Building code (IRC R806) requires at least 1 square foot of attic ventilation for every 150 square feet of attic floor — half of it as intake at the eaves, half as exhaust at the ridge. Too little ventilation cooks your shingles from below, traps moisture, and can void your shingle warranty. Enter your attic size below and get the exact numbers.
Attic ventilation calculator
Based on typical manufacturer net free area (NFA) ratings: ridge vent ≈ 18 sq in per linear foot, continuous soffit vent ≈ 9 sq in per linear foot, standard static roof vent ≈ 50 sq in each. Always confirm the NFA printed on the products you buy — it varies by brand and model.
Why the 50/50 split matters
Ventilation only works as a system. Hot air rises and exits at the ridge; that only happens if cooler air can enter at the soffits to replace it. An attic with plenty of ridge vent but blocked or missing soffit vents barely moves air — and worse, the exhaust vents can start pulling conditioned air from your living space through ceiling gaps. Intake (soffit) should always equal or slightly exceed exhaust. Never mix exhaust types on the same roof section either: a ridge vent combined with a powered fan or gable vents short-circuits the airflow and can pull rain in.
Signs your attic is under-ventilated
A baking-hot attic in summer (over ~130°F), frost or damp insulation in winter, mold on the underside of the deck, shingles that curl or age years ahead of schedule, and ice dams in cold climates. If your shingles failed early, ventilation is one of the first things a manufacturer checks before honoring a warranty claim.
Planning roof work? See what the full job costs in the roof cost calculator, and check your roof’s remaining life with the life expectancy calculator.
Frequently asked questions
How many box (static) vents do I need?
Divide your required exhaust NFA by ~50 sq in per vent. A 1,800 sq ft attic under the 1/150 rule needs 864 sq in total, 432 at the ridge — that’s about 9 static vents, or 24 linear feet of ridge vent. Ridge vent usually wins on cost and looks.
Can an attic have too much ventilation?
Too much exhaust without matching intake is the real problem — it can depressurize the attic and pull moist household air up. Balanced additions on both sides are safe; lopsided ones aren’t.
Does a metal roof change ventilation requirements?
No — the code requirement is based on attic area, not roofing material. What changes is the detailing: some metal systems use above-deck ventilation channels, which is a different (valid) strategy your installer should explain.