The NSPC permits s-traps?

Discussion in 'NSPC Plumbing Code Questions' started by North Jersey, Jun 30, 2010.

  1. North Jersey

    North Jersey New Member

    Messages:
    107
    Am I mistaken, or does the following language permit the use of an s-trap?:

    12.8.1 Venting of Fixture Drains
    Fixture drains shall have a vent so located that the vent connects above the top weir of the trap and the
    developed length of the trap arm is within the limits set forth in Table 12.8.1.

    EXCEPTIONS:
    (1) Water closets and similar siphonic fixtures.
    (2) Combination waste and vent systems. (see Section 12.17)
    (3) Vents may be connected below the top weir of the fixture trap if the following conditions are met:
    a) The vertical section of the drain pipe shall be at least one pipe size larger than the trap inlet size.
    b) The horizontal pipe connected to the trap outlet shall be at least two pipe diameters long.
    c) The developed length of the trap arm shall not exceed the values in Table 12.8.1.
  2. hj

    hj Moderator & Master Plumber Staff Member

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    26,837
    Location:
    Cave Creek, Arizona
    It appears to be what it says, but I would question whether it is what its intent is.
  3. North Jersey

    North Jersey New Member

    Messages:
    107
    What do you think they're going for? If I could drop below the floor before I vent, I could avoid butchering a kitchen cabinet and losing a bunch of storage space. I was thinking I could meet the exception by putting 2" traps on my sink and my disposal, since the inlets would be 1-1/2".

    On a different note, can I use the elbow that came with the disposal and put a p-trap on each fixtures, or should I make a straight shot to a sanitary tee connected to the sink tailpiece? I kind of got the impression from something Terry wrote that the disposal elbows should be avoided.
  4. Terry

    Terry Administrator Staff Member

    Messages:
    15,135
    Location:
    Bothell, Washington
    Most plumbers will use a disposer kit and one p-trap. The disposer kit comes straight off the disposer.
    We use the supplied 90 on single bowl sinks.

    I worked on a kitchen sink the other day that had three p-traps. The third trap for the dishwasher was so full of gunk that the dishwasher could no longer drain.
    I cut out the three traps, took out the supplied 90 and ran everything with one trap. Now she can use her dishwasher.

    I don't know why anyone would revert back to old style plumbing. There was a good reason we quit doing things that way.
    On the job above, one of the complaints was kitchen smell. Does a smelly kitchen sound good to anyone? Or is it just me?
    I like smell containment.
  5. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    You don't need a 2" trap you need the VERTICAL section of the drain (IE after it connects to your vent) to be 1 size larger then your trap inlet size.
    Last edited: Jun 30, 2010
  6. North Jersey

    North Jersey New Member

    Messages:
    107
    Terry,

    I was contemplating individual p-traps based on this fellow's theory:

    http://www.nettally.com/palmk/GDplumbing.html

    What's wrong with his line of thinking? Does the baffle in the disposer kit constrict drainage that much?
  7. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    If you have two traps wouldn't you either need:

    A. Both traps protected by their own vet,
    B. Both traps connect to their shared vent via a double fitting so that each trap is connected to the vent symetrically?

    I would think with 2 traps tied togeather like that, and one of the traps constantly has rushing water going through it, that you'd tend to siphon the sink's trap dry when the dishwasher is draining.

    Up here a setup like that would be against code.
  8. North Jersey

    North Jersey New Member

    Messages:
    107
    Couldn't I just make everything downstream of the trap 2"?

    Whatever the case, does this code provision sound practical?

    Based on the exception I could run the drain down through the floor (long sweep, sanitary 1/8, sanitary 1/4?) and jog horizontally to the vent as long as:

    1) I size up the vertical drain one pipe size
    2) I make a 3-4" (twice the pipe diameter) horizontal run out of the trap outlet before turning down
    3) I keep the developed length (whatever that is) of the trap arm under 8 feet for a 2" drain and under 5 feet for a 1-1/2" drain (see table 12.8.1 in http://www.phccweb.org/files/Depts/Technical/2006NSPCNonIllustratedWeb.pdf)
    Last edited by a moderator: Jun 30, 2010
  9. Terry

    Terry Administrator Staff Member

    Messages:
    15,135
    Location:
    Bothell, Washington
    Plumbers don't run two p-traps on the same trap arm.
    We wouldn't be able to sneak that by a plumbing inspector. And there is a good reason why.

    As a side note, if your disposer isn't grinding well enough to drain through a baffle tee, it's old and worn out. It's time to replace the disposer.
  10. North Jersey

    North Jersey New Member

    Messages:
    107
    I'll refer the author of that page to this thread. So, the idea is that water rushing through the wye from one fixture would siphon out the p-trap on the other, correct?

    Any thoughts on the s-trap exemption in the NSPC?
  11. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    Correct.

    As to why they all s-traps who knows....
  12. North Jersey

    North Jersey New Member

    Messages:
    107
    Going up a size and tying into the vent within about four feet should prevent of siphoning, right? Would you size it up just one size, or would you go to three inch on that vertical piece? :) Thank you for the invaluable information!
    Last edited by a moderator: Jun 30, 2010
  13. MACPLUMB 777

    MACPLUMB 777 TROJAN WORLDWIDE SALES RP

    Messages:
    679
    Location:
    Houston, Texas, United States
    The proper way to run pipe is 2" vertical with a 2"x11/2"x11/2" tee on top !

