Roof Ventilation Calculator

Inputs

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Use the ceiling/attic floor area over the vented space, not the sloped roof surface area.
ft²
Required total NFA = vented floor area / ratio. Verify the adopted local code and project conditions.
Share of total required NFA assigned to upper/ridge/high exhaust vents. The balance is intake/soffit/low vents.
%
Manufacturer net free area for one intake vent, or per linear foot for continuous soffit venting.
in²
Manufacturer net free area for one upper exhaust vent, or per linear foot for ridge/high roof venting.
in²
Planned count of intake vents, or planned linear feet when the NFA value is per foot.
Planned count of upper exhaust vents, or planned linear feet when the NFA value is per foot.

Results

Default result
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Required total NFA(N total)
720in²
Pass

1:300 selected. Many code paths pair this reduced ratio with climate/vapor-retarder and 40-50% upper-vent conditions; verify the adopted local code.

Also computed

Required intake NFA(N intake)Pass360in²

50% of total required NFA assigned to lower/intake vents.

Required exhaust NFA(N exhaust)Pass360in²

50% of total required NFA assigned to upper/exhaust vents.

Required intake units/ft(Qi req)Pass40

Round up to 40 intake unit(s) or linear feet at the entered product NFA.

Required exhaust units/ft(Qe req)Pass20

Round up to 20 exhaust unit(s) or linear feet at the entered product NFA.

Provided intake NFA(Ni prov)Pass360in²

Intake utilization: 100.0%.

Provided exhaust NFA(Ne prov)Pass360in²

Exhaust utilization: 100.0%.

Roof ventilation preview: 50 percent upper exhaust share and 100 percent limiting utilizationRoof ventilation NFAIntake at the eaves, upper exhaust near ridge or high roof areaintake reserve0%exhaust reserveScreen only. Product NFA, blocked openings, vapor retarder conditions, WUI/fire rules and adopted code control final venting.
Method notes 4 notes
  • Required total net free area is calculated as vented floor area divided by the selected ratio.
  • Manufacturer NFA values should be net free area after louvers, screens and product restrictions, not gross opening area.
  • The intake/exhaust split is a sizing balance screen. Blocked soffits, insulation baffles, ridge length, gable vents, roof shape and wind exposure can control real performance.
  • Local adopted code, climate/vapor-retarder conditions, WUI/fire rules, product listings and roof assembly details control final attic ventilation.

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