Roof Ventilation 1:300 Rule Explained (Soffit vs. Ridge Balance)
Proper attic ventilation extends roof shingle lifespan and prevents winter ice dams. Here is the Net Free Vent Area (NFVA) formula and code requirements.
1. The IRC Code Requirements for Attic Venting
Section R806 of the International Residential Code (IRC) regulates roof ventilation. Without continuous airflow, summer attic temperatures reach 150°F, cooking roof shingles and increasing AC cooling loads.
Net Free Vent Area (NFVA) Formula
Total NFVA (Sq Ft) = Attic Floor Area (Sq Ft) / 300
Soffit Intake (50%) = Total NFVA x 0.50 x 144 (Sq Inches)
Ridge Exhaust (50%) = Total NFVA x 0.50 x 144 (Sq Inches)
2. Worked Math Example: 1,800 Sq Ft Attic
Let us calculate ventilation requirements for an 1,800 sq ft home footprint.
Attic Footprint = 1,800 Sq Ft
Step 1: Total NFVA = 1,800 / 300 = 6.0 Sq Ft (864 Sq Inches)
Step 2: Soffit Intake (50%) = 432 Sq Inches (3.0 Sq Ft)
Step 3: Ridge Exhaust (50%) = 432 Sq Inches (3.0 Sq Ft)
RESULT: 432 Sq In Soffit Intake + 432 Sq In Ridge Exhaust
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Launch Roof Vent CalculatorFrequently Asked Questions
What is the 1:300 attic ventilation rule?
The 1:300 rule dictates that an attic requires 1 square foot of Net Free Vent Area (NFVA) for every 300 square feet of attic floor space, provided at least 40% to 50% of the venting is positioned near the ridge.
What happens if you have too much exhaust and not enough soffit intake?
If exhaust vents exceed intake capacity, negative air pressure draws conditioned air and moisture from the living space into the attic, increasing energy bills and creating mold hazards.