P-Trap Weir and Vent Line Height

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by OldSalt, Feb 3, 2017.

  1. OldSalt

    OldSalt New Member

    Joined:
    Feb 3, 2017
    Occupation:
    IT Consultant
    Location:
    Northern Idaho
    I'm building a house in Northern Idaho, and without getting into all the details, it's been a rough build. Budget is largely exhausted and I had problems with my original plumber, hence I'm completing trim and finishes on my own.

    A professional plumber completed rough in, and passed inspection. However, when the vanities came in, the drain lines were positioned too low on the wall. I would have had to cut the bottom of the vanities in half to put the P-trap down low enough to meet the drain line. So I've made modifications, and am in the process of installing the finish plumbing and drains for the Master. I believe that I've messed up, and want to verify this before pulling out vanities, messing with granite, and ripping out walls (again!!).

    20170203_110010_resized.jpg

    I first realized the problem when the granite company arrived to set the counter tops. The vanities are "stand alone" and were not set when they arrived. The granite guy is an outstanding contractor, and attempted to help me by setting the first two vanities. He cut the base of the vanity to meet the drain height, and we agreed that he should stop there and let me finish plumbing and vanity install. It just wasn't working out.

    This is what I worked out. (Get ready for some "plumbing porn".)

    I realized when researching the Master vanity drain install that the Weir is not supposed to be higher than the vent. The vent is at the level of the top of the Sani-tee, which is lower than the P-trap weir. Thus this will be a "fail" at inspection (i.e. water could siphon out of the P-trap, because there's no vent break). I'm pretty sure that I need to tear this whole thing out, raise the sani-tee up six inches or so, and re-install with a straight pipe out of the p-trap angles slightly down to the sani-tee. Can someone confirm this, or offer a better option?

    Second, similar issue. As illustrated in this rough (emphasis, ROUGH) drawing, the original installation had the double vanity drains split at the vent/drain intersection, with an elbow at either end (and a straight pipe stub from the elbow out the wall). To raise the drain lines height to meet the required vanity height, I opened the wall and added two elbows at either end of the drain line, one pointed upward, an approximately six inch pipe, and one at the top of the pipe pointing out of the wall. The problem is that the weir is now also six inches above the vent pipe.

    DrainDesign.jpg

    Second, I apparently can't use a measuring tape, because the drains are too high. As illustrated in the picture below, the straight pipe from the P-trap back to the wall is angled up about 1/4", rather than down. Again, while I want my home plumbed correctly, a larger part of this is the final plumbing inspection. This is angle is slight enough that an inspector might not notice unless he puts a level on the pipe. And to be honest, having 1/4" water retention in the pipe just makes the p-trap an extra six inches or so long. The over riding issue is the design of the drain/vent/weir. Even if the inspector pass this, I'd know it's wrong behind the wall. Then again, how big an issue is this from a practical perspective. There are two vanity drains connected to the same 2" drain pipe.

    1) What's the likelihood of a siphon being created that pulls the water out of either vanity drain p-trap?
    2) What's the likelihood of the 1/4" rise from p-trap to the drain causing a future drainage problem. The sink drain is inches above the wall drain, and the water should still drain without problems.
    3) Recommendation? Pull the vanity, open the wall, and raise the vent/drain "Y" intersection up to the level of the drain pipes in the wall? Of connect the drains as pictured and don't worry about it unless the inspector requires the change?

    20170203_105842_resized.jpg


    Thanks for reading, and for the feedback. One thing I've learned during this build is that there's a whole lot more knowledge associated with proper drain construction and I realized. I've learned a lot. (Paid the price, too.)

    Scott
     
    Last edited by a moderator: Mar 4, 2019
  2. Terry

    Terry Administrator Staff Member

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    Plumber
    Location:
    Bothell, Washington
    [​IMG]

    You may need to cut the bottom of the cabinet. The trap arm can't slope up, or it creates an S trap that will siphon the trap dry, letting sewer gas into the home.
    Or you may be able to open the wall, and if the line is vertical, raise the santee in the wall. If it's a horizontal line, you either need to keep chopping wall out, or chop the cabinet.
    A third way would be to add an AAV behind the trap before it enters that wall.

