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Thread: "weir must not be above trap opening"

  1. #1
    DIY Junior Member billyh's Avatar
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    Default "weir must not be above trap opening"

    I've read quite a few posts and the rule that I am having the most difficulty adhering to is the:

    "weir must not be above the trap opening"

    of course, I will adhere to it but it is a pain. I am in Ontario so, assume international plumbing code (until I find out otherwise)

    --- Now, is the reason for this because you don't want vent air to be trapped inside the fixture arm?

    --- Does the WC have to adhere to this rule as well?
    It seems rather impossible as the trap is inside the toilet.

    --- If I have a main 4" vertical stack in my bathroom wall tying to the main waste out, can I stack 4" Sani-Tees (4"->2" on top for sink, 4"->2" for tub, 4"->3" for WC on bottom) ?

    --- Will the vertical stack suffice as a vent for all three of these? (it is vented through the roof)

    --- If this looks good thus far, can the main waste be tied to the WC using a 4"->3" Wye instead of a Sani-Tee + 1/8 + horizontal run to under WC + vertical to flange? I saw this posted somewhere and was wondering if there is only one official way to do this.

    Any questions, let me know or point me to relevant posts.
    Thanks for any help!

  2. #2
    Plumber Winslow's Avatar
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    I believe the rule you are reffering to actually reads the weir of the trap can not be above the vent pipe opening. The reason is to prevent siphonage of the trap. This doesn't apply to water closets as the trap is above the vent and actually relies to a certain degree on the siphoning action to suck water from the trap. The ballcock then ensures a trap seal. The answer to your other questions depends largely on the code that has jurisdiction in your area. According to the UPC all fixtures are for the most part individually.

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    Moderator & Master Plumber hj's Avatar
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    quote; "weir must not be above the trap opening"

    That statement makes absolutely no sense, because the "weir" establishes the height of the trap's exit and water level, which HAVE to be above the weir or the fixture will stay full of water.

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    DIY Junior Member billyh's Avatar
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    Thanks for the help.

    If you search for "Fixture arm defined" in google, the first hit will have a diagram.

    It states that figure B is correct while figure C is not because the vent opening cannot access the gasses that are isolated at the weir (or exit water level in p-trap).

    Is this not correct?

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    Last edited by Terry; 02-06-2011 at 01:17 PM.

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    DIY Senior Member dlarrivee's Avatar
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    In Canada you're under the Canadian NATIONAL PLUMBING CODE... not the IPC....

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    DIY Junior Member billyh's Avatar
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    Thanks! Found that out a bit after I posted.

    So, as for this WC connection....

    Can someone point me to a good post that explains what bends you are, are not allowed to have before it hits a vent?

    Are you allowed to angle down 45 degrees to the main waste? Or do you have to drop vertically from the flange then 1/4 turn until you hit a sanitary tee that vents up and wastes down?

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    Plumber jimbo's Avatar
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    Quote Originally Posted by billyh View Post
    Thanks for the help.

    If you search for "Fixture arm defined" in google, the first hit will have a diagram.

    It states that figure B is correct while figure C is not because the vent opening cannot access the gasses that are isolated at the weir (or exit water level in p-trap).

    Is this not correct?


    Yes...the point is that in B the vent opening breaks the siphon, whereas in C it cannot break the siphon. The reason that toilets are an exception to this is that the toilet by normal operation refills the trap after each use. A faucet or tub etc. does not do that.

    I dont understand their arrow in A pointing to the "trap weir" of the closet bend.

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    Forum Admin, Expert Plumber Terry's Avatar
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    There are a lot of ways to plumb for a toilet or a water closet. The drawing above on the closet flange is just "one" of those ways.
    The trap on a toilet is designed to siphon, and then it is refilled after the flush.
    Other traps are prevented from siphoning by the vent.

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    Moderator & Master Plumber hj's Avatar
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    The closet bend does NOT have a 'weir'. Even drawing "B" would be incorrect if the trap were moved further away from the vent connection, thus raising the "weir". By definition the connection to a vertical line MUST either be with a sanitary tee, or in the case of two sinks, a "back to back fixture fitting", which while it LOOKS like a double Y-1/8 bend is configured to prevent the problem with the trap weir.

  10. #10
    DIY Junior Member billyh's Avatar
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    Thanks for the clarification on these. Hopefully it will help others as well!

    Now, for something a little simpler... I have a joist that is partially in the way of my flange. Assuming that I would rather not use an offset flange (due to the restriction), is it commonly allowed to attach a 1/8 turn right onto the flange to angle it away from the joist then another 1/8 to get it vertical again?

    From there, can I 45 degree slope right to a Y on the main waste line or do I have to drop and run parallel to a Sanitary Tee?

    Thanks for the responses thus far!

  11. #11
    Moderator & Master Plumber hj's Avatar
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    just go from the 45 to the next one UNDER the joists going horizontal to the sanitary tee. Going to the main line would probably negate any vent for the toilet.

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