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I'm laying out my shower drain and it initially was going to look like this:

enter image description here

Fairly straightforward. Shower into p-trap, vent coming out of top of drain line over to wall to be vented, all connected into the main. This is the TOP drawing below.

The catch is that the total height from the top of the vent elbow to the bottom of the p-trap is much taller than the height I have to work with in the joist bay.

Can I do what the BOTTOM drawing is showing? Instead of the vent coming off the drain line vertically, it's horizontal over to the wall, where it then goes vertical. This would shorten the total height allowing me to raise the whole shower drain assembly up. I can then connect that to the main run with a 45 degree connection.

enter image description here

In researching this, I've seen some informal mentions that this is OK as long as that vent branch is also sloped the requisite 1/4" per foot like any drain line. And I don't see a physical reason why this wouldn't work. But I can't seem to find a definitive "yes, this is OK" from a code perspective. Is this OK code-wise?

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  • Do you have enough space to go with the 1st diagram, but instead of a 90-degree elbow use a 45 (1/8) or 22.5 (1/16) instead? – brhans Aug 27 '19 at 11:21
  • @brhans possibly...good question! I will investigate! – DA01 Aug 27 '19 at 13:47
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    I have done the second set up more then 15 years ago with no problems. I can not speak to code as the house i was in was not regulated by any codes. – Alaska Man Aug 27 '19 at 18:08
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The only thing I see in the code is “A dry vent connecting to a horizontal drain shall connect above the centerline of the horizontal drain pipe.”, so as long the wye is angled up (which it would be to get the slope you described), I think you would be good.

I am not a plumber, so I advise waiting on some more experienced advise.

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  • That does seem to clarify things. Good find! Looks like I would have to use an angled connection, at the very least. – DA01 Aug 27 '19 at 13:49
  • That's 3104.3 Check the very next code section. 3104.4 – Ecnerwal Oct 23 '20 at 0:50
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Could you try angling the wye 45deg (at most) between the trap arm and drain, then using a 45 or 22 to connect at the vent?

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THE INTERNATIONL PLUMBING CODE REQUIRES A DRY VENT TO RISE VERTICALLY TO A POINT AT LEAST 6 INCHES ABOVE THE FLOOD LEVEL RIM OF THE FIXTURE IT SERVES BEFORE TURNING HORIZONTAL. THUS THE ILLISTRATED WOULD NOT PASS CODE REQUIREMENTS.

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  • Please edit your post so that it's not SHOUTING AT PEOPLE :) – ThreePhaseEel Oct 23 '20 at 0:32
  • You also need to recall that "vertical" in the IPC is defined as 45 degrees and up. And it's not just the fixture it serves, it's the highest fixture on the FLOOR it serves. Neither of the vents as drawn meet IPC, but we don't know what code the question is under. – Ecnerwal Oct 23 '20 at 0:46

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