Guide
Ice dam prevention
Ventilation, insulation and membrane, in that order.
Attic ventilation calculator
Under-ventilating an attic is the cheapest way to take five years off a shingle roof. The code sets a floor of 1/150 of the ventilated space in net free area, and this calculator turns that into the two numbers you actually order against: how many soffit vents low, and how many feet of ridge vent high.
Net free area
Attic floor area, not roof area. The code default is 1/150; 1/300 is permitted only under the two conditions below. Pre-filled with a 1,700 sq ft attic.
1,632 in²
total net free area required at 1/150 · 11.33 sq ft
| Line | Value |
|---|---|
| Required NFA, total | 1,632 in² |
| Low (intake) half | 816 in² |
| High (exhaust) half | 816 in² |
| Soffit vents needed | 91 |
| Ridge vent to install | 46 ft |
How the estimate is built
Attic floor area divided by the ratio gives the required net free area in square feet; multiply by 144 for square inches, which is the unit vents are rated in. Split it in half: the low half at the soffit, the high half at the ridge. Divide each half by the rated NFA of the product you are using and you have a count and a length.
When 1/300 is allowed
The reduced ratio applies in exactly two cases. Either a Class I or II vapour retarder is installed on the warm-in-winter side of the ceiling in Climate Zones 6, 7 and 8; or at least 40 and not more than 50 per cent of the required venting area sits in the upper portion of the attic, with the upper vents no more than 3 feet below the ridge measured vertically. Neither is optional wording — an inspector will ask which one you are relying on.
Intake is the half everyone gets wrong
A ridge vent with no intake below it does not ventilate — it pulls conditioned air out of the house through every ceiling gap. If the soffits are blocked by insulation, no amount of exhaust fixes the attic. Baffles at every rafter bay are part of the job, not an extra.
Never mix exhaust types
A ridge vent and a powered fan on the same attic short-circuit each other: the fan pulls its air from the nearest opening, which is the ridge, and the far end of the attic ventilates not at all. Pick one exhaust strategy. The same applies to box vents added alongside a continuous ridge. What under-ventilation costs a roof is on the shingle lifespan guide, and the winter version of the same failure is on the ice dam guide.
Related
Guide
How long asphalt shingles last
Heat from below is the first thing that shortens a roof.
Tool
Roof inspection checklist
Ventilation is one of the six zones on the walk.
Sources
- International Building Code 2021 §1202.2.1, attic ventilation — read 2026-09-04