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Thread: Odd P trap

  1. #1
    DIY Junior Member Bob C's Avatar
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    Default Odd P trap

    Hi all. I've learned a lot just lurking on this forum but I ran into something I haven't seen before.

    I'm tearing out our bath tub and installing a shower in it's place. The drain system is copper but the trap appears to be cast brass. My question concerns the union in the center of the trap. There was no washer or other type of seal at the joint when I took it apart. Is this merely a friction seal? I plan on continuing to use this trap. Can I just clean up the joint with a wire brush and reassemble? Is there some sort of sealant I could put there?

    Thanks for the help in advance!!

  2. #2
    In the Trades Gary Swart's Avatar
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    When you say "union", do you mean it's just a slip together joint? A union is a plumbing fixture that is a screw together connector that can be unscrewed to disassemble the joint. If it is a slip together joint, then is should be soldered. Brass and copper are the same basic metals and are joined with a solder joint. Problem is, the cast brass will be at least 3" and possibly 4" and is quite heavy. This will not be a solder job for a novice. You really will need a plumber to make this connection. There is no sealant that will suffice, including the liquid solder that is hyped to replace traditional heat. And no, silicone won't work either. You have gotten by in the past with this slip joint because there is no pressure in a drain line.

  3. #3
    Forum Admin, Expert Plumber Terry's Avatar
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    A union on a p-trap is just that. When they are tight, they should hold. Though if you're concerned about it, you can always replace it with new.

  4. #4
    Home Builder ckl111's Avatar
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    I thought p-traps with a union joint weren't allowed under the floor since it would be sealed in or is that just for ABS p-traps?

  5. #5
    DIY Junior Member Bob C's Avatar
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    Thanks for the quick replies guys. I've lived in the house 23 years so I don't know how old it is. It looks somewhat corroded. I guess I'll clean it up and test it before I go on. I'd really like to keep it as my connectiion to the old drain system if I can. Yes, it willl be enclosed when I'm done but can be accessed through the living room ceiling (how I got there).

  6. #6
    Moderator & Master Plumber hj's Avatar
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    Normally a 'ground joint' union, the kind being asked about which does not use a washer or gasket, is considered a "solid" connection., except possibly in MA.
    Licensed residential and commercial plumber

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