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Thread: 3 inch Pipe for a new toilet upstairs through 2x8's or headers?

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  1. #1
    DIY Junior Member HayesFreeman's Avatar
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    Mar 2013
    Atlanta Ga

    Default 3 inch Pipe for a new toilet upstairs through 2x8's or headers?

    I've got a problem that I can't figure out.
    I'm building a new bathroom upstairs.
    The bathroom is up against the back wall of the house and the toilet I need to install would be up against that wall
    After taking out the end space for a shower, there is only about 4.5 feet left to put a toilet.
    I'm having a problem with how to drain the toilet.
    My floor joists are only 2x 8. yet I need to drain with 3 inch pipe minimum.
    I do not have any place where I can put the toilet and just drain straight back and down due to a door header that I run into
    in the wall below. There are a couple of spots that I can run the pipe down, but that would require boring through
    two floor joists, to get to that area
    3 5/8 inch holes for the pipe in the 2x 8's is breaking every rule I've found for boring.

    I don't think i can run the drain straight back and down due to the header below, and yet I don't think I can run it through the two floor
    joists either.

    Am I wrong about one or both of those statements?

    Is it possible that I could bore those holes in two joist. Would the fact that it's only two joists and that I could do it close to the wall help my case? Could I support those two joists with additional 2x8's on both sides, and then drill through the 3-layered 2x8?

    The only other solution I can think of would involve putting the toilet at the end of the bathroom, but that would put the shower in the middle. but the bathroom is tiny. That would be like a "hall shower" with doors on both sides that you'd have to walk through to get to the toilet.

    Any ideas or help would be appreciated

  2. #2
    Forum Admin, Expert Plumber Terry's Avatar
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    Aug 2004
    Bothell, Washington
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    You will have to find your way around those joists. Even if you have to drop below the joist. When I'm plumbing with 2x8 joists, and I have to go sideways to them, I drop below.
    It looks best if you put that near a wall where it can be boxed out. You can run "with" the joists, but when you cross them, drop under.
    You're thinking way too much. It isn't going to change physics.

  3. #3
    Moderator & Master Plumber hj's Avatar
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    Aug 2004
    Cave Creek, Arizona


    I would have to be there and actually see it, in order to determine whether the joists have enough support to allow the drilling. You might also consider cutting them off and headering the joists to create a "box" for the pipe.
    Licensed residential and commercial plumber

  4. #4
    ACO Shower Drain Sales johnfrwhipple's Avatar
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    Jul 2009
    North Vancouver, BC
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    Drilling out 3 1/4" holes in a 7 1/4" (2"x8") floor joist sounds like a really bad idea to me. What is the floor surfacing material you plan to use overtop of this floor?

    What is the span of the floor joists?

    I like HJ's idea of creating a box. You might increase the framing to double 2"x8"'s on 12" centers and using triple 2"x8"s to create something to box out the toilet rough in. Kind of like installing a sky light in a roof. You increase the floor joists to become strong enough to support a few bays

    en........nen......................nen Plumbing run through this new space

    e=existing floor joist
    n=new floor joist (2"x stock or LVL)
    X=new framing (2"x stock or LVL)

    Of course Simpson strong tie joist hangers and joist hangers should be used.

    A structural engineer can advise you of the best way to specify the right framing upgrades and fasteners to use. We have seen an increase in the request for LVL mini beams 1.75" x 8" type for these framing upgrades. Glued and screwed to existing floor joists make for a much stronger floor with a much more increased deflection rating. If those are old fir floor joists they may be quite rigid and stronger than today's lumber.

    Is the floor bouncy now? Can you stomp around without spilling a full glass of water on the floor?

    jfrwhipple@gmail.com - www-no-curb.com - 604 506 6792

    Always get construction advice double checked by your local city hall. Flood Test Every Shower - Every Time.

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