I've started a kitchen remodel and it turns out this is a wet vent arrangement with a lavatory sink from the second floor draining through the current kitchen sink and laundry standpipe drain. This is moderately inconvenient as I am attempting to shift my sink about 2 ft to the right (so it sits in the corner of an L-shaped kitchen).

After further investigation, I can work around this issue by connecting the new vents to the existing 2" vent coming out of the powder room, which vents through the adjacent garage (single story) roof. The rest of the house vents through the main roof. However, the vents for the kitchen sink/dishwasher and laundry standpipe have to merge in a horizontal tee configuration between the joists (a few feet above the highest flood line on this floor). I cannot find a prohibition in the NSPC 2009 code, but it is not an arrangement I have seen before in any references. Has anyone else implemented a similar solution that passed inspection? This requires a round about path though a wall as seen in the attached isometric image because there is a forced air duct running between the joists.

To clarify the attached drawing:
- The existing 2" 2nd floor lavatory sink will be slightly rerouted through the wall beneath the laundry standpipe to the lowest connection of the trio. Going to the 3" drain in the basement is possible, but potentially impossible (down to sub 1/4" clearance) due to 4" rigid dryer venting running through the same joist bay.
- The kitchen sink will drain next. A 24" sloped horizontal run will connect the sink to above the 2nd floor connection with a combo-fitting or tee. A separate vent with cleanout (any recommendations on positioning and on where to hide it?) is routed up and reconnects to the laundry standpipe vent.
- The laundry standpipe will be the top with a 31" sloped horizontal run and connect with a combo-fitting or tee.
- The combined 1-1/2" vent is through this perpendicular wall past the forced air duct (where it meets a new 2" vent from the basement and is upsized to 2" from there on out), then up into between the joists, and finally merges using a tee with the existing powder room vent. If allowed, are there recommendations on if the powder room should be the branch or if the roof-vent should be the branch side of the tee?

- As presented, I do not think I can wet-vent the first floor fixtures due to height constraints of the sink/disposal. If I could swap the kitchen sink and laundry standpipe, then I could wet-vent the standpipe because it only requires a 1-1/2" vent and the sink requires a 1-1/2" drain, but I would be using a 2" pipe (1 size increase). The limiting factor in performing the switch is the deep sink requiring a slightly lower drain location and extra 1/2" rise due to the drain slope.

- I'm looking into using a double-fixture fitting to join the kitchen sink/dishwasher and laundry standpipe, but I believe I am unable because the inlets are not at the same opening level. The top of the standpipe is about 42" off the ground (16" + 24" standpipe height), but the floodrim of sink is about 36". Is this correct or have I misinterpreted what constitutes inlet openings?

Based on the DFUs, I will have enough capacity in the 2" vent (and the 2" drain the lavatory sink, kitchen sink/dishwasher, and laundry) to cover this along with enough capacity to add a basement rough-in to cover a future bathroom down there.

The code is NSPC 2009 and none of the walls are load bearing. I plan on using Simpson SS- or HSS-series brackets to repair the notches in the wall. As a bit of background, this is phase 2 of my house project. I have already passed the first phase structural, electrical, and plumbing, but in that case the plumbing was supply-side only (much more forgiving in the layout).

Thanks for any suggestions. It's a lot, but I generally only ask when exhausted relevant forum posts to read.

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