Sharing trap arm for 2 drains

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Philip.I

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Hi all,
I'm replacing my shower/tub combo with separates and had a question on drainage. As you can see in image, original combo had a 1 1/2" pipe tying into the vent and drain line. I'm wondering for plumbing shower and bath separates:
1. Can I drain shower and tub down same trap arm (with both having separate p traps to tie into it)? Shower drain would reduce from 2" to 1 1/2" after ~4'.
2. Or can I have separate trap arms join right at the 1 1/2" opening shown in the photo?

Note: Shower is further and would slope 1/4" per 12".
 

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wwhitney

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Those are the UPC answers. The OP is under the IPC, which is more lenient on some of these questions.

Both the tub and shower are 2 DFUs (assuming the showerhead(s) put out less than 5.7 gpm), and so can use a 1-1/2" trap, although 2" is more standard for showers. But where the drains combine, you have 4 DFUs, so you'd have to use a 2" branch drain (1-1/2" is limited to 3 DFUs under the IPC). Given that, you might as well use a 2" trap for the shower (and you could use a 2" trap for the tub if you like).

That means you can't connect both fixtures to the existing 1-1/2" pipe. You'll need to replace fittings back to where it's at least 2". The 1-1/2" vent is still fine, though.

As for the vent, the IPC does actually allow "horizontal common venting" of two fixtures, where their trap arms can combine and then continue to a single vent for both fixtures. (IPC 911.2)

https://up.codes/viewer/colorado/ipc-2018/chapter/9/vents#911.2

Each fixture has to individually meet the maximum trap arm length limit relative to that common vent. That's 6' for a 1-1/2" trap and 8' for a 2" trap. Note that achieving those lengths requires a perfect 1/4" per foot slope, no extra slope. That's to comply with the trap weir rule, which limits the total fall of the trap arm to one pipe diameter.

Cheers, Wayne
 

Jeff H Young

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Good call Wayne. I'd run a 2 inch because that's code here . but if minimum is 1 1/2 over there go for it
 
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