venting questions

Hi all,
I am building a new home in middle TN and have a question about venting my DWV system. I have a plumber and his work is impeccable, but I am not sure about his plan for the vents. I certainly don't want any more roof penetrations than needed, so..
He has installed 3" vents for all the toilets-2 at one end of the house, and 3 at the other. He intends to neck down the 3" and make it 2" right above the top plate, then manifold all together, and go out the roof at 2". Bear in mind that there are lavs, and showers in these locations as well. Effectively, under his plan, I'll have 2-2" roof penetrations. Is there a fixture count per roof vent...or??..that you all might enlighten me with? Thank you for your time

Kendal

1. He does NOT need a 3" vent at each toilet, and if he used 2" he would not have to reduce it in the attic
2. In most jurisdictions, the total AREA, of the vents through the roof has to equal the AREA of the incoming sewer, meaning that a 4" sewer line would need FOUR 2" vents or some combination which was equal to at least 12.56 sq. in. If he does not know this, then I question the "impeccable" part of your statement.

Thank you for your response.
1. In Metro Davidson County (one county away), I know they require at least 1-3" vent penetration, but here there isn't even a plumbing inspection! Interesting on the 2" vent as you state..I see the 3" as not detrimental however.
2. I have done more research since my post and haven't seen the Area approach at all..very simple. What I have seen mainly is the "fixture unit" approach which interestingly coincides with your area formula quite well. As far as impeccable, I mean clean and organized, no spaghetti, not drilling in the band of an I-joist, etc. I certainly am no expert beyond that, but it's nice to see someone who seems to care about how it looks even if there will be drywall over it!

Kendal

quote; I know they require at least 1-3" vent penetration

That might coincide with an alternate, but older, requirement, that at least one vent has to be carried full size from the incoming sewer to the roof.

At least one major plumbing code states that "All vent pipes shall extend undiminished in size above the roof ...".

What year was that one written? It was a code issue in the 40s and 50s, but only local codes still have that requirement.

our code doesn't aloow a reduction in size either....you would not need 3" for toilet vent, but if thats what you choose then you can't reduce. also has to be min. 3" before going through roof and extend 6" above roof surface on highside...

Places where it gets below freezing for potentially days on end often require the roof penetration to be bigger than more temperate areas. The hot, moist air can freeze into hoar frost and plug up a vent line if it is too small, so a 2" vent might not be allowed as it is likely to close off, whereas a 3" pipe is less likely to.

precisely...and i believe in northern Ontario this size has to be 4"

In Chicago, when I was there, the penetrations had to be 1" larger than the vent size, but a minimum of 4".