We're in the middle of renovating our upstairs bathroom. My lovely and amazing wife has bought two vessel sinks and in-wall mounted faucets. After removing the drywall we have sorrowfully discovered that the main stack is right in the way of one of the sinks. I don't think there's any way around it, the only way to put the sink where she wants it is to move the plumbing.

This is in the upstairs, so the only fixtures draining out from this point will be the two sinks, so we're only really moving vents.

My question is I have to go through 2x6 studs with 3" pipe, can I do 90° bends for below the sink drains like in my illustration, or do I need to do 45° bends?

old plumbingNew plumbing

Update in response to answers:

enter image description here

  • What on earth is this extra pipe for… & why? i.stack.imgur.com/nCnLY.jpg
    – Tetsujin
    Dec 31, 2021 at 18:39
  • 2
    Dry vent to below, most likely. Most codes don't let you use a pipe that serves a toilet/wc as a wet vent below the point that the "soil" enters it.
    – Ecnerwal
    Dec 31, 2021 at 18:41
  • @Ecnerwal _ I've never seen such a structure… I'm guessing we've got transpondian differences again ;)
    – Tetsujin
    Dec 31, 2021 at 18:52
  • @Tetsujin That's the dry vent from the downstairs bathroom.
    – ShemSeger
    Dec 31, 2021 at 19:14
  • Shem - I think it's another of those UK/US differences. I have never seen anything coming up to a stack.
    – Tetsujin
    Dec 31, 2021 at 19:16

1 Answer 1


Unless your plumbing code is unusual, you have to use 45's until you are

6" above the flood rim of the highest fixture on the floor served by the vent

(From memory quote of IPC 2018 language)

Vents must be "vertical" below that point but "vertical" for vents allows up to 45 degrees off vertical.

So you could move the upper 90's up a bit and pass, while the lower deviation would have to be 45's to pass. Presumably the dry vent serves only the floor below, and that could be 90's on this floor, as it would meet the language of the code (that's why floor served is in there.)

I suspect it might be possible to meet code and use smaller pipes for the deviation - you'd have to calculate the required vent load, but you could perhaps go to a 2" dry vent and a 2" drain for the deviation, rather than maintaining 3" all the way, then rejoin the 3" pipe above (with the new dry vent, the lower floor dry vent, and the extension of the 2" drain (which is also a dry vent at that point.)

The capacity of a pipe serving as a dry vent is considerably greater than the same size pipe serving as a wet vent, per the vent rules/tables. So that's why a dry vent might let you shrink acceptably here. That also lets you make a 90 on the drain part, as it's not a vent if the dry vent is big enough to handle the vent loading.

Starting from your revised picture: Green as drain, your black (45& vertical) as dry vent, you move the drain cross over to the drain, you offset the 2" dry vent and presumed 1.25-1.5" lavatory drain going to the drain cross so both fit in the wall without connecting there.

As commented, depending on the total vent loading, your revised drawing might work as is.

enter image description here

  • I've added an updated image using smaller pipes as you recommended. Is that along the lines of what you were describing?
    – ShemSeger
    Dec 31, 2021 at 21:18
  • Almost, not quite. IMHO, you (probably) want two 2-inch pipes coming off the 3" stack. One is a drain to connect the sinks to, one is a dry vent. It may be the case that the arrangement shown provides adequate venting despite the wet-vent leg, but I don't know that.
    – Ecnerwal
    Dec 31, 2021 at 22:04

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