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Thread: P-trap question

  1. #1
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    P-trap question

    Thanks to LazyPup's expertise and advice, I'm about finished with the DWV piping part of my project. Except for one lingering question about the tub/shower drain: Given that the tub will be above the drain line, can the p-trap for the shower drain be below the drain line?

    As a reminder, this installation has a 3" main drain line (to the toilet & main vent) and 1.5" drain lines to the lav, tub/shower & kitchen sink.

    Logic tells me "yes" because water runs down hill and, while an inch or two of the line above the p-trap (leading to the tub waste/overflow kit) will have water in it, it can't have water any higher than the drain line.

    Make sense? Or am I off in my thinking?

    Thanks in advance .. once again.

    John

  2. #2
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    Good Morning John,,

    Before we begin with this trap, Please accept my deepest appology as it appears I made a grevious error on the layout. When I wrote the text description for the line to the right hand side of the main drain I was mistakenly thinking the sink on the far end was a bathroom lavatory sink.
    A tub/shower is rated at 2DFU and a lavatory(1DFU) would have made the combined load 3DFU which is the maximum allowable load on a 1-1/2" line, however while reading this post I realized that the sink is a kitchen sink, not a lavatory sink, therefore the sink is also 2DFU so the combined load downstream of the Wye at the tub location is 4DFU's and will require a 2" output on the Wye and a 2" line from the wye to the main drain.

    I have edited both the text and isometric in the previous post to show the correction.


    Now for your trap...

    See Attached Illustration:

    When computing both pitch and length the critical point of a trap is the "Trap Weir". The trap weir is the point where the inverted U section of the trap turns horizontal and where the water actually spills out of the trap and into the drain line. The term 'Weir" is derived from the proper name of a spillway for a dam.

    The actual depth of standing water in a trap is predetermined by the manufacturers design for the trap, however, for the sake of illustration I have pointed out that the code requires the standing water depth of a trap to be a minimum of 2" and not more than 4" vertical when measured from the top of the trap Dip to the trap Weir.

    By necessity a fixture arm is wet vented from the vented drain or stack to the trap. In order to insure proper venting the code establishes a minimum length from the vent to the trap weir of 2 x the pipe diameter.

    Under the International Residential Code the maximum length of a fixture arm in feet can be computed by dividing the pipe diameter by the required pitch. A 3" drain line may be pitched at 1/8" per foot with the expressed written consent of your local inspector, but as a rule all lines 3" or less must be pitched at 1/4" per foot.

    As you can see from the illustration the bottom of the pipe at the trap weir may not be higher than the top of the pipe at the vent opening.

    Your Tub/shower drain is an 1-1/2" line which requires a 1/4" per foot pitch so the maximum length would be 1.5" / .25" = 6 feet.

    You can also see that the actual elevation of your trap will be determined by the length of the trap arm.

    Once you have established the elevation of the trap you extend a vertical riser from the trap input to the tub Waste & Overflow output however, with the singular exception of a laundry standpipe, the vertical rise from the trap input to the actual drain opening may not exceed 24". (Some local codes limit this to 12").
    Attached Images Attached Images  

  3. #3
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    Richard ..

    First, thanks so much for the illustration. Answers my question perfectly.

    To respond to various items in your message:
    • Re using 2" instead of 1.5" for the kitchen drain: Thanks for the update. Don't believe it'll be a problem.

    • Re 6' max length from the 1.5" vent to the tub trap @ 1/4" pitch: I'm good to go, I believe. My run will be a foot or less, and my little torpedo level tells me I've got at least 1/4" pitch.

    • Standing water in the trap: The trap/elbow(weir) assy I'm using will have 2", so that's up to code.

    • Minimum length from the vent to the trap weir of 2x the pipe dia: That's 3" for a 1.5" pipe. I'm thinking my length will be somewhere around 8".

    • Vertical rise from the trap input to the actual drain opening may not exceed 24" (not counting local code exceptions): I estimate mine to be around 20-21".


    So, I think I've the right things checked off and, once again, thank you so much for your guidance.

    Regards,

    John

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