Back-to-back WCs with upstream tub and shower. What's wrong?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Nathan Ladd, Jul 27, 2019.

  1. Nathan Ladd

    Nathan Ladd New Member

    Joined:
    Jul 27, 2019
    Location:
    Vancouver, WA
    Hi all,
    I'm hoping someone can help explain to me if something is wrong with my plumbing configuration and why. (And maybe even the relevant code sections). The attached diagram will probably explain this better, but I have a 3" horizontal drain branch off my main 3" stack. Starting downstream and working up, I have a 2" lav drain and vent vertically connected to the 3" branch, then a 3" double wye connection back-to-back WCs, a reducer from 3" to 2", then a 1-1/2" drain for a second floor tub (tub has separate dedicated 1-1/2" dry vent), a 2" dry shower vent and finally a 2" shower drain (with a clean out on the end).

    I'm in Vancouver, WA and must adhere to the 2015 UPC. I failed inspection because the inspector said I could not have the 1-1/2" upstairs tub draining upstream of my WCs and that I needed to move it down stream of the WCs. I though I did the proper research and that my configuration was ok. The inspector didn't say anything about the upstream 2" shower drain though and I'm wondering if he just didn't notice it or if because the 1-1/2" drain is on a different level if that's the issue. He also mention that my back-to-back WC was technically in violation too, but said he would let it pass, any thoughts on why he made that comment? Appreciate any insights, thanks!
     

    Attached Files:

    Last edited: Jul 27, 2019
  2. wwhitney

    wwhitney Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    The issue is the WC venting, as everything else is individually vented. By not individually venting the WCs, you are relying on the horizontal wet venting rules [908.2]. Those rules say that only a single bathroom group can use a horizontal wet vent. That means the wet vented portion (between the WC and the lavatory) should only have one WC on it, and no fixtures from upper floors.

    So you could fix this by individually venting each WC before the double wye. Or you could taking advantage of the leniency the inspector offered you to have two WCs on the wet vent, and just reroute the upstairs bathtub drain.

    Cheers, Wayne
     
  3. Sponsor

    Sponsor Paid Advertisement

     
  4. Nathan Ladd

    Nathan Ladd New Member

    Joined:
    Jul 27, 2019
    Location:
    Vancouver, WA
    Thanks for the response, Wayne. So there isn't a problem with the shower being on upstream of the WCs, but I can't have the 2nd floor tub upstream of the WCs because the WCs are wet-vented and I can only have a single bathroom group on the wet vent (and the inspector is granting leniency by allowing me to have 2 WCs on the same wet vent even though that's technically two bathroom groups).

    Can I bother you for a follow-up question? It would be easiest for me to re-route the upstairs 1-1/2" drain into the 2" lav drain as shown in the attached diagram. Would that be ok or does the 1-1/2" drain have to go downstream of lav?

    Thanks!
     

    Attached Files:

  5. James Henry

    James Henry In the Trades

    Joined:
    Jul 23, 2019
    Occupation:
    Plumber
    Location:
    Billings, Montana.
    No, the " one bathroom group wet vent theory " is based on the assumption that only one fixture will be running at a time in the bathroom group.
    if you have a shower draining into a lav that is draining or a toilet that is flushing, then your vent wont work properly.
     
  6. Nathan Ladd

    Nathan Ladd New Member

    Joined:
    Jul 27, 2019
    Location:
    Vancouver, WA
    Ok, I think I understand now. So there's one more, probably important detail that I've forgotten to mention. This is a 1/2 bath, and 3/4 bath configuration and the shower and a WC are in the 3/4 bath, and the lav and other WC are in the half bath. Does that mean that if the shower is draining and the 1/2 bath WC is flushed, then it's also not properly vented?

    The inspector didn't say anything about the shower draining, but I wonder if he didn't even notice it? He focused in pretty quickly on the illegal upstairs tub drain and the shower drain is pretty hard to see from beneath. So is just re-routing the upstairs tub drain enough? Or I've attached another diagram which should definitely work, but it's going to be a lot harder to re-route both the shower and tub drain vs just the tub drain (second diagram)
     

    Attached Files:

  7. James Henry

    James Henry In the Trades

    Joined:
    Jul 23, 2019
    Occupation:
    Plumber
    Location:
    Billings, Montana.
    Your drawings don't match. I see 3 toilets on one drawing and 2 toilets on the other and I only see 1 sink to wash your hands.
    try drawing a floor plan.
     
