I don't know why I am struggling to mind a definitive answer on this. Once a vent pipe is above the flood line of fixtures and goes into the attic, can it run completely level or does it require the same slope as the waster lines, or does it need to be sloped so gravity works but no actual slope run is required per most local codes?

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    I'll let the plumbing experts answer as to code, but I believe a vent must only have a slight slope so no water from rain or such collects in the horizontal section.
    – RMDman
    Jul 15, 2023 at 14:50
  • Does that section of pipe get down to freezing temperatures? Is there something that makes having a small slope difficult?
    – crip659
    Jul 15, 2023 at 15:01
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    It has to slope enough to shed condensation, rain water, and backed up drain water. You don't want water to collect and restrict it. It does not have to slope enough to act efficiently as a drain. I can't express that in code terms but hopefully it helps.
    – jay613
    Jul 15, 2023 at 15:57
  • I can get SOME slope, but not 1/4" per foot, to at least to run it where I want to.
    – Evil Elf
    Jul 15, 2023 at 18:24

1 Answer 1


contact your local AHJ for a definitive answer, but IPC table 909.1 has vent slopes same as waste slopes (1/4" up to 2" pipe, 1/8" over)

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    And it specifically needs to slope towards the drain it serves. And it can't be "horizontal" (actually sloped) until it's "6 inches above the flood rim of the highest fixture on the floor served" - merely being above the flood rim is NOT enough.
    – Ecnerwal
    Jul 15, 2023 at 21:53

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