Back-to-back Lavs on Same Side of Stack

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Still_Learning

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I've read on this forum about using a double fixture fitting for back-to-back lavs but what should I do if both sinks are on the same side of the stack and close to back-to-back in location (think two mirrored bathrooms sharing a wall).

Both lavs will be pretty close to the main stack (within 30-36 in'). Should I combine them as in option 1 (in the picture) or stack two san-tees on the main stack as in option 2? Or is there a better way to achieve this?

plumbing4.png
 

Still_Learning

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Many thanks!

I should've clarified. The main stack is 4". If I go with option 2 and stack the san-tees, should the drain line for each lav be 2" or 1.5"? These are bathroom sinks.

I'm wanting to make sure I both get it right and there's enough room in the wall for the drains and the revent.
 

wwhitney

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Presumably there are no fixtures draining into the stack above the lavs, as if there are, it would not be a dry vent.

IPC or UPC?

IPC allows Option 1 as drawn (with 1-1/2" horizontal drains), UPC does not. For IPC Option 1, each lav trap arm measured from trap to the stack must be a compliant trap arm.

For UPC, you can do Option 1 if you add a revent (like the green line in Option 2) to one of the trap arms upstream of the wye. Then the drain between the wye and the stack should be 2", but each individual trap arm can be 1-1/2". The revent dry vents one lav, which horizontally wet vents the other.

For either UPC or IPC, you can do Option 2 without the green revent. The lav on the upper san-tee is dry vented; the lav on the lower san-tee is wet vented by the upper lav. The lav trap arms only need to be 1-1/2".

Cheers, Wayne
 

Still_Learning

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Sounds good! Yes, no fixtures above this and I'm under UPC jurisdiction.

So for Option 1, I'd need to add the revent between the branch of the wye and the p trap and make the section from the wye to the stack 2". I think I got it.

I'll probably go with Option 2 with 1.5" drain lines without the revent as that seems easiest to accomplish and fit into the wall, but it's nice to know there is an alternative if I need to go a different route. Thanks so much!
 

wwhitney

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So for Option 1, I'd need to add the revent between the branch of the wye and the p trap and make the section from the wye to the stack 2". I think I got it.
To be clear, the revent can be between the wye and either trap. That dry vents one lav and lets it wet vent the other.

Cheers, Wayne
 
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