đ¨ Roof Ventilation Calculator
Calculate required attic ventilation per building code. Get balanced intake and exhaust recommendations with NFA calculations.
How to Use This Calculator
Enter your attic floor area in square feet. Select the attic type. Enter your current ridge vent length in feet and soffit vent NFA (net free area) in square inches. These are found on the vent manufacturer specs or by measuring existing vents.
Understanding Your Results
The 1:300 ratio applies when you have balanced intake and exhaust ventilation. Without balance, the stricter 1:150 ratio applies. Adequate ventilation prevents moisture damage, ice dams, and premature shingle aging. Always maintain at least 50% of ventilation as intake (soffit vents).
Frequently Asked Questions
How much ventilation does my attic need?
Building codes require 1 sq ft of net free ventilation area (NFA) per 150 sq ft of attic space. If you have balanced intake and exhaust, the ratio improves to 1:300. Split ventilation evenly between intake (soffit) and exhaust (ridge).
What is NFA in roofing?
NFA stands for Net Free Area â the actual open area through which air can pass in a vent after accounting for screens and louvers. A vent's NFA is always less than its physical size.
How long should my ridge vent be?
Ridge vent should cover as much of the ridge as possible for maximum effectiveness. Most ridge vents provide 18 sq inches of NFA per linear foot. Divide your required exhaust NFA by the vent's NFA per foot to find minimum length.
What happens if my attic is not ventilated properly?
Poor ventilation causes moisture buildup, leading to mold, wood rot, and damaged insulation. In summer, trapped heat can exceed 150°F, degrading shingles from underneath and increasing cooling costs.