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Thread: Soil Stack Horizontal Venting

  1. #1

    Default Soil Stack Horizontal Venting

    I'm a Do-It-Yourselfer with a question. I'm remodeling my bathroom and want to install a small lavatory/utility cabinet in the wall behind and above the toilet tank. In doing so, I want to take advantage of the 3.5" stud cavity of the wall behind the toilet. As such, I will box-frame the rough opening of the cabinet space in the wall behind, and above, the toilet tank.

    PROBLEM
    My problem is that my main soil stack vent runs vertically right behind my toilet tank where I want to put my box-frame. (See attached .pdf pic.) Also, the main stack is cast iron for the first 35" above the floor. It then junctions with a with a no-hub fitting with PVC connection (all 4"" pipe) out through the roof.

    QUESTION
    My question: Can I put in a 90 degree fitting to turn the main soil vent horizintally over two stud cavities (approx 24"-30"), run it vertically for about 3.5 feet, and then run a 90" turn to tie back into the original soil stack vent?

    The toilet tank is 29" above the floor so the 35" where my cast iron/no hub fitting ties in is 6" above the highest flood plane (the toilet tank). I've also read that you need venting to be 42" above the floor prior to horizontal runs. However, I thought that was applicable only to branch venting which ties into the main vent. If a 90" fitting won't work, could I run a 45 degree fitting (to fit drainage requriements) then go vertical and back with a 90 degree to tie into the main vent?

    Any comments would be appreciated.
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  2. #2
    In the Trades kordts's Avatar
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    It's not the tank. That's not drainage, it's the rim of the bowl that the vent needs to be 6" higher before it goes horizontal.

  3. #3
    Moderator & Master Plumber hj's Avatar
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    The 42" parameter is for the point where vents join together. your plan is fine.

  4. #4
    Aspiring Old Fart, EE, computer & networking geek Mikey's Avatar
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    Does he have to maintain a 1/4" slope in the "horizontal" sections?

  5. #5
    Moderator & Master Plumber hj's Avatar
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    He doesn't HAVE to do anything, but it would be better to do it that way.

  6. #6
    DIY Junior Member Stainedrat's Avatar
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    Something to think about if you haven't already removed the wall. Where is the tie-in for your lavatory and shower for their vents?
    This is usually where the 42" rule of thumb comes into play. (6" above the flood rim of your lavatory.) You may have to redirect those revents. It is always best to stay 6" above the highest flood rim of your system for when you revent.
    And 1/8" fall per foot is adequate slope unless your inspector wants different.

  7. #7

    Default Thanks

    Thanks for the feedback/input. I'll proceed as advised - with caution.

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