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Roof Ventilation Calculator

Get your Net Free Vent Area (NFVA) needs in seconds.

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Step 1: Select Your Roof Pitch

Steeper roofs have more attic volume and require additional ventilation

How to determine roof pitch:

Roof pitch is expressed as rise/run (e.g., 6/12 means 6 inches of rise for every 12 inches of horizontal run). Most residential roofs are 5/12 to 8/12. If unsure, select "Standard Pitch" for a conservative estimate.

Understanding Roof Ventilation

Proper attic ventilation is essential for your roof's longevity, energy efficiency, and preventing moisture damage.

Why Ventilation Matters

  • •Extends roof life by reducing heat damage
  • •Prevents ice dams in winter
  • •Reduces moisture and mold growth
  • •Lowers cooling costs in summer

How Balanced Ventilation Works

Effective attic ventilation uses the natural convection cycle:

  1. 1.Cool air enters through soffit vents at the eaves
  2. 2.Air warms as it rises through the attic
  3. 3.Hot air exits through ridge or roof vents

Building Code Requirements

The International Residential Code (IRC) specifies:

  • •1/150 ratio: Standard requirement
  • •1/300 ratio: With vapor barrier + 50% upper vents
  • •50/50 split: Equal intake and exhaust

Ventilation Calculator FAQ

Common questions about attic ventilation and this calculator

Net Free Vent Area (NFVA) is the unobstructed area of a vent that allows air to flow through. It is measured in square inches and accounts for screens, louvers, and other obstructions that reduce airflow. Building codes specify minimum NFVA requirements to ensure adequate attic ventilation.
The 1/150 ratio is the standard building code requirement, meaning you need 1 square foot of NFVA for every 150 square feet of attic space. The 1/300 ratio (half the ventilation) is allowed when you have a vapor barrier installed in the ceiling AND at least 50% of the ventilation is in the upper portion of the attic. Most homes without a vapor barrier should use the 1/150 ratio.
Balanced ventilation means having equal amounts of intake (soffit vents) and exhaust (ridge/roof vents). This creates a natural airflow where cool air enters at the soffits and hot air exits at the ridge. Unbalanced ventilation can cause moisture problems, ice dams, and reduced energy efficiency.
Ridge Vents are best for most homes as they provide continuous ventilation along the entire roof peak. Static Box Vents are a good alternative when ridge vents are not possible. Powered Fans are ideal for hot climates or high-moisture areas. Turbine Vents are effective in windy areas but less reliable in calm conditions.
This calculator provides estimates based on industry-standard building code formulas. Your actual ventilation needs may vary based on roof complexity, climate, existing moisture issues, and local code requirements. For an accurate assessment, we recommend scheduling a free roof inspection.
Common signs include: excessive heat in the attic (over 20 degrees warmer than outside), ice dams in winter, moisture or mold in the attic, peeling exterior paint near the roofline, and higher than normal energy bills. If you notice any of these, your ventilation may be inadequate.

Need Help With Your Attic Ventilation?

Our experts can assess your current ventilation system and recommend improvements. Schedule a free inspection today.

Call (360) 947-7119Request Free Inspection