vents is specified in the code as are the minimum sizes of the drains and vents for various combinations of "fixture units." It's hard to tell, but if this is to scale-ish you might be too far away tub and/or the toilet. I'm not clear on where the "wet vent" actually goes since it's not drawn, just annotated.

You might be able to tie into the one for the shower but the size has to be big enough to accomodate both. You might also be able to use a Studor Air Admittance Valve in some places if you really can't get to a real vent to the outside. I know they get used, but you'll need an "existing conditions exception" and it also probably depends on the inspector.

If you're not pulling a permit, you might save yourself a whole lot of pain by paying a plumber to review what you've got and help with the parts you don't know how to do. A flooded toilet or stealthily leaking fixture, 'cause you missed something a plumber would see right away, can cost waaaaaaay more than what you'll spend to get it done right by having someone who knows what's right/to code take a look at it even if you end up doing it yourself.

  • 1
    "Hire a pro, don't DIY" - on the DIY site, and vague handwaving / fear mongering.
    – Ecnerwal
    Apr 8, 2023 at 2:15
  • Difference, @Ecnerwal, between paying a plumber to review the plan and paying a plumber to execute the plan. Of course, finding a plumber willing to just do a review might be difficult... In general, I agree with your assessment.
    – FreeMan
    Apr 8, 2023 at 13:25

You are not allowed one upstream fixture, but it's an easy fix.

If the squares are 6" then it appears good, other than where you connect the vent. Properly sloped 2" lines can go 8 feet from the trap to the vent.

But section 912.1 states that the wet vent shall be from the connection to the dry vent "downstream in the direction of flow" which does NOT bless your tub connection.

If you relocate the dry vent to the tub drain, all becomes code legit for a "bathroom group wet vent" - 5 or 6 DFUs depending on flush volume per table 709.1 so you'll need 2-1/2" pipe (per table 912.3) to the tub drain downstream of the dry vent connection. The tub drain before the vent connection can be up to 8 feet from the dry vent connection on 2" pipe, per table 909.1, and if the scale is right, that means you could attach the dry vent right where the tub and shower drains join (apparently about 6-7 feet of pipe from the tub drain) and either stick with 3" or use 2-1/2" until you get to the toilet connection, as you prefer.

Without any modifications Virginia may have made, https://codes.iccsafe.org/content/IPC2018P5/chapter-9-vents#IPC2018P5_Ch09_Sec905

The 1-1/2" dry vent is good for far more fixture units, varying with effective length per table 906.1 - no need for (an eventually stinky) Studor with a proper dry vent to connect to.

I suggest reading it a bit beyond just looking at cited locations, if you can stand that sort of thing. Having two windows open can be handy to track down where it references other sections of the code.

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