Poor or improper attic ventilation can result in severe damage to many elements of your home.  It will ruin the insulation, warp the roof decking and/or rot wood framing,destroy the shingles, and even cause the growth of dangerous mildew. Further, neglecting to ensure proper roof ventilation will raise your energy bills!

The proper attic ventilation system allows a continuous flow of outside air through the attic of your home, thereby protecting the efficiency of the insulation and helping to lower temperatures in the living space as well.  It consists of a balance between air intake and air exhaust.

The Federal Housing Administration recommends a minimum of at least 1 square foot of attic ventilation (both intake and exhaust) for every 300 square feet of attic space.
For example, if your attic is 900 square feet you will need a total of 3 square feet of ventilation. This amount is generally divided equally between intake and exhaust ventilation (i.e., 1-1/2 feet of each), to insure proper air flow through the attic.

To calculate the amount of ventilation you need for your home, use the following chart.

Roof Ventilation Calculator

Step 1
Calculate attic square footage.
How: Multiply length of attic (in feet) times width of attic (in feet)
30’ x 40’ = 1,200 square feet
Step 2
Calculate NFA (Net Free Area) needed for this attic by using the “1 in 300” rule.
How: Divide attic square footage by 300
1,200 sq. ft. ÷ 300 = 4 square feet of NFA needed
Step 3
Convert square feet of NFA to square inches.
How: Multiply square feet of NFA by 144
4 sq. ft. x 144 = 576 square inches of NFA needed
Step 4
Split the amount of NFA needed equally between the intake and the exhaust.
How: Divide square inches of NFA needed by 2
576 sq. in. ÷ 2 = 288 square inches of NFA needed equally for exhaust & intake
Step 5
Calculate the number of vents needed.
How: Divide the square inches of NFA needed at the ridge by the NFA of the SSB960 vent
288 sq. in. ÷ 60 = 4.8 vents (round up to 5 vents)

Once you determine the recommended amount of ventilation you should have a qualified roofing contractor go on to your roof to determine the number of intake and exhaust vents that are currently in your home to see if there is sufficient ventilation according to the square footage of your attic.