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Thread: Shower Drain P-trap Depth?

  1. #1
    Mech Engineer Fightintxag's Avatar
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    Default Shower Drain P-trap Depth?

    This may belong in the shower/bath forum. Feel free to move it if necessary.

    I'm remodeling my master bath. As part of this bath redo, the shower will be expanded. The old shower floor is recessed in the slab. So in order to change the shower footprint, I'll have to bring the recess up to level. That means the shower drain's riser will be extended up another 6" or so.

    I chipped up the concrete to extend the riser and replace the shower drain (going to use the kerdi drain and membrane) and saw that I have a cast iron riser. After seeing a lot of suggestions that it'd be wise to replace the cast iron p-trap with pvc while I have the concrete busted up, I decided to do that. I finally uncovered the p-trap about 3' below the drain. Here's a couple images:


    You can see 2 horizontal and 1 vertical connection of some sort (the pipe flares out and there's a rubber ring that normal 2" diameter pipe come out of). I'm planning to cut the pipe between the 2 horizontal fittings and transition to pvc using a no-hub.

    After extending the riser, I'm looking at the p-trap being 3'6" or so below the shower drain.

    Question 1Is there any problem with my plan to cut the pipe and reconnect pvc using the no-hub in the area between those 2 horizontal couplings?

    Question 2Is there a problem with having the shower p-trap 3'6" or so below the drain? Seems awfully deep to me, but I never had any issues with the old setup 3' below the drain.

    Question 3If the answer to Question 1 is "yes, there is a problem", what should I do? Should I jog the horizontal pipe up with elbows so that the p-trap is shallower than the original is?

    Thanks,
    Scott
    Last edited by Fightintxag; 03-26-2006 at 08:50 AM.

  2. #2
    Builder/Remodeler Kelly's Avatar
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    Default

    C'mon, one of you sure-nuff plumbers, help this fella out, wouldja?

    He's a good and valued member of the John Bridge Tile Your World forums and is really needin' to get to work on his tile if he can get this nasty hole covered up.

    Or y'all can come look at his thread here and post your comments.

    Thanks, you know we'd do the same for your customers, eh?

    CX

  3. #3
    Forum Admin, Expert Plumber Terry's Avatar
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    Default

    I would extend the drain straight out to the new p-trap using the no-hub couping.

    If you use fittings to raise the line, you create an S-trap, which does not work right.

    3'6" if really too high for the riser, but it's better than creating the S-trap.

    If there is a way to add a vent to the line before the p-trap and have a shorter vertical, that would be best.

    That would more than likely mean breaking out more concrete to keep the vent in the wall when you raise the entire line.
    The pipe coming from the vent can raise on a 2% grade, but not with fittings.

    Oh, and lose the water fittings and use waste fittings.

  4. #4
    Mech Engineer Fightintxag's Avatar
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    Thanks guys. Probably saved me from creating a problematic shower drain.

  5. #5
    Mech Engineer Fightintxag's Avatar
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    Unhappy Stinky Shower Drain

    Ok, so I got my bathroom all finished up. But if the shower goes unused for a few days, the house begins to smell funky and it seems to be worse in the shower stall.

    I'm pretty confident that my new P-trap isn't leaking.

    The old P-trap was ~3'-6" deep, so I set the new one that same depth. Now my shower heads come out of the ceiling (instead of the wall) so the water is falling a greater distance (~8' instead of 6') before hitting the drain with greater velocity. Then you have the water shooting down the riser a good 3'-6" more. This is the same as before, but the inside of the new PVC is much smoother than the old CI (especially with the 30+years of buildup). Finally the PVC P-trap has a much more gentle bends than the old CI one did. If you look at the pictures above you can see how tight the CI P-trap bends were. There was actually CI webbing connecting the two vertical parts of the U in the P-trap.

    I think all of this is conspiring to make the water shoot right on through the P-trap, and it's not staying full. What can I do? Is there some sort of baffle I can put in the riser that will slow the water without causing back-up issues?

  6. #6

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    Quote Originally Posted by Fightintxag
    I think all of this is conspiring to make the water shoot right on through the P-trap, and it's not staying full. What can I do? Is there some sort of baffle I can put in the riser that will slow the water without causing back-up issues?
    This is what I'm thinking is happening. Somehow the water is not staying in the trap to keep the air sealed out... which is the basic flaw of S-traps. I've never seen one in normal residential plumbing but in commercial plumbing I've see lots of trap primers. This is a funky device which attaches to the cold water line at (usually at it's highest point and mounted vertically). A line runs from the primer to the trap. Every time there is a quick change in water pressure it gives a little squirt in the trap to keep it full of water. Something like this could be your solution unless you want to pour a cup of water in your shower drain every day or two to replace any evaporated water. I'm suspecting that your drain water has too high a velocity and is over running the trap leaving it unsealed. You might try some insert or a finer screen in your drain to slow the flow but as you said it may cause your drain to back up and to clog.