    On a trap arm you are allowed 42" in length, graded at 2%, or 1/4" per foot.
    The reason for that, is so that the vent breaks the siphon on the trap.
    If you 90 up, you will create a siphoning condition that will suck the traps dry.
     
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  4. OldSalt

    OldSalt New Member

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  5. OldSalt

    OldSalt New Member

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    Thanks for the feedback, Terry.

    It's pretty clear that I need to do some work behind the walls.

    The first picture (original post) covered the situation with two different bath vanities. All I need to do is raise the sani-tee up a few inches add a soft coupling above the tee to reconnect the vent, and then patch the walls. However, the AAV option would seem to work, also. I see options with the AAV and p-trap integrated into the sink drain, and I see a $10 Oatley AAV that I would expect could be mounted on a tee between the p-trap and wall. It's a bit pricy, but it would save me some time.

    The second picture was of the master bath. I don't really have a choice there since I have both the "s" trap situation (due to elevated trap arm, caused by the drain pipes at the wall being too high), and the vent problem (weir higher than vent, since vent double sani-tee is 4-6" lower than the drain pipes at the wall). I'll just find the drywall screws, and do it all again.

    Another thing that I've learned from this is that there is at most, about two inches of variance on the height allowed for a wall drain pipe. The pipe is 1.5" ID, and the p-trap can be raised and lowed an inch or two on the sink drain pipe. Those are pretty close tolerances, when dealing with the lower drain pipe, vent sani-tee, the drain pipes exiting the wall, and the p-trap. If not using a conventional vanity, the vanities should be on site during plumbing rough in so that the plumbers can install at the correct height.

    This is a picture of one of the vanities, which explains why prefer not to cut the bottom and hang a p-trap drain below it. The vanities were not cheap, and both aesthetics and functionality would suffer with a hole in the bottom, and p-trap hanging out the bottom.

    vanity.jpg

    I picked up the drain parts to make the changes last night. I think I'll sleep on this another day before I dive in and start tearing things apart. The "up side" is, the first faucet I mounted leaked like a sieve at the brass tee union where the handset valve hoses connected to the spout. I couldn't figure out of the three parts which was leaking, or why, due to space constraints. If I pull the vanities, I will install the faucets working from behind the vanities, and test for leaks before re-installing the vanities/drain p-traps.

    Thanks again, for the advice.

    Scott
     
  6. Terry

    Terry Administrator Staff Member

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    Plumber
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    Bothell, Washington
    [​IMG]

    This wasn't too bad. Not noticeable from the room looking at it.
     
  7. Smooky

    Smooky In the Trades

    Joined:
    Apr 4, 2011
    Location:
    North Carolina
    If I can, I just cut the wall and fix the pipe but on occasions I have had to cut a shelf or two.
    If only the p-trap is going to be below the base or shelf, I like to cut a round hole. I find a pan that I can drop in the hole so it gives it a more finished look and prevents things from falling through the hole. You need a pan with a rim so it want fall into the hole.

    http://www.lindysstainless.com/lindys-stainless-steel-pie-pan.html
    http://www.webstaurantstore.com/american-metalcraft-989-9-x-1-3-4-deep-aluminum-pie-pan/124989.html

    For a deeper p-trap you could use a loaf pan
     
    Last edited: Feb 4, 2017
  8. OldSalt

    OldSalt New Member

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  9. OldSalt

    OldSalt New Member

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    So, I suppose it's been done. I guess plumbing is like art. What you get depends on what you want, and who's paying for it. :)

    Thanks again.

    (And thanks to Smooky for his tip.)
     
  10. OldSalt

    OldSalt New Member

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    Follow-up:

    I decided to open the walls, and started with the master double vanity. I thought that I'd post this follow-up if any are curious about the end result (hey, I know I am :) ).

    This picture is "rough plumbing done wrong" by the homeowner/builder. Originally, as inspected and approved, the rough plumbing had the double sani-tee (i.e. a sani-wye?) routing pipes left and right, both with 90 degree elbows at the end pointing the drains through the wall. I opened the wall, added double elbows to raise either end up about six inches.

