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Thread: Size of Soffit Vents

  1. #1
    Join Date
    Sep 2002

    Default Size of Soffit Vents

    I did a search and could not find an answer to my question, so I apologize if it has been asked and answered.

    I own a 1932 brick home that had vinyl and aluminum added in the ?1950s over the window trim, soffits, fascia, and the porch. I have removed all the aluminum/vinyl and am in the process of replacing any rot, stripping, priming, painting etc.

    I plan on having a new roof placed next - removing the ugly turbine vents (currently the only vents for the roof) - and replacing them with a ridge vent and soffit vents. My questions are:

    1) how much surface area should the soffit vents encompass? I was thinking of using the round ones (they would be less conspicuous and easier to install). However, most people I have asked about how many of them to install tell me to instead install a continuous soffit vent.

    2) How big of a deal is it to have an underventilated attic? (My wife really hates the look of all the potential soffit vents we have looked at so far.) I understand it should provide lower cooling bills and extend the life of the roof; however this house has been without them for 70 yrs and seems to have done ok. Does underventilation cut the life of a roof short by a year or so, or cut the life in half?

    I hope these are answerable questions.



  2. #2


    Code requires 1 square foot of vent for every 150 square foot of attic.

    Under venting is lack of the house ability to breath. Baring constantly getting wet from plumbing leaks or lack of rain shields almost all the rot you find will be related to an area that has no way to breath to get rid of moisture. Don't short sheet your venting. May be a lot of reason why you are repairing rot now.

    Rot is affected not just the roof decking but entire attic, soffitt, area above walls, and much more.

    Good Luck


  3. #3
    Join Date
    Sep 2002


    Does one square foot of vent = one square foot of soffit vent OR one half square foot of soffit vent plus one half square foot of ridge vent?

    It seems that the ventilation should take into account the volume of an attic space not just the square footage. Since my house is a one and one half story structure there is not that much actual square footage of the attic but since the roof pitch is so great the volume would be more substantial.

    Thanks for the answer.


  4. #4



    SECTION R806
    R806.1 Ventilation required. Enclosed attics and enclosed
    rafter spaces formed where ceilings are applied directly to the
    underside of roof rafters shall have cross ventilation for each
    separate space by ventilating openings protected against the en-
    trance of rain or snow. Ventilating openings shall be provided
    with corrosion-resistant wire mesh, with 1/8 inch (3.2 mm)
    minimum to 1/4 inch (6.4 mm) maximum openings.

    R806.2 Minimum area. The total net free ventilating area
    shall not be less than 1 to 150 of the area of the space ventilated
    except that the total area is permitted to be reduced to 1 to 300,
    provided at least 50 percent and not more than 80 percent of the
    required ventilating area is provided by ventilators located in
    the upper portion of the space to be ventilated at least 3 feet (914
    mm) above eave or cornice vents with the balance of the re-
    quired ventilation provided by eave or cornice vents. As an al-
    ternative, the net free cross-ventilation area may be reduced to
    1 to 300when a vapor barrier having a transmission rate not ex-
    ceeding 1 perm (57.4 mg/s m2 Pa) is installed on the warm
    side of the ceiling.

    R806.3 Vent clearance. Where eave or cornice vents are
    installed, insulation shall not block the free flow of air. Amini-
    mum of a 1-inch (25.4 mm) space shall be provided between
    the insulation and the roof sheathing at the location of the vent.

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