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Old 01-07-2020, 11:52 AM   #1
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What's normal diameter of main vent stack?


The one that goes through the roof. Or both if there's multiple.

Are these 3" pipes?
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Old 01-07-2020, 12:28 PM   #2
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Re: What's normal diameter of main vent stack?


Pretty much. Some codes now have moved to 4 inch. Ask your local municipal plumbing inspector for the correct information for your location.
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Old 01-07-2020, 12:48 PM   #3
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Pretty much. Some codes now have moved to 4 inch. Ask your local municipal plumbing inspector for the correct information for your location.
Does the vent for an individual fixture equal half the diameter of the drain for that fixture? So if I have a 3 inch drain for a toilet, then that would have a 1.5 in vent? And so I wouldn't need to worry about having a 2x6 wall because I can enlarge the vent up in the attic?
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Old 01-07-2020, 04:04 PM   #4
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Re: What's normal diameter of main vent stack?


here is video where they transition from 3 to 4 inch vent pipe in the attic. This is new Hampshire, so this would follow the ipc plumbing code. check with your local municipality plumbing inspector for the proper code in your area.



Please wait for ghost or eplumber to respond, one follows the ipc and one follows the upc plumbing code. They will be along shortly.


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Old 01-07-2020, 04:49 PM   #5
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here is video where they transition from 3 to 4 inch vent pipe in the attic. This is new Hampshire, so this would follow the ipc plumbing code. check with your local municipality plumbing inspector for the proper code in your area.



Please wait for ghost or eplumber to respond, one follows the ipc and one follows the upc plumbing code. They will be along shortly.


At 11:05, he says he used an 1.5" vent for a toilet. I suppose that's all I need to know.

Thank you.
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Old 01-07-2020, 05:07 PM   #6
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Re: What's normal diameter of main vent stack?


Don't remember exactly or for sure, but I think Cali's adoption of IRC at least 3-5 years ago required toilet venting to be 2".....FWIW


(Reason I remember is that we were in 2x4 framing, but the BI (who was not to good) said code was 1.5, and we were worried that a subsequent inspector might call us on it... which luckily did not occur.)
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Old 01-07-2020, 05:12 PM   #7
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Don't remember exactly or for sure, but I think Cali's adoption of IRC at least 3-5 years ago required toilet venting to be 2".....FWIW


(Reason I remember is that we were in 2x4 framing, but the BI (who was not to good) said code was 1.5, and we were worried that a subsequent inspector might call us on it... which luckily did not occur.)
Right now, I'm just making sure it doesn't have to be 3" so that I need to make the walls 2x6.
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Old 01-07-2020, 05:54 PM   #8
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Re: What's normal diameter of main vent stack?


Fair enough farmerjohn, glad the video helped you with the information you needed to know.
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Old 01-07-2020, 06:06 PM   #9
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Re: What's normal diameter of main vent stack?


The vent size is usually based on how cold your states winters are.
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Old 01-07-2020, 09:03 PM   #10
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Re: What's normal diameter of main vent stack?


OK, I haven't looked in that section for a while, but last time I checked, the big-box home improvement stores (in NH) only had 1.5" roof flashing pieces. Well, I think some were 1-1.5" and others were 1.5-2". I see on line they do sell 3-4" flashing, too. I just never remember seeing it in the store.

My point is, the 1.5" seems to be the normal size. It's also what I see on just about every house. If I've ever seen a 3-4" vent, I don't recall it.

Thinking of what a vent is supposed to do, it's hard to imagine ever needing one that big. Think of the vent in a water jug or old-style gas can. A very small hole lets in enough air to let out a relatively large stream of water or gas.

So, are there any places where a 3-4" vent would be desirable (or required by code) in a normal residential building? I'm just curious.
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Old 01-07-2020, 09:35 PM   #11
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Re: What's normal diameter of main vent stack?


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Originally Posted by farmerjohn1324 View Post
Right now, I'm just making sure it doesn't have to be 3" so that I need to make the walls 2x6.

Well...Ya can't run 2" thru 2x4 that I know of......
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Old 01-08-2020, 01:14 AM   #12
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Re: What's normal diameter of main vent stack?


NY requires one full sized vent, usually 3” but if 4” is used in the system the vent is 4”. Other vents can be smaller but must have a long increaser so they are at least 3” when they penetrate the roof. The reason is that in the winter frost can close up a smaller pipe. In warm climates smaller vents work.
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Old 01-08-2020, 09:34 AM   #13
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Re: What's normal diameter of main vent stack?


Vents are sized based on Drainage Fixture Units (DFU) This is the GPM flow of any fixture.
7.5 gpm = 1 DFU
In UPC land, a vertical 1-1/2" vent can handle 8 DFU. That's more than adequate for a bathroom- except for the exception that a toilet is a mandatory 2" vent. Therefore a bathroom group is vented by 2"
Also, the cross sectional area of the vent system must meet or exceed the cross sectional area of the building drain..... So a 1 bath home would then need a 3" or 4" vent- like you see in older homes......
Frost closure does come into play but only in severe cold climates- not enforced everywhere. And is not considered when sizing vents for fixtures

I often drill 2x4 bottom and top plates 2-9/16" diameter hole for a 2" pipe. Nobody seems to mind as long as I nail plate. Cheap builders don't want to buy 2x6's anymore
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Old 01-08-2020, 09:37 AM   #14
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Quote:
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Quote:
Originally Posted by farmerjohn1324 View Post
Right now, I'm just making sure it doesn't have to be 3" so that I need to make the walls 2x6.

Well...Ya can't run 2" thru 2x4 that I know of......
Why not?

2.5" OD of pipe. 3.5" wall thickness.
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