Hello I live in Northern New Jersey and my town follows the 2009 National Standard Plumbing code from the PHCC which is what my town follows:

So they have this snippiet of verbose code:

12.8 FIXTURE VENTS

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.

Table 12.8.1
MAXIMUM LENGTH OF TRAP ARM
Size of Trap Arm (Inches) Length – Trap Arm to Vent Slope – Inches per Foot
1- 1/4 - 3' 6" - 1/4
1-1/2 - 5' - 1/4
2 - 8' - 1/4
3 - 10' - 1/8
4 - 12' - 1/8

NOTES FOR TABLE 12.8.1:
This table has been expanded in the "length" requirements to reflect expanded application of the wet venting
principles. Slope shall not exceed 1/4" per foot.
12.12 FIXTURE REVENTING

12.12.2 Horizontal Branches
Three lavatories or one sink within 8 feet developed length of a main vented line my be installed on a 2" horizontal
waste branch without reventing, provided the branch is not less than 2 inches in diameter throughout it's length
and provided the wastes are connected into the side of the branch and the branch leads to its stack connection with a
grade of not more than 1/4 inch per foot. [/B]


So, the first section where it talks about the vent being above/below the trap weir. If I do indeed increase the size of the drain by 1 pipe size than the trap itself, does this mean I can connect the vent to the drain below the floor using a wye as long as I follow the maximum thresholds of 8' developed length and 1/4 inch per foot pitch?

In addition if the above assmption is true, I would envision at least a small trap arm size before it goes vertical, since if you didn't have a horizontal trap arm, the trap would turn into an s-trap, no?

A couple of weeks back you guys helped me out deciding on a kitchen revent design if you remember, and in that thread I could have done two things:

1) Use a wet vent as long as it was one pipe larger than the trap and within 8' feet of a stack.
2) Or, Build out a secondary vent stack that was less than 5 feet away, at that point the vent would be 1 1/2" and the drain 2". THe only problem I had with this solution, was my concern with boring through several 2x4's since this plumbing was being done on a load bearing exterior wall over a window. Eventually I figured I put the trap arm flush with the studs instead of boring through them and offset the back of the kitchen cabinetry to make it all fit.

But if I can connect the vent underneath the floor with the drain, this would make this a whole lot easier since I would have to concern myself with a trap arm going through studs at all.


Thank You,
Amado

Amado...

You either use a mechanical vent (AAV) or you drill through the studs (we do it every day), or surface mount the pipe, or install an island sink vent as previously discussed.

I can see you worked hard thinking this out, but you cannot do what you are proposing... not anywhere that I know of... ;)

Choices are as above!

Mark

So obviously they are not saying what I thought they were saying. Can you, for my own personal knowledge, let me in, on what the above code is really intended for?

Thanks again for your advice and patience,

Amado

In rereading it a bit, I can understand why you are asking about it, for sure.

Unfortunately, all I can tell you is that all sinks with PTRAPs require that the vent come off above the trap.

If you have found some kind of exception here then it would be a first in 30 something years that I've ever heard about this, and with that being said, I suppose you could have found an exception so my best advice would be for you to call a local plumbing inspector and specifically ask if this section of code pertains to kitchen sinks.

I'm betting it doesn't pertain to kitchen or bathroom sinks, but would you please let me know what they tell you, OK?

If you call now they may still be in the office... ;)

Mark

I am just following this thread from distance. Have question : Why would you want to connect under the floor ? Any specific reason for it ?

Hey Milo:
Going below the floor allows me to connect the vent to drain without worrying about running the trap arm along the exterior supporting wall. At least that was my idea. But like massplumber2008 stated this would mean vent below trap weir. I still have to verify if this bit of code is what it says it is. As always, at least in my town, code department out to lunch. I'll try to get an answer to this later today, or tomorrow.

Amado

The basement is underneath... I forgot to mention this.

Thank you for your response. Can you draw a simple sketch of your situation so I can visualize it better. Maybe we can find better way to connect it... Milo

My initial post on the kitchen revent has a very detailed picture. The post is called Reventing a kitchen sink, it's one of my only three questions! :-)

Even though I was all set to do the flush trap arm technique, that massplumber2008 thought was very kosher, but once I saw this code snippet, it kind of pulled me off on a tagent I have to admit. I will likely do the flush trap arm in th end. Thx.

Again, thanks to all the plumbing professionals helping me answer and resolve my questions. You would be surprised how so many so-called "plumbing" forums are very UNFRIENDLY to inquiring minds.

Amado

We enjoy helping Amado... :) Still curious on that code above... let me know when they respond to you, OK?

Thanks!

The way that I interpret the language of 12.8.1 sub. sect. 3 a,b,c that applies to the post, is that it describes an island sink loop vent. If massplumber has already explained he how-to's of it, then go with it. Examples could be found if you Google images of a 'foot vent'.