Vent sizing

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by chuck b, Sep 6, 2012.

  1. chuck b

    chuck b sea-bee

    Jan 19, 2011
    levering, michigan
    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. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    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.
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  4. Tom Sawyer

    Tom Sawyer In the Trades

    Nov 29, 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.
  5. hj

    hj Moderator & Master Plumber Staff Member

    Aug 31, 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.
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