Roof Ventilation Calculator (NFA)

How It Works

Proper attic ventilation prevents moisture damage, ice dams, and premature shingle aging. The FHA standard is 1 sq ft of NFA (Net Free Area) per 300 sq ft of attic floor when a vapor barrier is present, or 1:150 without one. NFA must be split equally between intake (soffit vents at eaves) and exhaust (ridge vent or roof vents at peak). The key principle is that hot air rises — cool air enters through soffit vents, flows upward along the underside of the roof deck, and exits through the ridge. Never mix different exhaust vent types (ridge vent + gable vent) as this can short-circuit airflow.