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Thread: Vent sizing

  1. #1
    sea-bee chuck b's Avatar
    Join Date
    Jan 2011
    levering, michigan

    Default Vent sizing

    Doing DWV for 800' square foot cottage with a septic system. Need to know if 2" vents will be acceptable (with 3" PVC PIPE 1' below and above roof line) for the following:

    Full Bath: Toilet, Sink and Tub/Shower
    Laundry: Drain
    Kitchen: Sink (w/disposal and dishwasher)

    Would prefer to use only 2" if possible for venting. Is a 3" "main line vent" required (Michigan)?


  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    Don't know, it's a matter of two things: first, adding up the fixture units, and then, often, at least for the roof penetration, it often needs to be bigger to prevent hoar frost from closing it off in the wintertime. So, you may get by with 2" until it goes through the roof then bumping it up to 3" or even larger. Your local codes will determine this.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    In the Trades Tom Sawyer's Avatar
    Join Date
    Nov 2010


    Michigan is IPC so you do not need to run a full size vent through the roof. There needs to be at least 1 vent that is 1/2 the diameter of the building drain which if your main is 4" then you need a 2" vent. All the other individual vents can tie into that one before it exits the roof. The stack size has to be increased to 3" starting 18" below the roof line and extend at least 18" above the roof line to prevent frost closure.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  4. #4
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona


    THis is a code issue. Some require a "full size" vent from the incoming sewer to the roof, others require that the total AREA, not diameters, of the roof vents must equal the area of the house sewer outside the building. Your area could have a different requirement so you have to contact your building department, not us.
    Licensed residential and commercial plumber

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