I inherited a small mess that I am cleaning up. The basement washing machine and utility sink wet vent into a 1-1/2" riser that goes thru the concrete slab, w/ an air admittance valve. I am stuck with that situation (these were installed by a plumber for the previous owner.) I don't know the DFU but if not large enough I'll change the AAV (or add a second.)

The W/M trap was cracked -- see pix -- (and the drain for the U/S is in a horrendously stupid location thru the legs - and notice the copper to galvanized to pex -- so I am starting anew with it all.) I may finish that room at some point, so might install a W/M supply/drain box - not sure.



Q1: The wye for the U/S drain - isn't that a No-No -- should have been a San Tee??

Q2: I'll locate the W/M riser/trap 12-18" above the slab - should I use 2" (and also there to the slab) even though the thru-slab is 1-1/2?

Q3: I plan to use an S-Tee for the W/M drain. OK?

Q4: Concerned w/ the high W/M flow/pumping - do I need to back vent the W/M drain (or I could add a second riser/AAV??)

Q5: Any reason to lower the U/S drain and stack the S-Tee above the W/M S-Tee (I'd think it's best to keep as much separation as I can)?

Q6: What's the reasoning for the W/M trap 12-18" and a max 30" riser (just curious)?

Q7: This pic floats around - isn't the wye for the W/M a no-no? Or is that only with a wye on the trap arm itself? And the sink drain looks too steep - is the vent entry below the trap weir level?

W/M  plumb-in

Thanks for looking at this!

  • If what they've done to the studs in that last photo is any indication, I wouldn't trust it very much. Sep 23, 2019 at 16:59
  • We're typically an "ask 1 question per post" type of site. There is some flexibility, but a bulleted list of 7 questions is a bit much. You might try to focus this question in on what is most important to you and ask another question for a separate part. The good news is that you've done some research and presented a well formatted question which is more than we normally get! Also, the abbreviations are hard to read. I would just use the whole word, or "washer" and "sink".
    – JPhi1618
    Sep 23, 2019 at 17:30
  • Is there a p trap under the sink?
    – Kris
    Sep 23, 2019 at 18:14
  • yes the sink is wet trapped.
    – Ken K
    Sep 23, 2019 at 18:25
  • If there is a trap under sink good. if not add one and drain the washer into the sink. Doing this eliminates risk of flooding because of under sized drain line or inadequate vent size
    – Kris
    Sep 23, 2019 at 18:27

2 Answers 2


Q1: the code always allows a combo wye whenever a san tee is permitted. The reverse is not the case: vertical to horizontal turn must use a combo wye and may not use san t.

Q2. do not do that. code NEVER permits reducing pipe diameter in the direction of flow. So if the below grade pipe is 1.5 inch everything upstream from it must be 1.5 or smaller.

Q3: yes, san tee is permitted horizontal to vertical.

Q4: there's no need for a second vent merely because there is a high discharge volume. wet venting may be a problem, though. Also, the fact that the washer drain is 1.5 inch and not the code-required 2" is the bigger problem.

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. Nov 16, 2019 at 10:48

To bring this back around for others.... Wet venting the washer is not allowed, and especially not with the laundry tray together. The pics (installed by a plumber) are totally illegal per the UPC and IPC. The washer must have its own 1-1/2" vent, but it can combine (above 42") with the sink wet vent to become a common vent. (For residential washers) the standpipe, trap, and trap arm must be 2"; a fixture branch can be 1-1/2". Answers: Q1 -> Q4 = Yes; Q5 = not allowed; Q6 & Q7= ?dunno (yet)

Thanks y'all! and to terrylove.com/forums - another great resource - for the answers.

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