Can a horizontal wye connect to a drain upstream (backwards)?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Michael Bluejay, Sep 10, 2021.

  1. Michael Bluejay

    Michael Bluejay New Member

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    Austin, TX
    I'm installing a shower but the 18" between its drain and the wall is not enough space to run its trap arm, wye for a vent, and wye to tie into the main drain. If it's acceptable, I'd like to connect the shower drain to the trap arm coming from the toilet, but the wye from the shower would be connecting upstream to the toilet trap arm. I'm guessing that's not acceptable but if it is it would simplify my install.

    Venting of both fixtures isn't shown in my drawing because it's hard to draw, but I do plan to vent.
     

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  2. wwhitney

    wwhitney Well-Known Member

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    Definitely not.

    How about a floor plan showing the fixture locations (including lavatories), any obstacles like which way the joists run if the drains are in between joists, and the fixed targets (existing drain to connect to) and resources (existing vents, drains you don't want to move)?

    Cheers, Wayne
     
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  4. Michael Bluejay

    Michael Bluejay New Member

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    I'm sorry, I should have been more specific.

    It's a concrete slab. I'm installing both the shower and the toilet. The only existing is the vertical part of the drain and vent.
     
  5. wwhitney

    wwhitney Well-Known Member

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    A dimensioned floor plan as requested will allow us to advise you on how to get everything to fit. 18" should be enough. IPC or UPC?

    Cheers, Wayne
     
  6. Jeff H Young

    Jeff H Young In the Trades

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    No you can't do that.
     
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  7. Reach4

    Reach4 Well-Known Member

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    What is below the floor may differ from what you suspect. Make that floor plan sketch.
     
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  8. Michael Bluejay

    Michael Bluejay New Member

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    Thank you for the replies.

    My area uses UPC.

    Here's the dimensioned floor plan. The all dimensions are measuring the inside, from finished wall (drywall) to finished wall. The old toilet is roughly where the new shower is going. I removed the toilet and dug down through the slab and can see the old horizontal drain pipe, which is how I know how far down it goes.

    The left wall is exterior, and I'm happy to vent one or both fixtures off through that wall rather than through the existing vent, if that makes things any easier.
     

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  9. wwhitney

    wwhitney Well-Known Member

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    Under the UPC the WC needs to be vented within 6' of pipe run from the closet flange. So you can't easily vent the WC from the existing 2" vent on the right.

    Where's the lav and how is it vented? Can you easily vent it with a 2" vent and run its drain by the WC to wet vent the WC via the lav?

    Cheers, Wayne
     
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  10. Michael Bluejay

    Michael Bluejay New Member

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    Thank you again.

    The center of the current sink is 31" from the left wall, with the drain running through the top wall, where it meets up with the existing vent shown in the diagram.

    I don't really understand your second question, but since the sink drain is in the wall, I think the answer would be no.

    Making a new vent on the left or top walls is an option.
     
  11. wwhitney

    wwhitney Well-Known Member

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    So you've got basically two options to vent the WC.

    Option one is to install a dry vent takeoff near the WC with a new dry vent in that left hand wall. The challenge with that is that the dry vent is supposed to rise vertically until 6" above the flood rim of the WC. "Vertically" here means up to 45 degrees off plumb. Since you're meeting up with a drain whose center line is 15" below top of slab 8' away, the WC fixture drain will need to be no more than 13" down when it turns horizontal.

    You could take a vent off as the WC drain turns from vertical to horizontal by using the following sequence: closet flange, vertical pipe, the branch inlet of a wye, then a street 45 for the drain to turn horizontal. The straight inlet on the wye is your vent (with a 3x2 reducer or bushing as you only need a 2" vent) and is at 45 degrees off plumb. But I think it doesn't end up low enough, given that the closet flange is 12" off the wall, I think the vent would emerge from the slab before it makes it into the wall.

    An alternative way to take a dry vent off the WC drain would be to route the horizontal portion in plan towards one of the two side walls, so then a dry vent could rise up out of the slab under one of those walls. There's also an allowance in the UPC for the dry vent to turn horizontal below the slab when structural conditions preclude its rising vertically. But with the allowance for wet venting now in the UPC, I'm never clear when that horizontal vent allowance applies.

    Option two is to wet vent the WC from the lav, which is the typical solution. The WC drain goes to the right as normal. The lav drain is 2" (with a 2" vent) and you reroute the lav drain to go down into the slab and then down the page, joining the WC drain with a horizontal wye or combo. That joint is the vent for the WC, so it has to be within 6' of pipe length from the closet flange.

    Either way, the shower gets vented separately; if the existing stack on the right is a vent stack only with no drainage from a story above (lav drainage is OK under option 1), then the shower trap and trap arm can be at a higher elevation than the WC drain, joining the stack with a san-tee, which vents the trap arm.

    Cheers, Wayne
     
  12. Michael Bluejay

    Michael Bluejay New Member

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    Thank you.

    I'd like to have an idea of how I'm going to do the drains before I think about venting. Once I know how the drains are going to be plumbed, then I'll think about where the vents can connect to them. As in my original post, I was thinking of installing the wye from the shower backwards/upstream into the main drain if that were acceptable, though it seems like it's not. And if it's not, I still need a plan to get the shower and toilet tied into the main drain. Any ideas about that?
     
