Please see attached vent and waste layout. I have back to back bathrooms that I'll be attempting to layout as such.

My concern is the main vent going to through the roof. In the image, I have it a 4", and in the existing bathroom it is 4".

My question is can I use the 2x4" bushing to reduce in size to meet the 4" vent? Does the UPC require the vent to be 4" for the two bathrooms? This will require some furring in order to fit the shower mixer valves because of the 4" going through a 5.5" wall. The kitchen sink has its own vent.

Also, did I get my fittings correct? Any other concerns? Code is UPC. enter image description here

2 Answers 2


Well, I think 4" vent is overkill. Let's see.

You have kitchen sink on its own vent, so that does not count.

You have 2 showers at 2 DFUs each

You have 2 toilets at 4 DFUs each

You have two handwash/lavatory sinks at 1 DFU each.

Total of 14 DFUs on this vent, going into 3" soil pipe.

Per table 916.1 of the UPC, you could vent that with 1-1/2" pipe if the total developed length was less than 32 feet. I'd choose 2" which allows 110 ft developed length and feels like more safety factor.

But you have an existing 4" vent - let's ask why rather than assume the prior plumber was a total idiot. The only reason I could think to use 4" would be in the attic and above the roof if having the vent freeze is an issue in your climate, so a larger diameter to resist freezing might be advisable there. In conditioned space, 2" will carry FAR more venting DFUs than you are applying to this branch. So you can probably skip messing with the wall framing and just bump it up above the top plate. The sort of weather that vents freeze in is exactly the sort of weather you don't want a plumbing problem in...

Likewise, unless you just wanted to stick with mostly 2" vent to save on getting various pipe/fitting sizes (and maintain overkill) you should be able to use 1-1/2" to vent the toilet/sink combinations to the stack.

  • Ok great. Thank you. The waste system is about 15 years old but the vent system is much older maybe 40+yrs. Vents are cast iron and plumbing system is abs. Freezing is not an issue. House is in hawaii.
    – Pdhawaii
    Apr 28, 2020 at 4:46
  • I think trying to make sense of the old work is a really good idea, but as you do that remember that saving money by using material you have on hand might be the reason too.
    – dlu
    Jul 1, 2021 at 4:40

Back in the day all homes required a 4" stack.So no,the plumber was not an idiot. You can snap a 4x2 combination into existing stack rather than drill another hole through roof.

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know the details of contributing here. Sep 27, 2020 at 12:14
  • Can you be more specific about which day "back in the day" was? To some, that was last week, to others, it was 30 years ago...
    – FreeMan
    Sep 27, 2020 at 13:06
  • A 4" stack was required in the 40's,50's,60's.....up until around the late 80's,early 90's.
    – user123780
    Sep 28, 2020 at 5:14
  • A 3" stack is still a code requirement today.
    – user123780
    Sep 28, 2020 at 5:16

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