Code tool
Roof Ventilation Calculator
Code requires 1 square foot of net free ventilating area per 150 square feet of attic floor, or 1 per 300 when a vapor retarder is present and intake and exhaust are balanced. Half must be intake at the soffit and half exhaust at the ridge.
Enter attic floor area to get the net free ventilating area your roof needs, balanced between soffit intake and ridge exhaust, plus how many vents that means.
Calculate attic ventilation
Net free area is the open area of a vent, not its physical size.
Total net free area
square inches required
- Intake required
- 720 sq in
- Exhaust required
- 720 sq in
- Ridge vent needed
- 40 ft
- Soffit vents needed
- 80
Net free area is not vent size
A vent that measures 16 by 8 inches does not provide 128 square inches of ventilation. Louvers, screens and baffles block most of the opening. The usable figure is net free area, published by the manufacturer, and it is often less than half the physical dimension.
Always size from the NFA on the spec sheet. Sizing from the physical opening is the most common way an attic ends up under-ventilated despite looking well vented from the ground.
Balance matters more than total
Ventilation works by convection: cool air enters low at the soffit, warms, and leaves high at the ridge. That only happens if intake at least equals exhaust, which is why the vent type you choose matters as much as the total area. An attic with generous ridge venting and blocked soffits will pull its makeup air from inside the house instead, dragging conditioned air and moisture into the attic.
Under-ventilated attics cook shingles from below and shorten their life, and in cold climates they are the direct cause of ice dams, because warm attic air melts snow that then refreezes at the cold eave. If you have recurring ice dams, ventilation and insulation are the fix, not heat cable.