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.
Code requires 1 square foot of net free ventilating area per 150 square feet of attic floor, or 1 per 300 when a vapour retarder is present and intake and exhaust are balanced. Half must be intake at the soffit and half exhaust at the ridge.
Calculate attic ventilation
Net free area is the open area of a vent, not its physical size.
Total net free area
1,440 sq in
square inches required
Intake required720 sq in
Exhaust required720 sq in
Ridge vent needed40 ft
Soffit vents needed80
Net free area is a budget to split, not just a total to hit. Roughly half
belongs at the ridge and half at the eaves. Exhaust without matching intake
does not ventilate the attic harder, it pulls conditioned air up out of the
house through every ceiling gap it can find.
Net free area is not vent size
A vent that measures 16 by 8 inches does not provide 128 square inches of ventilation. Louvres, 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.
Frequently asked questions
How much roof ventilation do I need?
One square foot of net free area per 150 square feet of attic floor, halved between intake and exhaust. With a vapour retarder and balanced venting, the ratio relaxes to 1:300.
What is net free area?
The actual open area of a vent after louvres and insect screening, published by the manufacturer. It is typically well under half the vent's physical dimensions.
How many soffit vents do I need?
Divide the required intake NFA by the NFA of one vent. A 1,500 sq ft attic needs 720 sq in of intake, which is 80 vents at 9 sq in each, or a continuous strip vent instead.
Can you have too much roof ventilation?
Excess exhaust without matching intake is genuinely harmful. It short-circuits, pulling air from the house or from other roof vents rather than the soffits. Balance matters more than raw quantity.
Do ridge vents work without soffit vents?
Barely. A ridge vent needs low intake to draw from. Without open soffits it will pull makeup air from the living space below, carrying moisture into the attic.
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