Wet Venting possible with raised fixture on horizontal branch?

Discussion in 'UPC Plumbing Code Questions' started by Manuel, Sep 23, 2021.

  1. Manuel

    Manuel New Member

    Joined:
    Sep 3, 2021
    Location:
    Hawaii
    So I've got a section of a level here and it shows what I currently have modeled out for this bathroom. Is wet venting in this fashion possible, where the fixture is branching off vertically or rolled @ 45 degrees from the main branch pipe acting as a wet vent? From what I can tell in the UPC, fixtures on the same level should be horizontally connected to the branch line so this shouldn't be viable but I wanted to make double sure by asking.

    The reason I have it like this in the first place is that the slab joists below are keeping me from raising the main branch from coming any closer to the finished floor and I'm already pushing the 2' max drain head to p trap on those floor fixtures due to the raised floor.
     

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  2. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    No, that doesn't work. It violates the trap weir rule, the trap arm needs to be vented before it falls more than one pipe diameter from the trap elbow outlet.

    If the bottom of the slab joists is more than 24" from the finished floor I don't have a solution for you. Unless you have room in the raised deck sleepers to run a 2" drain across, rerouting the lav drain to run towards the tub and shower, and can fit the traps above the slab.

    Cheers, Wayne
     
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  4. Tuttles Revenge

    Tuttles Revenge In the Trades

    Joined:
    Oct 15, 2014
    There's nothing there that prevents a horizontal wet vented system from working as long as the standpipe to the trap isn't too long. You still need trap arm lengths that meet code like any horizontal system.

    The drawing being out of perspective is making it difficult to understand your intentions. A plan view/Top Down drawing of the layout of the bathroom that also shows the direction of the beams might help. Something simple.
     
  5. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    As I understand it, the OP's drawing is a true section, and the floor framing is more than 24" tall.

    So the problem is that with the horizontal branch drain below the floor framing, it's not possible to comply with the two different constraints on the trap elevation. On the one hand, it has to be at most one pipe diameter above the horizontal branch drain for horizontal wet venting. On the other hand, the tailpiece from the drain to the trap is limited to 24".

    Cheers, Wayne
     
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  6. Tuttles Revenge

    Tuttles Revenge In the Trades

    Joined:
    Oct 15, 2014
    OK.. I read that again and see that. That was between my first and third cup of coffee ;)

    So it could be done with horizontal wet venting if the standpipes were'nt too tall. Reading my illustrated manual with explanations as to the Why.. they explain that the tail piece length rule has 2 parts.
    1- to prevent odor from material built up in an excessively long tail piece (meh)
    2- that an excessively long tail piece could create velocities that could siphon a trap despite it being vented.

    Personally I would ask the inspector for a variance if you were on the cusp of that distance and install it like a true horizontal wet vent with the entire system on the 1/4" slope to maintain that venting.
     
  7. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    Looking at Hawaii's UPC, they've amended it to allow AAVs. https://up.codes/viewer/hawaii/upc-2018/chapter/9/vents#new_912.0 So an AAV could be put inside the joist bay, if vented access can be provided without violating whatever floor to floor fire separation is required.

    They also allowed combination waste and vent systems where "structural conditions preclude the installation of conventional systems." https://up.codes/viewer/hawaii/upc-2018/chapter/9/vents#910.0 The section on vertical waste pipes in a combination waste and vent allows a horizontal branch at a 45 degree angle.

    Cheers, Wayne
     
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  8. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
    1. Can you drill through the joists?

    2. Would the void be big enough to contain the horizontal wet vent/drain and trap arms? Let the U of each trap extend though subfloor holes between joists.
     
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