Main stack vent horizontal offset question

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by DavidTu, May 1, 2010.

  1. DavidTu

    DavidTu Member

    Messages:
    239
    Location:
    Seattle, WA
    The waste stack in my remodel had to be positioned such that above the floor-level of the 2nd floor (2-story single family home) the stack's vent cannot continue straight upwards but must go horiztonal around 3-4ft before continuing upwards. There are two bathrooms on the 2nd floor. I am aware of the restrictions for horizontal offsets of vents to be 6" above the flood level rim of all fixtures serviced by them.

    However, I have two questions: 1) First, does that restriction even apply to the main stack vent (as opposed to fixture trap-arm vents? and 2) the UPC says that the restriction can be waived if there is essentially no choice in the matter due to the building's construction (which I believe is the case here)--I'm in Seattle, how will I fare w/ inspector on this one?

    Helpful Plumbing Hints for Residential Construction by Bert Polk Plumbing Inspector Lincoln County
    Last edited by a moderator: May 24, 2010
  2. Terry

    Terry Administrator Staff Member

    Messages:
    15,136
    Location:
    Bothell, Washington
    Can you post a picture?
  3. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,843
    Location:
    Cave Creek, Arizona
    If you can angle the offset rather than go horizontal, it will not apply to you. But the 42" restriction applies to EVERY pipe that has a fixture ATTACHED to it, which also might not apply in your case, but only a picture would tell what you can do.
  4. DavidTu

    DavidTu Member

    Messages:
    239
    Location:
    Seattle, WA
    Picture

    Below is a picture/sketch of what I am trying to do. It is not exact, but representative. I can post a picture of the site, but not sure it'll help. The question is, can the 4' offset shown be done w/ the main stack vent at the floor level (below the flood level of fixtures upstream)--but note that the fixtures of course are vented/revented themselves.

    drains..jpg

    Attached Files:

  5. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,843
    Location:
    Cave Creek, Arizona
    The vent COULD be flooded if the vertical pipe were obstructed, therefore the vent should NOT be offset below the 42" level. BUT, using your diagram, that vent has NO FUNCTION since everything else has its own vent which ties back in above the ceiling. I am not sure how extensive your remodeling is, but if you should be in one of those locations which require a "full size vent" all the way to the roof, one of the toilet vents might be changed to a full sized one.
  6. DavidTu

    DavidTu Member

    Messages:
    239
    Location:
    Seattle, WA
    In Seattle we are under UPC, which requires the cross-section vent area to match the building drain area--other than that I am unaware of any further requirement. I was under the impression that the main stack had to have its own vent, but according to your comment that is incorrect. The low-heel 90 coming off the toilets are 2" vent. Are you saying it is ok to start at 2" then increase to 3" and have that be the through-the roof vent w/ other vents back-vented into it? (The building drain is 4" but we actually have two TTR 3" vents b/c there is another bath way on the other side of the house).
    Last edited: May 4, 2010
  7. DavidTu

    DavidTu Member

    Messages:
    239
    Location:
    Seattle, WA
    Here's an update, having talked to the inspector (by phone) he said that it is not required to have the 4" vent continuous so I can simply follow HJ's advice and get rid of the superfluous "venting" and then simply back-vent each fixture (with normal sized vents). I did get the feeling he thought upsizing the first toilet vent to 4" would be beneficial, but not required by code.

    Since the two toilets are so close together (their vents are only about 1.5' apart) I am simply going to keep them at 2" vents but combine into a 3" (VTR) just over 42" above floor level. Seems like a reasonable compromise w/o hasseling trying to fit the 4" Sani-tee in place of the Low-heel 90 4x2 I currently have in there (on the WC's). (Side note: there are two 3" VTR's rather than a single 4" so 3" is all that makes sense to up-size to in any case.)

    Thanks for the help.
  8. Terry

    Terry Administrator Staff Member

    Messages:
    15,136
    Location:
    Bothell, Washington
    In Seattle, as long as you have four 2" vents through the roof, you would be fine, or you can combine them for a single 4"
    That's if you have four toilets.

    Most homes that have three toilets have been required to have a 3" vent, or two 2" vents and one 1.5" vent.
    Since your home is in Seattle, it may have four inch waste inside, thus the requirement for the 4" vent or equivalent.

    You can run quite a bit of plumbing on a single 2" vent.


    [​IMG]
  9. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,843
    Location:
    Cave Creek, Arizona
    There is no need to even increase those vents to 3" as long as a 2" vent out the roof complies with the area requirement for all vents through the roof.
  10. DavidTu

    DavidTu Member

    Messages:
    239
    Location:
    Seattle, WA
    We already have set up two 3" vents thru roof and will use them both. We do have 4 toilets and also a 4" inside building sewer, drain & stack. I haven't done the math yet, but I assume once we back-vent everything we'll have to be at 3" min vent anyway (and of course we are required to have 4" area VTR) so combining the two 2nd fl toilets into 3" vent right away will only be at most 10' early.

    Thanks for the replies guys.
  11. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,843
    Location:
    Cave Creek, Arizona
    Two 3" are larger than the 4" so all you would NEED is a 2" vent on this system.
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