Attic Ventilation Formula

1 square foot (or 144 square inches) of net free area (NFA) ventilation for every 150 square feet of attic space.

The single most important variable in determining the longevity of a roof system is maintaining a properly ventilated attic. Having a properly ventilated attic requires 50% intake and 50% exhaust of air flow. Use the 1 in 150 Rule as shown below (hover over the tooltip to see calculation):