DWV for 2nd floor with 2x10 joists 12" OC

[TouchOfGrey]

I'm in the process of planning the 2nd floor DWV lines for my house. Or more accurately redoing them with the reality of very limited space between floor joists that are 2x10s 12" OC. The attached drawing represents my current plan (1st floor plumbing is omitted). The first floor sits on a 4 foot crawlspace with open-web trusses, so lots of freedom for any plumbing once it hits that area. Any feedback would be appreciated but the particular question I have regards the section highlighted in red for bathroom 1. That toilet is already wet vented by the lavatory but I'm unsure if I need to carry the red section up through the roof as the vent stack, tying in the lavatory and shower vents (or if the red section in bath 1 isn't needed and I can just connect the shower and lavatory vents and go through the roof with that). Bathroom 1 is physically isolated and can't connect to the main 4" vent stack that goes through the roof for bathroom 2/3 and the rest of the house. The house is in NC so the relevant code is their version of IPC.

 

[wwhitney]

My comments are in red marked up on the diagram below. Hopefully it is clear which fitting/line each comment applies to.

As to the "could be avoided" comment on the dry vent above the 3x3x3x2 san-tee with side inlet: as currently drawn, that vent is required (but only needs to be 1-1/2"). It's only job is to dry vent the WC. The shower/tub/lav drain coming in could wet vent the WC, but that's 5 DFUs, so the wet vent/drain, from where the lav joins to be the 5th DFU, would need to be upsized to 3".

So a way to avoid that dry vent and that 3" wet vent, and to replace the 3x3x3x2 san-tee with side inlet with a 3" quarter bend, would be to (a) move the lav drain so it joins the 3" WC drain a couple feet before the stack, and (b) move the 2" lav/tub drain so it joins the 3" drain between the lav joining and the stack.

Cheers, Wayne

 

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[TouchOfGrey]

Much appreciated, all the comments make perfect sense. For the main vent reduction to 2", is it fine to just reduce from 4" to 2" coming out the top of the san-tee? Didn't know if doing so would technically violate the main vent stack running "undiminished in size" from the building drain to open air but guessing that doesn't count since it occurs near the drain/vent connection

 

[wwhitney]

For the main vent reduction to 2", is it fine to just reduce from 4" to 2" coming out the top of the san-tee?

Yes.

Didn't know if doing so would technically violate the main vent stack running "undiminished in size" from the building drain to open air but guessing that doesn't count since it occurs near the drain/vent connection

Are you quoting a specific section of the IPC? The IPC in general only requires a vent through the roof of size 1/2 of the minimum required building drain size. NC has amended IPC 909.1 IIRC to make that a 2" minimum vent through the roof as well.

Cheers, Wayne

 

[TouchOfGrey]

I mistakenly assumed I was quoting the IPC but it looks like it may be an NC specific code (section 904.1) which states the 2" minimum through the roof and adds "Such stack shall run undiminished in size and as directly as possible from the building drain through to the open air or to a vent header that extends to the open air."

 

[wwhitney]

Yes, NC's IPC 904.1, thank you, I misremembered the number. Now that you've mentioned your interpretation, it's not clearly to me which is right.

Chapter 9 Vents: Vents, 2018 North Carolina Plumbing Code | UpCodes

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If you do want to run the Bath 3 stack undiminished in size from the building drain up through the roof, I'd suggest making the whole thing 3". There's no need to match the size of the building drain; that much at least is clear from NC IPC 904.1. In fact for a house with 3 WCs or less, there's no need for any 4" drainage piping. [The building sewer, which refers to the drain outside the footprint of the house, might still be required to be 4", not sure.]

Cheers, Wayne

 

[Reach4]

Regarding the "undiminished in size", that is for the vent, and not the pipe that is vented. So you could use a 2 inch vent pipe venting a 3 inch drain line, or maybe a 1.5 inch vent pipe, and you could increase to 2 or 3 inches going up from the start of the vent.

You would not be allowed to be venting with 2 inch, and then reducing that to 1.5 on the way up.

 

[TouchOfGrey]

Wayne, that's basically what I think I'll do regarding changing the bathroom 3 stack to 3" and then I'll likely run that through the roof due to the ambiguity. Not pictured, there are two additional bathrooms on the first floor which is why I have the building drain as 4".

Reach, certainly agree with what you're saying regarding the pipe being vented vs. the vent pipe. But my question/concern is that the drain ends and the vent begins at the 3x3x3 san-tee here (what was a 4x4x3 san-tee in my drawing) and immediately reducing the 3" top to 2" could technically be considered diminishing the vent section. Would a 3x3x2 low heel inlet work in place of the 3x3x3 san-tee immediately reduced to 2"?

 

[wwhitney]

Reach, certainly agree with what you're saying regarding the pipe being vented vs. the vent pipe.

I'm not sure I do. The wording of 904.1 does not actually use the word vent. And the phrase "undiminished in size" could be taken to refer to the actual diameter of the stack where it connects to the building drain, not the minimum required size of the stack.

But my question/concern is that the drain ends and the vent begins at the 3x3x3 san-tee here (what was a 4x4x3 san-tee in my drawing) and immediately reducing the 3" top to 2" could technically be considered diminishing the vent section.

If you use a 3x2 bushing to reduce it, I wouldn't say that the vent portion has any reduction in size.

Would a 3x3x2 low heel inlet work in place of the 3x3x3 san-tee immediately reduced to 2"?

A 3x3x2 low heel inlet quarter bend is just a 3x2x3 san-tee by another name.

Cheers, Wayne

 

[TouchOfGrey]

Thanks for the clarifications and pointing out that 904.1 doesn't use the term vent which adds to the possible interpretations of that section. Seems like I should talk to the AHJ to get some kind of definitive answer on that.