Minimum vent stack pipe size

[KBecker57]

I'm in Massachusetts. I'm confused with the apparent contradiction in section 10.16 in the code re: venting: First here:

(5) Vent Stacks and Stack Vents. (a) Vent Stack Required. 1. Any structure, in which a building drain is installed, shall have as a minimum one full size main stack vent or a vent stack no less than three inches in diameter.

vs. this:

(b) Connections at Base and Top. 1. All main vents or vent stacks shall connect full size at their base to the drainage of the building or to the main soil or waste pipe, at or below the lowest fixture branch. 2. All vent pipes shall extend undiminished in size above the roof, or shall be reconnected with the main soil or waste stack above the highest fixture connection discharging into it. The minimum size of any vent through the roof shall be two inches in diameter.

I plan on having a 3 inch horizontal waste pipe in my crawl space. The main house vent stack would connect to it and go through the roof. I want to hide the vent stack pipe inside a new interior 2x4 wall. A 3 inch pipe would require a 2x6 wall. So, a 2 inch vent pipe connecting to the waste pipe would allow a 2x4 wall but is it compliant. Thoughts?

It's a very small house. 600 square feet and only one floor with a crawl space. One toilet, bathroom sink, shower, washer, and kitchen sink.

 

[arctic bill]

The horizontal pipe should be 4 inches , You can reduce to 3 inches once you a running a stack riser. You can fur out a wall to a full 4 inches .

 

[JG plumbing]

I don't see the contradiction. They are talking about main stacks and their size. Also they mention anything that isn't a main stack has to be 2"when it comes through the roof.

 

[KBecker57]

The horizontal pipe should be 4 inches , You can reduce to 3 inches once you a running a stack riser. You can fur out a wall to a full 4 inches .

OK, thanks. The sewer pipe is 4 inches when it exits the building. It transitions to 3 inches inside my crawl space because the 4 inch pipe and fittings don't allow for giving me enough slope. And, I only have 17 DFUs for all of my fixtures and 3 inch inch pipe allows for 34 DFUs, according to my reading of the code. Is this wrong?

 

[KBecker57]

The horizontal pipe should be 4 inches , You can reduce to 3 inches once you a running a stack riser. You can fur out a wall to a full 4 inches .

Can you point me to a link that shows how I would fur out a wall to a full 4 inches? I've looked and I cannot find anything. Thanks.

 

[KBecker57]

I don't see the contraction. They are talking about main stacks and their size. Also they mention anything that isn't a main stack has to be 2"when it comes through the roof.

So, the main stack vent must be 3 inches all the way through the roof and additional stack vents can be 2 inches, correct? Since I only have one, it must be 3 inches, correct?

 

[JG plumbing]

Yes

 

[arctic bill]

Can you point me to a link that shows how I would fur out a wall to a full 4 inches? I've looked and I cannot find anything. Thanks.

Just put any piece of 1x3 on top of any 2x4 stud

 

[PerplexedPlumber]

With a 3" vent pipe, why wouldn't he use only a 1 x 2 or 2 x 2?

(in small space homes, saving an inch can be valuable)

 

[arctic bill]

With a 3" vent pipe, why wouldn't he use only a 1 x 2 or 2 x 2?

(in small space homes, saving an inch can be valuable)

A 2x4 is 3 1/2 inches wide , if you put a 1x2 on top then add 5/8 inch to total is 4 and i/8 , Plenty of room ,

 

[TomFOhio]

Do you have an attic that gets you to your vent pipe. You can tie your other vents into the main stack instead
of penetrating the roof again.

 

[PerplexedPlumber]

A 2x4 is 3 1/2 inches wide , if you put a 1x2 on top then add 5/8 inch to total is 4 and i/8 , Plenty of room ,

Apologies. I misread/misunderstood your original reply.