How to calculate roof ventilation

Written by ricky gee
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How to calculate roof ventilation
Most building codes require roof ventilation. (Hemera Technologies/ Images)

The Federal Housing Administration requires that for every 300 square feet of attic space you have, you must have at least 1 square foot of ventilation. Each vent actually takes up 2 square feet of the requirement, since the outtake and intake of each vent takes up 1 square foot each. The number of vents determined by this method of calculation is the minimum number of vents allowed.

Skill level:
Moderately Easy

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  1. 1

    Calculate the size of your attic in square footage:

    Length x width = attic size

    30' x 40' = 1200 square feet

  2. 2

    Use the 1/300 rule to determine the amount of square inches of ventilation required for proper exhaust. Divide the square footage by 300:

    1200 / 300 = 4 square feet

    Divide the number of square feet by 2 since each outtake and intake takes up 1 square foot of the requirement. In this example, 4 / 2 = 2 square feet.

  3. 3

    Convert the square footage into square inches. Multiply the square footage in step 2 by 144:

    2 x 144 = 288 square inches

  4. 4

    Divide the number of square inches by the area of the vent size to determine how many vents are needed:

    288 / 50 = 5.76 vents

    Round up to the nearest whole number. In this example, you need 6 vents for proper ventilation.

Tips and warnings

  • Vents come in different sizes. Make sure to divide the number you receive in step 3 by the size in square inches of the vent.

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