Please Critique Basement Bathroom Drain Rough Plumbing Plan

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by rcalliott, Feb 18, 2021.

  1. rcalliott

    rcalliott New Member

    Joined:
    Feb 18, 2021
    Location:
    St. Louis, MO
    Originally posted this in the Code Questions Forum...not sure if that's the right place, so also posting here...

    Looking for feedback on my planned drain rough ins before I start busting concrete to build a basement bathroom with a sink, toilet, tub/shower combo.


    IMG_4901.jpg

    Blue tape represents existing 4" PVC drain pipe under slab.

    PVC Fittings from left to right:


    • AAV (in the box) atop 2 inch tee serving the sink drain (will be recessed behind the drywall in the vanity)

    • Next is a 2" long sweep 90 degree turn from vertical to horizontal.

    • Then a 2" 45 degree fitting to get the pipe slotted into the 3x3x2x2 Double Wye.

    • Double Wye serves toilet above it via 3" pipe, and connects to 4x4x3 45 degree wye attached to existing 4" pipe 12" to 15" under the slab.

    • Fernco couplings with PVC stubs to attach wye to existing drain pipe, which flows from left to right in the picture. I've been advised to replace flexible couplings with the shielded variety (planning on these https://www.menards.com/main/plumbi...exible-coupling/1056-44rc/p-1444430427234.htm)

    • From the right of the Double Wye is a 2" pipe with various 45 degree fittings to get the angle correct to the 2" p trap for the tub/shower combo (furthest item on the top-right).


    With a wet vent from the sink, my understanding of the toilet NOT being the last fixture on the pipe requires the Double Wye/Symmetric Fitting.

    Seems like some folks don't care for the AAV. on the other side of the top wall is my workshop with access to the main stack and a vent that goes up through the roof. If the AAV functions poorly in this configuration, I can tap into this 1.5" vent pipe...its just a pain I'd like to avoid.

    Any feedback and general comments/suggestions are appreciated.
     
  2. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    Which plumbing code are you under? The "toilet must be most downstream on the wet vent" requirement is from the UPC, which generally doesn't allow AAVs. While the IPC does allow AAVs and doesn't require the toilet to be most downstream on the wet vent.

    To my knowledge the 4" long unshielded underground couplings are OK, but the 4" shielded underground couplings are also OK, and obviously more secure. What is to be avoided are the 2" long shielded above ground couplings. Conversely, above ground you'd have to use the latter and couldn't use any of the 4" long underground couplings.

    If you do use a horizontal double wye, I suggest you pitch the barrel of the fitting at least 3/8" per foot. That will ensure that the two branch inlets can each be at least 1/4" per foot (but you have to get the two inlets at exactly the same level). If you can use separate wyes, that would be a better option.

    Cheers, Wayne
     
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  4. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    P.S. If you are following the UPC, so you need the WC to join the wet vent last, you could run the lav drain parallel to the back wall behind the WC, hit a LT90 to go towards the 4" line, hit a wye for the shower drain, and then hit another wye for the WC.

    Cheers, Wayne
     
  5. rcalliott

    rcalliott New Member

    Joined:
    Feb 18, 2021
    Location:
    St. Louis, MO
    Much appreciated! I am under UPC as far as the city's ordinance is concerned. Many other munis in the area are IPC. Practically speaking, occupancy inspections are not required for sale of residential dwellings, and from speaking to the County code enforcement department (who does any inspections for City I live in), apparently its about 50/50 in terms of folks who get permits for home renovation vs those who do not.

    I freaked out when we were buying the place in 2017 after finding out the kitchen renovation 10 years earlier had no permits pulled. The RE agent and private-company home inspector we hired laughed and told me this was quite common and not to worry; that its an affluent area where the vast majority of home owners have professionals do the work, so quality/workmanship is generally fine.

    My thinking on the AAV was that if it ever became a performance problem or if inspections ever became enforced and I had to remove it before selling the house, I could connect to the 1.5" PVC vent behind that wall. Difficult to verbalize - the PVC vent behind the wall is close by, but it will be a maze of piping to get it around obstructions and through the stud wall cavity behind the lav. Doable, just terribly annoying, lol.
     