  13. wwhitney

    wwhitney Well-Known Member

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    But the need to have a vent connection within a certain distance from the trap or closet flange and the need for a dry vent to rise vertically until above the flood rim level of the fixture served are constraints on the system. So those constraints need to inform how you layout the drains.

    Below is a quick sketch of the wet vent option. I'm assuming that where you labeled "horizontal drain 15" below top of slab" that horizontal lines run underneath the circle you labeled "existing vent in wall" and that they are connected with an upright 3x3x2 combo.

    Then in the sketch below the new lav drain and the new WC drain are buried at a depth to meet up with the horizontal drain while falling at 1/4" per foot. So at the closet flange the 3" WC drain is about 2" higher than at the tie in about 8' away. The pipe distance from the closet flange to the wye where the lav drain joins (and vents the WC) has to be 6' or less; I think it is as drawn, but if necessary you could point the lav drain perpendicular to the 3" WC drain and join them with a horizontal combo instead of a horizontal wye.

    The lav is dry vented as currently, but it needs to be a 2" vent. That vent presumably goes through the wall horizontally at a height at least 6" above the lav flood rim and joins up with the "existing vent in wall".

    Then the shower trap and trap arm are shallower, so they can connect to a san-tee on the below grade 2" vertical line above the assumed 3x3x2 combo. That vents the shower and the 2" vertical line above that is a dry vent, no drainage coming from above.

    So that's one option.

    Cheers, Wayne

    2601 bath plan.jpg
     
  14. Michael Bluejay

    Michael Bluejay New Member

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    Thank you as usual.

    I can tell you that the sink drain doesn't vent until it connects to the Existing Vent in Wall, so I think that's wrong and the sink needs to have either a separate, higher horizontal arm for venting, or a vertical piece above the 90° turn in the wall.

    I don't know how the pictured existing horizontal drain connects to the existing vertical vent because I haven't dug out that far. I presume there's a tee at the bottom of the vent.

    I attached a picture of my dry-fit shower drain. It's 12" tall, plus I guess 1.5" for the lip of the tee that'll go below it, plus 1.5" to reach the center of that 3" tee, gives me 15", so it would just barely fit. I guess if I need an extra inch or so I could just build up the floor so the shower floor is at a higher elevation. I guess I'll connect everything starting at the existing drain and working backwards+up, and wherever the bottom of my shower drain fitting winds up, that's gonna be my shower floor.

    How's this for a venting plan?

    SINK: Install a new vertical vent in the top (north) wall. I don't know how hard it's gonna be to jackhammer under the wood bottom plate, though.

    SHOWER: Tie into the existing vertical vent on the right-hand wall.

    TOILET: Tie into the horizontal shower vent line in the slab.
     

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  15. wwhitney

    wwhitney Well-Known Member

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    On your picture, I understand up and down the picture to be vertical; then that's not a valid vent for the shower. The trap arm can only fall one pipe diameter (2") before the vent comes off (except for WCs). The vent has to be before the trap arm turns down at that elbow Generally where elbow is should be a san-tee. My proposal was that san-tee should be directly under and connect to the 2" vent line on the right in your diagram. And then there's no wye. That's a standard dry vent.

    On the WC, the shower is too far away to help with venting. The vent must be within 6' of the WC under the UPC. That's why the wet venting approach uses the lav drain to be the shower vent.

    Cheers, Wayne
     
  16. Terry

    Terry Administrator Staff Member

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    Venting for every fixture except the toilet must come off the top of the trap arm, not below it.
    The reason the toilet is different, is that the bowl is supposed to siphon, while everything else uses a trap to hold water to block sewer gases from entering the home.
    Also the toilet has a refill that puts water back into the bowl.
     
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  17. Michael Bluejay

    Michael Bluejay New Member

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    Thank you, I see.

    So, now that I'll be connecting the shower drain horizontally, that gives me a lot more vertical space to work with since I don't need a vertical wye for venting, so I'll be able to easily fit my shower drain. That solves the original problem I posted about.

    Here's my revised drawing for the shower drain. Do I finally have it right?
     

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  18. wwhitney

    wwhitney Well-Known Member

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    No, like below.

    2601-drain3.gif
     
  19. Michael Bluejay

    Michael Bluejay New Member

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    Okay, thank you, I think I've finally got it!

    For the sink, at the top of the trap arm in the wall, I'll install a vent that goes straight up.

    Once the sink drain line goes down inside the wall, I'll drill a hole through the bottom plate and tie the sink drain line into the toilet drain line.

    That will also serve as a vent for the toilet.

    Howzat?
     
  20. wwhitney

    wwhitney Well-Known Member

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    Yes, that is what my edit of your diagram was attempting to show.

    You may wish to post some pictures of your work before covering up or before gluing up, to get some more feedback.

    Cheers, Wayne
     
  21. Michael Bluejay

    Michael Bluejay New Member

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    Thank you again for your help. I'll either post pictures or hire a local plumber to inspect before I button it up.
     
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