Drain and Vent double vanity with window above it - Is this OK?

[Imnotsure]

Given the following, Would the following configuration generally be OK? I would probably just use 2" everywhere except p-traps.
Anything particularly wrong with it? Thank you!

 

[wwhitney]

Which plumbing code applies to you? There are at least 5 different codes in the US, probably a few more.

It is allowed by the IPC, although the 2x1.5x1.5 san-tee on its side you show needs to be a combo. The total fall from the far trap to the stack would need to be at most 1.5", so that trap arm could be at most 6' in length if you achieve the perfect minimum 1/4" per foot fall.

It is not allowed by the UPC. For the UPC you could stack two san-tees at the stack, with the upper san-tee being 2x1.5x1.5 and the lower being 2x2x1.5. Then each lavatory could have a separate trap arm to one of the san-tees. However, the maximum trap arm length for a 1.5" trap is 42" under the UPC, so you might have a problem with the far lavatory. The trap arm length includes the pipe outside the wall, anything between the trap outlet and the san-tee.

Cheers, Wayne

 

[Sponsor]

 

[Imnotsure]

Which plumbing code applies to you? There are at least 5 different codes in the US, probably a few more.

It is allowed by the IPC, although the 2x1.5x1.5 san-tee on its side you show needs to be a combo. The total fall from the far trap to the stack would need to be at most 1.5", so that trap arm could be at most 6' in length if you achieve the perfect minimum 1/4" per foot fall.

It is not allowed by the UPC. For the UPC you could stack two san-tees at the stack, with the upper san-tee being 2x1.5x1.5 and the lower being 2x2x1.5. Then each lavatory could have a separate trap arm to one of the san-tees. However, the maximum trap arm length for a 1.5" trap is 42" under the UPC, so you might have a problem with the far lavatory. The trap arm length includes the pipe outside the wall, anything between the trap outlet and the san-tee.

Cheers, Wayne

Thanks for much the reply and breakdown for each code. IPC applies for me, but am interested in how to make it pass with even the more restrictive UPC for this situation.

Would upsizing to 2” everywhere including traps help resolve the UPC limitation, allowing me to go a max of 60” (assuming separate trap arm config like you mentioned for UPC)?

 

[wwhitney]

Would upsizing to 2” everywhere including traps help resolve the UPC limitation, allowing me to go a max of 60” (assuming separate trap arm config like you mentioned for UPC)?

A 2" trap on a lavatory would be unusual, but I'm not aware of anything that would prohibit it, you'd need to figure out how to adapt to the 1-1/4" tailpiece at the trap inlet. That would extend the UPC 42" trap arm limit to 60".

But under the UPC, you still can't use the connectivity in your diagram in the OP. You'd need to use another arrangement. In addition to the two san-tees on the stack option, if you have room between the top of the lavatory flood rim and the bottom of the window (might need 9"+) you could have a vent take-off on the horizontal 1.5" trap arm in your diagram, which rises to 6" above the flood rim, then turns horizontal to join the stack. [In which case the 1.5" stack segment between the san-tee and where this vent-takeoff joins the stack could in fact be deleted.]

Cheers, Wayne

 

[Imnotsure]

A 2" trap on a lavatory would be unusual, but I'm not aware of anything that would prohibit it, you'd need to figure out how to adapt to the 1-1/4" tailpiece at the trap inlet. That would extend the UPC 42" trap arm limit to 60".

But under the UPC, you still can't use the connectivity in your diagram in the OP. You'd need to use another arrangement. In addition to the two san-tees on the stack option, if you have room between the top of the lavatory flood rim and the bottom of the window (might need 9"+) you could have a vent take-off on the horizontal 1.5" trap arm in your diagram, which rises to 6" above the flood rim, then turns horizontal to join the stack. [In which case the 1.5" stack segment between the san-tee and where this vent-takeoff joins the stack could in fact be deleted.]

Cheers, Wayne

Thanks! Yes, for UPC I’d imagine it something like this very roughly speaking (which is similar to what I believe you’ve mentioned:

 

[wwhitney]

Yes, the red portion of your diagram matches what I described in my first comment. If the lower trap arm is 2", then the lower san-tee would instead be 2x2x2. The point being that the segment of the stack between the two san-tees must be 2" for the UPC.

Cheers, Wayne