  7. #7
    Mech Engineer Fightintxag's Avatar
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    Unhappy

    Somehow the water is not staying in the trap to keep the air sealed out... which is the basic flaw of S-traps.
    I thought I kept the P-trap deep per Terry's suggestion to avoid an S-trap?
    I've never seen one in normal residential plumbing but in commercial plumbing I've see lots of trap primers. This is a funky device which attaches to the cold water line at (usually at it's highest point and mounted vertically). A line runs from the primer to the trap. Every time there is a quick change in water pressure it gives a little squirt in the trap to keep it full of water. Something like this could be your solution unless you want to pour a cup of water in your shower drain every day or two to replace any evaporated water.
    Oh my. With my slab foundation and newly tiled shower, I'm not sure this trap primer is something I want to undertake.

    Furthermore my job is forcing me to relocate and as of Dec 11, my home will become a rental property. Not sure I can count on the tenant to maintain the water level in that trap by carefully pouring cups of water down the drain on a regular basis.
    I'm suspecting that your drain water has too high a velocity and is over running the trap leaving it unsealed. You might try some insert or a finer screen in your drain to slow the flow but as you said it may cause your drain to back up and to clog.
    That's what I'm thinking is happening too. Right now there's not really any screen per say. There's just the grate that's provided with the drain (Kerdi drain). It looks like this:

    Could I put a mesh screen material or something underneath the grate to try to slow the water down? Could I somehow get that lower in the drain riser so it slows the water down closer to the P-trap? Of course it'll need to be removable in case of clogs wherever it is. Has anybody done something like this before?

  8. #8
    Rancher
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    Default P trap depth

    I plumbed a P trap 2'-3' below the drain and the only thing I notice is that it's loud when the water is trickling down the drain, if I had to do it over I would plumb it within 6" of the drain.

    Rancher

  9. #9
    Mech Engineer Fightintxag's Avatar
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    I plumbed a P trap 2'-3' below the drain and the only thing I notice is that it's loud when the water is trickling down the drain,
    Yup. Mine is pretty loud too - the trickling sound seems to echo and resonate in the drain riser.

  10. #10

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    Sorry... I didn't mean to confuse your design with an S-trap... I was merely pointing out "why" an s-trap is not a good idea. Not only does the velocity wash out the water which is meant to seal it but also the water exiting the down side can siphon water out of the trap weir (bottom of the trap).

  11. #11
    Mech Engineer Fightintxag's Avatar
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    So are there any other options aside from tearing up the shower floor and slab in order to install a primer or make an S-trap with vent?

    At this point I'm leaning towards putting some mesh screen under the grate to see if that'll slow the water down some. Are there any other options for reducing the water velocity? I don't suppose their are off the shelf baffles for this kind of thing?

  12. #12

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    NO S-TRAP! Don't have a clue about any baffles to be purchased. If you're gonna tear up the floor to fix it just raise the P-trap and put some distance between it and a 90 to turn it back down so as to avoid the effect of an s-trap.... I'd only guess at 12" or more, but all the distance you can get so air can break at the top and release enough water to run back into the trap to seal it...at a minimum give it a few inches of flat pipe at the top before turning back down.

  13. #13
    Mech Engineer Fightintxag's Avatar
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    Thanks, Randy.

    Anybody else have any creative solutions that don't involve tearing out my newly installed shower?

  14. #14
    Homeowner geniescience's Avatar
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    Default facts first

    scott

    could be good to tell us as much as you know. I will be glad to give you some good ideas on how to diagnose your problem, isolate the diagnosis from other options, and then figure out a way to fix it or "work around" it. Later. Facts first please.

    the main hypothesis is that something is sucking your p-trap water out, at least enough to let bad air move back up the drain ... (right? y/n)

    Where is the closest vent. downstream? how far?
    what else is on this branch? what else connects to it before the stack?
    did anything else change in the plumbing in the house in the last year?
    Etc. What else is there to know that is not covered above?

    Then, the 2nd thing to do is to verify that it really is the p trap seal that is "in play" here. Later.

    david

  15. #15

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    Genie... you are correct... I had assumed that the vent was adequate and in play working as it should.

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