Free Tool
Attic Ventilation Calculator: How Much Roof Venting You Need
A Texas attic in August can get far hotter than the air outside, and that heat cooks shingles from underneath. Venting is the fix, and building code tells you how much. Enter your attic size and the vent ratings to see the intake and exhaust area a re-roof should include.
Interactive Tool
Attic Ventilation Calculator
Enter the attic floor size (usually the house footprint under the roof) and the net free area printed on your vent products. The calculator applies the building-code ratio and splits it evenly between intake at the eaves and exhaust at the ridge.
Total vent area
768 sq in
Intake (eaves)
384 sq in
Exhaust (ridge)
384 sq in
Attic floor
1,600 sq ft
1,600 sq ft ÷ 300 = 5.33 sq ft (768 sq in) of net free area. At the NFA you entered, that is about 43 intake units and 22 exhaust units (vents, or linear feet for continuous soffit or ridge vent).
The 1/300 ratio only applies when 40-50% of the vent area is high on the roof (within 3 ft of the ridge) and the rest is low. Don't mix exhaust types such as ridge vents and powered fans on one attic - see ridge vent vs. static vent vs. attic fan. The NFA defaults are examples; read the rating on your vent product.
Why ventilation comes up on a re-roof
When the old roof comes off, adding soffit intake and a continuous ridge vent costs far less than it ever will again. Ask each roofer how they will vent the new roof and whether the quote includes it. Shingle manufacturers can tie their warranty to proper ventilation, so it is worth having in writing.
Compare the options in ridge vent vs. static vent vs. powered attic fan.
How the calculator works
Attic floor area ÷ 150 (or ÷ 300 with a balanced high/low split) = net free area in square feet. × 144 converts it to square inches, which is how vent products are rated. The result is split 50/50 between intake and exhaust, and divided by the NFA you enter to estimate how many vents or linear feet of vent you need.
Official ventilation references
The ICC's R806 text sets the vent ratios. IBHS research covers how roofs hold up in Texas heat and storms.
- International Code Council: IRC R806 roof ventilation
The 1/150 net free vent area rule and the 1/300 exception when 40-50% of the vents sit within 3 ft of the ridge.
- IBHS Roof 101
Insurance Institute for Business & Home Safety research on roof materials, hail and wind resistance.
- International Residential Code 2021, Chapter 9: Roof Assemblies
Section R905.2.2: asphalt shingles only on slopes of 2:12 or more, with double underlayment from 2:12 up to 4:12. Free to read online.
Questions
How much attic ventilation do I need?
The International Residential Code calls for 1 square foot of net free vent area for every 150 square feet of attic floor, or 1 for every 300 when the vents are split between high exhaust near the ridge and low intake at the eaves. A 1,600 sq ft attic needs about 5.3 sq ft (768 sq in) under the 1/300 rule.
What is net free area (NFA)?
It is the open area air can actually pass through once screens and louvers are accounted for. Every vent product lists its NFA, either per vent or per linear foot. Use that number, not the vent's outside size.
Should intake and exhaust be equal?
Roughly. For the 1/300 rule, code requires 40-50% of the vent area to be high (within 3 feet of the ridge) and the rest low. Many roofers aim for at least as much intake as exhaust so the ridge vent doesn't pull air from the wrong places.
Can I mix a ridge vent with box vents or an attic fan?
It's generally discouraged. Two kinds of exhaust on one attic can short-circuit each other, with one pulling air in through the other instead of from the soffits. Pick one exhaust type per attic space.
Does poor ventilation affect a roof insurance claim?
It can. Heat and moisture build-up from a poorly vented attic ages shingles faster and can blister them, and an adjuster may call that wear rather than storm damage. Fixing ventilation during a re-roof is the cheapest time to do it.