Context: I am located in California and doing an addition and moving the kitchen.

The kitchen sink and dishwasher drain into an existing line in the garage that has a laundry sink and a clothes washer.

I need advice on venting, pipe size, moving the laundry sink and clothes washer.

Existing - Garage has a laundry sink and clothes washer on a 2in drain with a 1.5 in vent. Note the clothes washer is downstream of the vent. The laundry sink is upstream.

New - Adding a Kitchen Sink and Dishwasher in the Kitchen with a 2in pipe flowing into an existing garage line with laundry and clothes washer. Adding a 1.5in vent in the kitchen. Moving clothes washer upstream of the existing 1.5 in vent, moving laundry sink downstream of the existing vent.


  1. Can the Laundry Sink be moved downstream of the existing vent and the clothes washer be moved upstream of the existing vent? See diagram
  2. How long can the Laundry sink arm be? 1.5in drain to 1.5in vent. 5ft?
  3. The existing 2” PVC drain to crawlspace does not have a vent. As the arm is over 13ft long, does it need a new vent?
  4. Is 2" PVC pipe drain to crawlspace big enough for Kitchen Sink, Dishwasher, Clothes washer, and Laundry Sink?
  5. Is the existing 1.5” Vent large enough for Kitchen Sink, Dishwasher, Clothes washer, and Laundry Sink?

See diagrams - Notice the difference between existing and new.

Existing Garage Piping

Proposed New Piping Setup

New V2 - based on Ecnerwal feedback

New V3 - based on Ecnerwal feedback

  • What plumbing code is in effect for where you are?
    – Ecnerwal
    Commented Oct 17, 2022 at 1:23
  • @Ecnerwal 2019 California Plumbing Code Based on the 2018 Uniform Plumbing Code
    – Max
    Commented Oct 17, 2022 at 1:24
  • The "new" laundry as sketched has several issues, which are probably sketch issues - You've drawn an S-trap on the standpipe, and you have the 2" line form the kitchen connecting to the 1.5" exsisting vent as a drain.
    – Ecnerwal
    Commented Oct 17, 2022 at 2:20
  • This question currently includes multiple questions in one. It should focus on one problem only.
    – isherwood
    Commented Mar 16, 2023 at 19:49

1 Answer 1


For International Plumbing code you would have:

  • Automatic clothes washer residential - 2 DFUs
  • Kitchen Sink, with dishwasher or food disposal or both - 2 DFUs
  • Service Sink (or Laundry Tray) - 2 DFUs

6 DFUs total. The Automatic clothes washer calls for 2" minimum drain

You do have a vent on the line to the basement right now. It's the vent at the laundry area.

A properly sloped dry-vented 2" pipe has a capacity of 21 DFUs, as a building drain, but is limited to 6 DFUs for a single horizontal branch. I'm not finding the precise point where they define horizontal branch .vs. horizontal building drain, but it would not matter for 6 DFUs.

And you've just clarified that you are under UPC, so this answer may not apply to you, and I'm going to leave it hanging here. Or put in a bit more; UPC will be similar, but differ in details, and I don't normally work with it given where I live.

1-1/2" dry vent on 2" drain is good for 12 DFUs if less than 75 feet or 20 DFUs if less than 50 feet developed length.

1-1/2" drain can be 6 feet long if properly sloped at 1/4" per foot. However, you appear to have drawn connecting a 2" drain to the 1.5" vent as a drain, which would be reducing the size of the drain, and making the 1.5" a wet vent - neither good. You may have meant to increase that lower section where the drains join in "new" to 2" - you need to, whether or not you meant to. 1.5" drain arm to 2" drain with 1.5" vent is fine.

You can only wet-vent 4 DFUs in your 2" drain line; You may not need more than one 1-1/2" dry vent, but you might need to connect it to your drain line at more than one point to limit the extent that the 2" drain is serving as a wet vent.

  • Hi @ecnerwal, I have added a new diagram to reflect your feedback. A few follow up points:
    – Max
    Commented Oct 17, 2022 at 3:14
  • Vents - one 1.5" vent re-vented at multiple points (what is the max arm distance for a re-vent?)
    – Max
    Commented Oct 17, 2022 at 3:14
  • Does the code permit adding a new re-vent into and exiting vent?
    – Max
    Commented Oct 17, 2022 at 3:14
  • Vent to Drain - Upgraded 1.5" drain to 2" drain at the base of the Existing Vent.
    – Max
    Commented Oct 17, 2022 at 3:15
  • P Trap vs S Trap - The existing clothes washer has a S trap. I understand I need to upgrade to P trap. I might not have the space for the kitchen drain and the washer p trap. Can the kitchen drain and the washer p trap be connected before reaching the vent?
    – Max
    Commented Oct 17, 2022 at 3:15

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