Context: Connecting a 1.5 in diameter 42in long sink + dishwasher trap arm into a 2in diameter drain.


  1. How far, from the trap to drain connection, does the vent on the 2in diameter draine line have to be located? from my research I am getting 5ft but also 8ft from the connection.
  2. Can the re-vent of the laundry sink be removed, if the drain is a 2" pipe rather than a 1.5in pipe?
  3. How long can the trap arm be if the 2" drain to the vent is 8ft?

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  • 1
    what's horizontal and what's vertical? please provide a sketch with orientation, diameters and lengths
    – P2000
    Commented Dec 10, 2022 at 0:52
  • 1
    8 feet (from the exit of the trap) is the distance if the whole thing was 2" to the trap, because that's the limit where the end of the pipe is still level with the trap weir.
    – Ecnerwal
    Commented Dec 10, 2022 at 1:53
  • Thank you P2000. See attached diagram. If the new laundry sink drain is 2in, can I get rid of the re-vent?
    – Max
    Commented Dec 10, 2022 at 3:03
  • Thank you Ecnerwal, this is what I thought. It should be 8ft for a 2in vertical change. But then, how long can the trap arm be, if the exit to the 2in drain pipe is 8ft from the vent?
    – Max
    Commented Dec 10, 2022 at 3:04
  • 8 feet is 8 feet from the exit of a 2" trap in a 2" trap arm. Not from where a long 1.5" trap arm enters a 2" pipe. So the answer to question 3 is 0 inches.
    – Ecnerwal
    Commented Dec 10, 2022 at 14:30

1 Answer 1



  1. 8ft max if the slope is 1/4in per foot. It would be less than 8ft if slope is steeper (8ft x 1/4in = 2in).
  2. Yes, if a 2in drain is used then the 1.5in re-vent can be removed.
  3. 0in because the trap weir elevation is right at 8ft for a 1/4in per foot sloped 2in diameter pipe. This is why it is recommended to use less than 8ft, to allow for a longer than 0in trap arm. The minimum trap arm length is 2 pipe diameter from the ptrap.
  • Curious to understand why the above answers are not helpful, when they are in fact the actual answers to the questions. Perhaps, the above answers did not address a different question the reviewer had in mind.
    – Max
    Commented Jan 10, 2023 at 22:52

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