The kitchen sink is directly below a window, so the vent has to go around it.

This is how it is currently routed:

Current Vent Route

Because I am installing an undermount sink, I need to lower the p-trap. I am also replacing the galvanized pipe with CPVC of the same size (2").

Which of the following two vent pipe routes are more appropriate? Note that I can't move or reroute where the drain enters the floor. It's already in a very tight space.

  • [Route A - vent arises below p-trap]:

Route A - vent arises below p-trap

  • [Route B - vent arises above p-trap]:

Route B - vent arises above p-trap

Route A is preferable because that gives me more clearance to run 45 degree pipe. I may or may not have sufficient clearance to run Route B.

  • Don't know plumbing code, but think route B is more in code than A.
    – crip659
    Commented Dec 22, 2022 at 21:43
  • @crip659 Agreed.
    – cryptic0
    Commented Dec 22, 2022 at 21:51
  • Those 45s would also be allowed to be 90s if that helps you move the drain inlet down and still keep it below the bend.
    – Chris O
    Commented Dec 22, 2022 at 23:06
  • 1
    I think the key here is which arrangements keeps the p-trap from being emptied by the suction effect of the draining water. And that criteria is met by option B, I think.
    – SteveSh
    Commented Dec 22, 2022 at 23:18
  • 2
    @SteveSh In that case, why not keep the original design? That way I don't have to make any additional holes in the studs. After all, it's been working fine for 60+ years. Whether it will pass inspection is another story.
    – cryptic0
    Commented Dec 22, 2022 at 23:50

3 Answers 3


Keep in mind the hight of the top of the sink , must use "B" if the turns are below the top of the sink... "A" is not allowed that creates a Not allowed s trap. That said you can use 90'above the top of the sink

  • 1
    Above the sink is insufficient. "6 inches above the flood rim of the highest fixture on the floor served" which may or may not be the sink, and even if it is the sink, 6 inches more than that.
    – Ecnerwal
    Commented Mar 25, 2023 at 2:34

Vents must be taken off from the top of trap arms, and before the level of the arm has dropped a certain distance according to the pipe size. So your A version would be contrary to all plumbing codes. There would be no purpose served by using 45 degree bends instead of ordinary 90's, except to make the work more difficult.

  • Couldn't I just keep the original routing then? Just swap out galvanized for PVC and move the trap lower a bit?
    – cryptic0
    Commented Dec 24, 2022 at 15:53
  • Not correct. Vents must rise vertically (which is defined as "within 45 degrees of vertical") until "6 inches above the flood rim of the highest fixture on the floor served" so 90's are a violation below that point.
    – Ecnerwal
    Commented Mar 25, 2023 at 2:30

Plumbing 90's are generally actually 89's because you always need some slope.

The trap doesn't end until it meets the vent.

so option A

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