How to Calculate Roof Ventilation

Why Ventilation Matters

Proper attic ventilation prevents moisture damage, ice dams, premature shingle deterioration, and excessive cooling costs. An improperly ventilated attic can void your shingle warranty and cause mold growth in as little as 2-3 years.

The 1/300 and 1/150 Rules

IRC Section R806 specifies two ventilation ratios:

Step 1: Calculate Attic Floor Area

Measure the footprint of the attic space (same as your building's upper floor area). This is not the same as roof area — it's the horizontal projection.

Example: 50' × 30' attic = 1,500 sq ft

Step 2: Determine Total NFA Required

Using the 1/300 rule:

Step 3: Split 50/50 Intake & Exhaust

The key to effective ventilation is balance. At minimum, 50% of NFA should be intake (at soffits) and 50% exhaust (at ridge). A 60/40 split favoring intake is acceptable.

• Intake NFA: 720 ÷ 2 = 360 sq in
• Exhaust NFA: 720 ÷ 2 = 360 sq in

Step 4: Size Your Vents

Exhaust Vents (Ridge/Box/Turbine)

Intake Vents (Soffit/Eave)

Common Mistakes

• Mixing powered and passive exhaust: A power attic fan can short-circuit ridge vents, pulling conditioned air up instead of attic air out.
• Blocking soffit vents with insulation: Use baffles to keep insulation away from soffit openings.
• All exhaust, no intake: Without adequate intake, exhaust vents pull air through leaks in the ceiling instead of through soffit vents.