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Thread: When does a bathtub trap become a running trap?

  1. #1
    DIY Junior Member smartnewb's Avatar
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    Default When does a bathtub trap become a running trap?

    I'm putting in a laundry closet on may main floor. It shares a wall with a bathtub in another room, which drains horizontally through several floor joists before joining the vent stack at a wall 5' away. (See picture.)

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    As you can see, it's a tight squeeze. I'd like to add drainage for the washer and laundry sink to the same horizontal pipe currently used by the bathtub. As I see it, there are two options.

    Option 1. Have all three fixtures (washer, laundry sink, bathtub) drain to the same trap. This would require moving the existing trap 6" to 1' for better accessibility.

    That brings me to the title of this post: I know running traps are not to code, and traps are supposed to be located directly below a drain. But modern bathtub drains run horizontally for a few inches before they drop into a trap. (See the horizontal section of pipe circled in red below.)

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    Surely, these bathtub drains are legal -- every single house in North America has one! So the question is: how much horizontal run is allowed before a trap becomes a "running" trap?

    Option 2. Add a second trap (for the washer and laundry sink) to the same horizontal drain pipe. According to what I've read, multiple traps are allowed to connect to the same horizontal drain pipe only if you install an extra circuit vent for each additional fixture. In my case, it would be extremely difficult to install an extra circuit vent. Can I get away without one?

    Thanks for reading. I'm a long-time listener and first-time caller. I've learned lots from this site so far. It's a great resource! (Notwithstanding the troubles I had uploading images...)
    Last edited by smartnewb; 01-19-2013 at 07:04 PM. Reason: Finally convinced the site to let me upload images

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    You need a vent for EACH device or you will siphon the other traps when you run the WM. You may also find that code would require you to make the drain line larger if you add other things to it. There's a limit, based on the drain line diameter, how far the trap can be from the vent. And, there are some pretty specific rules about where the vent and the rest of the drains can be installed. BOttom line, I don't think you'd find any inspector that would pass what you're interested in doing with your proposed plan, and for good reason. You need a separate trap on the WM drain, and it must also be vented. That doesn't mean you must run a new vent all the way through the roof, but there are some specific rules on how to join into existing ones, too.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    In the Trades Jerome2877's Avatar
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    The line for the tub is a trap arm and cannot be tied into. A laundry drain requires a 2" line and it must be vented.

  4. #4
    DIY Junior Member smartnewb's Avatar
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    Thank you for such prompt replies, gentlemen. It's appreciated. Further opinions are also welcome.

    I'm not sure how to interpret what's been suggested. Are you saying there's no good way to do what I want to do? I don't see what's wrong with Option 1 (if you ignore concerns about the pipe diameter -- see next paragraph.)

    I'm stuck with a 1.5" pipe -- that's what's already in the joist. The washing machine is currently located in my basement. It's currently drained with only a 1.5" pipe and the vent opens into the room, not to a stack vent. Also, it currently shares a trap with the nearby laundry sink. That's probably not legal, but it was like that when I bought the house six years ago and it's served me well.

    This project will not be inspected, but that doesn't mean I'll ignore the opinions of experts. Cheers.
    Last edited by smartnewb; 01-20-2013 at 06:46 AM. Reason: misunderstood suggestions the first time 'round

  5. #5
    Test, Don't Guess! cacher_chick's Avatar
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    I would be surprised if the plumbing code in Canada was all so much different than ours. Here, a trap for a washer cannot be below the floor. If it is a new install it has to be 2". A sink drain which is not a bathroom lavatory also has to be 2". Every fixture must have it's own trap, and each trap must be vented to prevent siphoning.

    It would not be in the best interest of anyone to suggest doing something which is not acceptable under the plumbing code, which is a minimum standard.

  6. #6
    Moderator & Master Plumber hj's Avatar
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    1. You could not legally, or sensibly, connect them all to the same trap.
    2. If you connect a washer to the 1 1/2" tub drain, your next question to this forum will be how to keep the washing machine water from filling the tub when it drains.
    3. The "horizontal" section of the tub drain has NOTHING to do with making it an "S" trap.
    4. re. Option #2. You do NOT connect multiple fixtures, other than a two compartment kitchen sink, to a single trap.
    5. HOW you connect multiple traps to the same line determines where, and how many, vents are required.
    6. THis is probably NOT a good DIY project.
    Licensed residential and commercial plumber

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    DIY Junior Member smartnewb's Avatar
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    Thanks for clarifying, cacher_chick and hj.

  8. #8
    Moderator & Master Plumber hj's Avatar
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    If it is any comfort, this job might be challenging even for a professional.
    Licensed residential and commercial plumber

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