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Calcimator

Roof Ventilation Calculator

Calculate the net free area (NFA) required for proper attic ventilation based on attic size, roof pitch, and vent type.

Inputs

sq ft
:12

Results

Total NFA Required

10 sq in

Vents Needed

1

Exhaust NFA5 sq in
Intake NFA5 sq in
How to Use This Calculator
  1. Enter attic floor area (sqft) and roof pitch.
  2. Set vent type (1:150 or 1:300 ratio) based on whether a vapor barrier is present.
  3. Enter existing intake NFA (sqin) and exhaust NFA (sqin).
  4. Review required total NFA, balanced intake and exhaust NFA, and vent count needed.
  5. Balanced ventilation (50% intake / 50% exhaust) is required -- do not add exhaust without adding intake.

How the result changes with Attic Floor Area

Attic Floor AreaTotal NFA RequiredVents Needed
1,0908 sq in1
3,56524 sq in2
6,53544 sq in3
9,01061 sq in4

What each input means

Attic Floor Area
Total attic floor area in square feet
Roof Pitch
Roof pitch as rise over 12 inches of run
Vent Type
Type of ventilation to install

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    Attic Floor Area = 1500, Roof Pitch = 6, Vent Type = 1 = 3 input(s) provided
  2. Calculate Total NFA Required
    Total NFA Required
    10 = 10
  3. Calculate Vents Needed
    Vents Needed
    1 = 1
  4. Calculate Exhaust NFA
    Exhaust NFA
    5 = 5
  5. Calculate Intake NFA
    Intake NFA
    5 = 5

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators.

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