  8. Nathan Ladd

    Nathan Ladd New Member

    Joined:
    Jul 27, 2019
    Location:
    Vancouver, WA
    Hi James, you're right, my shwr and tub reroute diagram doesn't show the upstairs toilet, but that drains and vents through the main stack and isn't attached to the branch drain I'm having issues with. I've attached a floor diagram, but again, the 3/4 bath sink and upstairs sink drain and vent on their own individual branches.
     

    Attached Files:

  9. wwhitney

    wwhitney Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    If you can add a 2" toilet vent, can you connect it to each toilet fixture arm before the double wye? That would eliminate any wet venting, and you don't have to move the tub drain.

    Given James's explanation of why wet venting is allowed for a bathroom group, I'd be wary of wet venting two toilets on the same wet vent, even if the inspector would let it slide.

    Cheers, Wayne
     
  10. James Henry

    James Henry In the Trades

    Joined:
    Jul 23, 2019
    Occupation:
    Plumber
    Location:
    Billings, Montana.
    I suspect you have an idea of how your going to do this but you just don't know how to draw it out.
     

    Attached Files:

  11. Nathan Ladd

    Nathan Ladd New Member

    Joined:
    Jul 27, 2019
    Location:
    Vancouver, WA
    I can't add individual vents to the toilet arms before the wye, there are isn't enough space. I can have a dedicated dry vent upstream of the toilet if i reroute the shower and tub drain as shown in the diagram attached to my previous reply. I guess I'll probably have to go that route
     
  12. Nathan Ladd

    Nathan Ladd New Member

    Joined:
    Jul 27, 2019
    Location:
    Vancouver, WA
    I appreciate all the responses. I guess I'll just try to give the inspector what he wants and reroute the upstairs tub drain down stream of the lav. Not sure how I'll do that, but I guess I'll have to figure something out
     
  13. James Henry

    James Henry In the Trades

    Joined:
    Jul 23, 2019
    Occupation:
    Plumber
    Location:
    Billings, Montana.
    It's impossible for me to give 100% correct advice without being there but here's something for you to consider.
     

    Attached Files:

  14. Nathan Ladd

    Nathan Ladd New Member

    Joined:
    Jul 27, 2019
    Location:
    Vancouver, WA
    Thanks James, that's actually a good idea and might be doable. I guess I'm still not 100% clear on why the inspector only told me to reroute the upstairs tub and not the shower that's also upstream of the WCs. Another option I just came up with that I think would be doable too (and maybe the easiest) is to add a new drain for the lav and then the existing wet vent would become a dry vent for the WCs. Diagram of that configuration attached. Do you think that would be valid? This is more in line with what Wayne was suggesting, but only a single dry vent for both WCs vs each WC getting a dedicated vent
     

    Attached Files:

  15. James Henry

    James Henry In the Trades

    Joined:
    Jul 23, 2019
    Occupation:
    Plumber
    Location:
    Billings, Montana.
    He told you to reroute the tub because it goes against the theory of only one fixture at a time being used with a wet vent.
    I noticed that the stack is 3". is the drain from the first floor down 4" ? it needs to be for you to be able to have 3 toilets.
     

    Attached Files:

  16. wwhitney

    wwhitney Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    2015 UPC Table 703.2 Footnote 4 (which applies to 3" drain lines) says "Only four water closets or six-unit traps allowed on a vertical stack or pipe; and not to exceed three water closets or six-unit traps on a horizontal branch or drain."

    Cheers, Wayne
     
  17. James Henry

    James Henry In the Trades

    Joined:
    Jul 23, 2019
    Occupation:
    Plumber
    Location:
    Billings, Montana.
    Your right. I should have thought longer before I spoke.
     
Similar Threads: Back-to-back upstream
Forum Title Date
Plumbing Forum, Professional & DIY Advice Correcting/improving current back-to-back bathroom drainage/venting Feb 17, 2020
Plumbing Forum, Professional & DIY Advice Back-to-back tile shower faucet repair Dec 2, 2019
Plumbing Forum, Professional & DIY Advice Re-plumbing back-to-back toilets & adding shower drain/vent Jan 2, 2019
Plumbing Forum, Professional & DIY Advice Moving back-to-back vanities 12" further from vent/drain Aug 18, 2018
Plumbing Forum, Professional & DIY Advice Back-to-back vanity DWV Oct 11, 2016

Share This Page