  6. rcalliott

    rcalliott New Member

    Joined:
    Feb 18, 2021
    Location:
    St. Louis, MO
    This was my backup plan if I didn't have enough space for the Double Wye between the WC and the 4" pipe under the slab. As you can see from the picture, with the Double Wye at an angle far from perpendicular, I can just fit it all while keeping the WC 12.5" from the studs.

    I may run out and get fittings for this orientation, lay it out, and post for review.

    Thanks again for the feedback!
     
  7. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    Flat double wye is probably discouraged, so the backup plan may be the better layout.

    If fitting spacing is tight, don't forget the option of street fittings.

    Cheers, Wayne
     
  8. rcalliott

    rcalliott New Member

    Joined:
    Feb 18, 2021
    Location:
    St. Louis, MO
    Backup plan it is. Please see the picture below and let me know if I've got this looking right.

    IMG_4904.jpg

    Lav fitting (acting as wet vent for WC and tub/shower) is out of frame on the left, but it will drain parallel against back wall and connect to the 2" LT90, then pick up tub/shower p-trap via 2" wye, then picking up WC via 3x3x2 wye. From there, 3" moves along and connects to existing drain under slab via 4x4x3 wye attached with shielded couplings.

    Really appreciate the help!
     
    Last edited: Feb 18, 2021
  9. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    That works, I have a few comments that may be of interest:

    - You don't show how the lav drain gets into the slab, but you have the option to have your vertical to horizontal transition (LT90) point at a 22.5 or 45 degree angle from the wall, rather than perpendicular. That way you only need a 22.5 or 45 degree elbow to get parallel to the wall, rather than a LT90.

    - How much shallower is your new work going to be compared to the existing 4" building drain? I.e. how much if any are you going to roll up the 4x4x3 wye? Seems like it could be flat, rolled up 45, or rolled up vertical, depending on the height difference. Then the angle you need to transition from the 3x3x2 wye to the 4x4x3 wye will depend on how much it is rolled up. If it's flat, you could eliminate the two 45s you show by just lining up the two wyes. I guess one advantage of using the two 45s is that you could roll the 4x4x3 wye any amount you need, rather than stick to a multiple of 45 degrees.

    - Seems like you can eliminate the first 45 after the closet bend by rotating the closet bend 45 degrees and hitting the 3x3x2 wye directly.

    Cheers, Wayne
     
  10. rcalliott

    rcalliott New Member

    Joined:
    Feb 18, 2021
    Location:
    St. Louis, MO
    For the lav drain, I am going to punch a hole in the pressure treated floor plate of the stud wall and another hole in the concrete beneath such that the 2" vertical drain pipe sits fully inside the wall. I think I understand what you're saying for the vertical to horizontal transition - I have a 90 and 45 set aside and will pickup a 22.5 as well. Not sure exactly how I'll connect, but I was planning to figure out the shortest/easiest path once everything else is in place.

    I believe the existing line is roughly 12" to 15" under the top of the concrete. I was intending to dig down and run everything 'flat' (with 1/4 slopes). Your notion of using the two 45s to allow for any amount of roll was my thought...the 45s give me some flexibility as I'm hooking everything together so I don't have to plan out all the angles in advance precisely. But I'll play around with this on the floor and see if I can reduce some of the 45s by moving wyes around a bit


    Same answer as above - the 45 gives me a bit of flexibility on the placement of everything, but I may be able to get rid of it with some adjusting.

    Thanks again.
     
  11. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    Agreed on the strategy of having the elbows on hand if you need them, but trying to minimize the number you actually use.

    On the PT bottom plate, it basically has no structural role other than under the studs. So you could cut out the entire bottom plate in the stud bay in which you are working, if that makes your life easier. (Although if the studs are toe nailed, it would be safer to leave a 1" stub past the stud on each side of the stud bay.) I might reconsider if there's a joint nearby so that would leave a short 16" or 32" length of plate (but I might not). Depending on how the plate is fastened to the slab and how frequently, it might be appropriate to add 1 or 2 more fasteners if you completely sever the plate.

    Cheers, Wayne
     
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