    That way you have 11/2" for trap arm and 11/2" vent up with a 2" drain out
  14. North Jersey

    North Jersey New Member

    Messages:
    107
    The unfortunate framing of my kitchen area would end up killing a lot of cabinet space in a small kitchen (2x4 framing, sink under the window, rim joist close to the center of the bottom plate, difficult spacing of the stud bays). If I can design an S trap that won't break the p-trap seal under the worst conditions, we'll be in good shape. I'm sure a 6" drain would do the trick, but I'll bet the threshold at which siphoning takes place is at least a couple pipe sizes smaller. I'm counting on giving up most of the space in my sink base, so I don't mind running a large drain if necessary.

    My main question is whether or not an increase of one pipe size and proximity of the vent would provide a viable system. The code doesn't appear to present a problem in this case, but siphoning would be unacceptable.

    The reason for the 2" vent is to accommodate the washer and laundry tub in the basement.

    Update: I just got off the phone with the inspector. My interpretation of the code is correct except for the size of the trap. If I don't want to go up to 3" for the trap arm and the rest of the drain, I need a 1-1/2" trap, sizing up to 2" on the outlet. The 1-1/2" size trap means that I have five feet to hook into the vent. For anyone else in New Jersey or Maryland (the only two states that have adopted the NSPC), this information should come in handy. According to the inspector, a 2" trap arm/drain is sufficient to avoid siphoning on a 1-1/2" p-trap. Can anyone corroborate with a real-life anecdote?
    Last edited by a moderator: Jul 1, 2010
  15. Terry

    Terry Administrator Staff Member

    Messages:
    15,135
    Location:
    Bothell, Washington
    What you describe on the framing is normal framing. We manage to plumb correctly under those circumstances daily.
    A plumber would have it done in less time that it takes to type this. It's that easy.

    Do you want pipes coming up through the cabinet taking up space?
    It seems a shame to cut up a new cabinet for a pipe that shouldn't have been there.
    The finish carpenter is going to love you for making his job harder.

    It sounds like you have it handled with the inspector though.
    Last edited: Jul 1, 2010
  16. North Jersey

    North Jersey New Member

    Messages:
    107
    I am the finish carpenter. :) I'm actually interested in learning the proper technique for my personal edification, so if I've missed an easy solution, I want to learn.

    Plumbers typically aren't interested in doing small increments of work, and rightly so, without charging me a hefty flat fee per service call. The nature of my remodel is such that the plumbing, flooring, and the drywall must be done in small increments.

    With a standard installation, I would have to punch a hole into an adjacent cabinet, saw a large hole for the 45 degree vent (I need to offset because I can't shoot straight through the sill plate without cutting a 1" deep channel down my rim joist), and punch a hole for the drain line through the bottom of the adjacent cabinet.

    I could also run the trap arm through three cabinets and into a bearing wall, but I believe this wall sits directly on the main girder.

    Going straight out the back of the sink base cabinet is not possible because the window sill is built up with horizontally stacked 2x4s to accommodate a window that was significantly shorter than the original rough opening. Even if we had cripples below the window instead of solid wood, I'm not sure I could get the vent up to 42" before going horizontal. I would also have to put a 2" hole through 2x4 king and jack studs, which would require some sort of Simpson tie reinforcement.

    If I'm missing a really obvious solution, please let me know. I would prefer to keep all the pvc in one cabinet and drill a single hole in the cabinets.
  17. Ian Gills

    Ian Gills Senior Robin Hood Guy

    Messages:
    2,777
    Location:
    USA
    A lot is to do with the distance between the trap and the vent. I have a 1.5 inch p-trap on a 1.5 inch pipe for my laundry tub. But it is just over 42 inches away from the vent. The trap gulps after the tub has drained. I am going to have to shorten the distance.

    If you have just shy of five feet of pipe between your trap and your vent then you will need 2 inch pipe. Any more than 5 feet and you will run into the same problems as me.

    Although Table 12.8.1 shows something different. Where am I going wrong here?
    Last edited: Jul 1, 2010
  18. North Jersey

    North Jersey New Member

    Messages:
    107
    Well, the table specifies lengths based on the trap arm size (1-1/2" in my case). A 1-1/2" trap corresponds to a developed length of five feet. If the drain is horizontal until you hit the sanitary tee for the vent, you should still have 18 inches to remain code compliant. In fact, according to the table, 42 inches should be fine for 1-1/4". Are you dipping below the crown weir before you tie into the vent? You can have up to 3 DFUs on a horizontal drain, and I believe a standard laundry tub counts for two. I'm not a pro, so I'm not the best one to help you troubleshoot, but I would make sure your vent is clear of obstructions and that the p-trap is not reversed.
    Last edited by a moderator: Jul 1, 2010
  19. MACPLUMB 777

    MACPLUMB 777 TROJAN WORLDWIDE SALES RP

    Messages:
    679
    Location:
    Houston, Texas, United States
    I would follow terry's recommendation come out of the top of the tee 90

    over below your

    stacked wood, then outside the window space 90 up though the roof is

    how real plumbers would get it done,

    or install aav on top of the tee as high as possible
  20. North Jersey

    North Jersey New Member

    Messages:
    107
    Mac,

    The trouble is those stacked 2x4s start at about 38 inches above the finished floor. I'm pretty sure the inspector will not allow me an exception from the requirement to keep that horizontal vent six inches above the flood rim of my fixture. In NJ, we have to have an engineer design the AAV. He might allow me to augment the combination waste-vent type installation, but I'm a little reluctant to introduce a mechanical vent that will one day fail and introduce sewer gas into the house. Do you know of an AAV with some sort of fail-safe, i.e., it automatically closes off when it fails?
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