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Thread: double sink: one trap or two traps?

  1. #1
    DIY Member adrianmariano's Avatar
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    Default double sink: one trap or two traps?

    I'm replacing my kitchen sink. The existing sink is a double sink installed with two traps. Each sink has its own trap (1.5") and the pipes then meet in a wye. But when I was looking at a plumbing book I saw they had illustrated a double sink where the garbage disposal waste line (1.25" pipe) traveled horizontally to the other sink's waste line above the trap and then there was only one trap (1.5").

    Which arrangement is better? Does it matter?

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    Forum Admin, Expert Plumber Terry's Avatar
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    You are only allowed one p-trap on a trap arm.

  3. #3
    DIY Member adrianmariano's Avatar
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    I'm not sure I know what exactly "trap arm" means. Does the trap arm extend all the way back to the vented pipe?

    My existing drainage has a vertical cast iron drain pipe that is (presumably) vented. The drain for the kitchen sink comes off of that cast iron pipe in the basement, runs vertically through the floor, then it turns 90 degrees and goes about a foot horizontally, then it splits into two pipes and each of those two pipes has a trap on it before connecting to the sink or disposal.

    So is my existing plumbing in violation?

  4. #4

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    i just did the same as you i have a disposal with a horizontal to a "T" that goes down into an Strap/Ptrap that goes into the floor/wall just put the trap below the T and you're fine but you dont need 2 traps. You remove the Y or cap one end and connect everything to the other side.
    Last edited by drocket; 01-16-2007 at 02:00 PM.

  5. #5

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    One trap only...that's it.

  6. #6
    Moderator & Master Plumber hj's Avatar
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    Default traps

    Two traps are a solution to a specific problem and are normally only used when the pipe from the wall is too high for the one trap installation. Many inspectors will reject an installation with two traps connected to one opening.

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    DIY Member adrianmariano's Avatar
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    Ok. I'll replace my existing setup with a single trap. I think routing the pipes for a single trap is much simpler than doing what was done before.

    Capping one end of the Y, as one person suggested, is not going to work because the drain location for the new sink is different, so the angle of the pipe needs to change, so I'll need to cut the pipe and replace it entirely. Everything except the trap itself is glued together.

    Is it usual to glue all this stuff. As I've been looking around I've seen some people show drain hookups using slip joints everywhere that can be unscrewed. If I use slip joints is it worth seeking out the chrome nut or do people think the plastic nuts are fine? (I hired a plumber to do the tricky parts of my bathroom remodeling job and he seemed very concerned that I install the right kind of trap for the bathroom sink. He left me one with chrome nuts.)
    Last edited by adrianmariano; 01-17-2007 at 07:32 AM.

  8. #8
    DIY Junior Member jcgun2's Avatar
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    On the note of two traps, can I use a washing machine drain to an a adjacent sink trap? Or is there a way to share the same drain line without the draining water coming up into the sink?

  9. #9
    Moderator & Master Plumber hj's Avatar
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    Chrome nuts are usually only used with a brass drain setup. They are too "short" to engage the plastic threads adequately. Glued drains are usually used by plumbers who want to make a replacement as difficult as possible, thus hopefully doing the work if the homeowner is stumped. Whether the washer will back up or not depends on the drain line in the wall.
    Licensed residential and commercial plumber

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