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Thread: Drain options when replacing an existing tub with a longer one (longer trap arm)?

  1. #1
    DIY Member jadziedzic's Avatar
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    Default Drain options when replacing an existing tub with a longer one (longer trap arm)?

    I have a typical 5-foot fiberglass alcove tub/shower combo in the 2nd floor bath that I'd like to replace with a 6-foot tub (Kohler Archer?), and I'm wondering what suggestions the experienced plumbers on the forum might have with regards to venting the longer trap arm that will be necessary.

    The tub is oriented parallel to the floor joists, left side of the alcove is flush with the exterior wall of the house, tub has a right-hand drain, with the drain line running within the joist bay under the tub from right to left where it connects to a sanitary tee in the exterior wall and heads downward (vent heads upward to join with other vents in the attic). (I could cobble up a picture or drawing if the description isn't specific enough.) We're under the IPC here, and my rough measurements give me about a 52-inch trap arm. The drain line is 1-1/2 inch PVC, and remains 1-1/2 inches all the way into the basement where it joins with the main drain line. Replacing the 1-1/2 drain line would require opening the wall below; possible, if necessary, but prefer to avoid if possible.

    If I install a 6-foot tub the trap arm will be about 64 inches, which is longer than allowed here. About the only option for venting I can come up with would be to re-route the drain so it goes through one of the (2x10) floor joists and heads downward (drain) and upward (vent) in the rear alcove wall which is behind the long axis of the tub; fortunately there is a wall below that wall in the 1st floor. I'm guessing there is probably another floor joist just to this side of the rear alcove wall. I considered a tee somewhere close to the outlet of the new trap with the leg of the tee angling up to run in the rear alcove wall to the attic, but I'd have to roll that at 45 degrees and probably chew up too much of the adjacent floor joist with the resulting hole.

    I could go with a left-hand drain tub, but that would put some portion of the supply lines in the (2x6) exterior wall, and given our winters in New Hampshire that's something I'm reluctant to do.

    Any thoughts? Right now the project is in the planning stage but I'd like to know if going with the longer tub is feasible before I get too involved in designs. Thanks!
    Last edited by jadziedzic; 03-12-2013 at 10:47 AM.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Do they make the tub with a center drain? Not sure if I follow, but the trap arm cannot go down before the vent comes off of it. The swing of a trap can place the outlet in a large circle - it works just fine nearly folded back on itself, if that would help.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    DIY Member Gordan's Avatar
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    Where do you get the maximum for trap arm length? IPC says 6' for 1 1/2" drains. (This is at the mandated 1/4" per foot minimum slope.)

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    Quote Originally Posted by Gordan View Post
    Where do you get the maximum for trap arm length? IPC says 6' for 1 1/2" drains. (This is at the mandated 1/4" per foot minimum slope.)
    At 1/4" per foot, 6' long, the vent may not work if the pipe were to be full. IPC may allow it, but it's not particularly reliable.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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