venting questions

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Kendal, Aug 7, 2012.

  1. Kendal

    Kendal New Member

    Messages:
    12
    Location:
    Nashville, TN
    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
  2. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,247
    Location:
    Cave Creek, Arizona
    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.
  3. Kendal

    Kendal New Member

    Messages:
    12
    Location:
    Nashville, TN
    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
  4. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,247
    Location:
    Cave Creek, Arizona
    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.
  5. kreemoweet

    kreemoweet Member

    Messages:
    377
    Location:
    Seattle. WA
    At least one major plumbing code states that "All vent pipes shall extend undiminished in size above the roof ...".
  6. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,247
    Location:
    Cave Creek, Arizona
    What year was that one written? It was a code issue in the 40s and 50s, but only local codes still have that requirement.
  7. mikeplummer

    mikeplummer Plumber

    Messages:
    190
    Location:
    Ontario, Canada
    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...
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,313
    Location:
    New England
    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.
  9. mikeplummer

    mikeplummer Plumber

    Messages:
    190
    Location:
    Ontario, Canada
    precisely...and i believe in northern Ontario this size has to be 4"
  10. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,247
    Location:
    Cave Creek, Arizona
    In Chicago, when I was there, the penetrations had to be 1" larger than the vent size, but a minimum of 4".
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