Roof Venting Requirements

There are all types of roof venting; gable, soffit, high mount, etc. The bottom line to roof venting is to make sure that you have enough square footage of travel to relieve the attic space of temperature.

Calculating how much venting your attic needs is relatively simple. All you need to know is the area of the attic floor. Include the garage, if you have one, and the soffited overhang because heat gets trapped above them, too. A common rule of thumb is the 1/300 rule, which means 1 square foot of net free vent area per 300 square feet of attic floor space. Say you have an 1,800 square foot home with a garage that that measures 20 feet by 22 feet. This will yield a total area of 2,240 square feet. You then divide this number by 300 - equaling 7.5. This tells us that we need 7.5 square feet of ventilation for the attic. Most attic vents are measured by square inches, so we need to convert the 7.5 square feet to square inches. This is done by multiplication. 1 square foot is equal to 144 square inches, so we multiply 7.5 by 144 - equaling 1,080. So we need 1,080 square inches of Net Free Vent Area (vent size minus obstructions).

If you are installing 12" by 16" vents, you would need 5 of them to accomplish the 1/300 rule.

More is not necessarily better, and it can actually hamper flow if you are using soffit vents to ridge vents. Lastly, the higher the venting, the better.