    MasterVanityRough.jpg

    I made the same change in another bath vanity, but raised the sani-tee up and left the pipe about 4.5 inches lower as well. That one worked out fine. I'm using that model for revising this installation. I need the bottom of the drain pipe where it exits the wall at maximum 21". I intend to move the "wye" up to where the bottom of the drain pipes going left/right are at 20". That'll allow about 3/4" to the left for appropriate drop (1/4" per foot, including the straight pipe from the sink drain), and about 1/2" to the right, so that both pipes end up at under 21" exiting the wall. At 21" or hopefully, a little less, the p-trap straight pipe will line up perfectly at 1/4" slope to the wall. I'm also going to reorientate the pipe exits to closer match the vanity requirements (i.e. drilling the left side to make about 30" of travel, and not drilling the right side, i.e. moving both drain pipes about six inches to the left. That'll align the p-trap straight pipe directly with the drain pipes.


    Complicating factors are:

    1) The new wye will be very near the electrical wires, but there shouldn't be a conflict.
    2) The old wye is too close to the clean out, so removing it means replacing the clean out too (hence, the yellow markings, my next drywall cut).
    3) I thought about just plugging the 1.5" left/right openings of the wye and leaving it there, adding the new wye above. However, even if I placed the new wye in direct contact with the old, it'd end up too high.
    4) This is an exterior wall, i.e. 2x6" exterior, with a double 2x4" interior wall. I have the equipment to core out the 2x4, and since it's a non-load bearing wall, it shouldn't be a problem to drill out the new 1.5" holes. I'll still reinforce the empty 2x4 holes by sistering the studs with scrap, just to ensure it doesn't break-up on me when I start pounding new holes 6" above the old ones.
    5) The closest plumbing supply is an hour round trip away, and I didn't plan on installing a new clean out. So, off I go again ..... (time sink).

    All this is just FYI, unless of course, I'm screwing something else up.

    For me, the moral of this story on my next build is to have the vanities ordered and on site when the rough plumbing goes in. Same goes for other cabinets, i.e. know the measurements for the kitchen sink and laundry (deep sink) cabinets. I really had no idea that rough plumbing had to be so precise. I thought that there was a general standard that was used for fixtures and such. All of that needs to be spelled out in detail, in the plans I suppose.

    Scott
     
    Last edited by a moderator: Mar 4, 2019
  11. Terry

    Terry Administrator Staff Member

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    Plumber
    Location:
    Bothell, Washington
    [​IMG]

    Bringing the double fixture fitting up higher, redrilling the studs to give you the 1/4" per foot rise should fix that.

    What you had previously was what I had found in a customers home. The lave with the upward 90 like in your picture was siphoning the lav trap. It was a pretty bad smell on that one.
     
  12. OldSalt

    OldSalt New Member

    Joined:
    Feb 3, 2017
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    IT Consultant
    Location:
    Northern Idaho
    Terry,


    Is there a difference between using a double sanitary tee versus a double fixture tee (i.e. the wye as shown in your picture)? I assumed that if code required a sanitary tee for a single sink, it would require a double sanitary tee if splitting a strain line. I like the larger wye for better flow purposes, but if the double sani-tee is code, I'll install that instead. (It's half the price, too)

    Thx.
     
    Last edited: Feb 7, 2017
  13. Terry

    Terry Administrator Staff Member

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    Bothell, Washington
    The double santee is not code.
     
  14. Dave Hesler

    Dave Hesler New Member

    Joined:
    Jan 19, 2019
    Location:
    Huntsville, Al
    These comments may in fact be really late. A double fixture tee could have been used instead of double santee. Double Santee's don't "force" a drain cleaning cable down the line however a double fixture fitting does.

    Double Santee: https://www.google.com/search?q=dou...BAgAEAE&biw=1097&bih=751#imgrc=70vlDuN_L9XH7M:

    Double Fixture Fitting: https://www.google.com/search?q=dou...BAgAEAE&biw=1097&bih=751#imgrc=AvErKF86Sxc89M:

    Having only done plumbing in two states (California and Alabama) I can only speak for code in those states.

    [​IMG]
     
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