A net free area calculation has two halves. The first converts attic floor area into the square inches of unobstructed vent opening the roof needs. The second divides that requirement by the net free area (NFA) each vent product is rated to deliver. The second half is where jobs go wrong, because NFA is a number the manufacturer declares, and two vents that look identical on a supply house shelf can carry ratings 30 percent apart.
By The Roofing Brief Team. Last reviewed: August 2026.
What is net free area?
Net free area is the clear open area of a vent, in square inches, after the screening, louvers, baffles, weather filters and slot geometry are subtracted. The US Department of Energy’s Building America Solution Center, hosted by Pacific Northwest National Laboratory, defines net free ventilation area as “the clear open area of a vent, taking into account the restrictions of the vent screening itself and the dimensions of the vent slots.” It is not the size of the hole cut in the deck.
Manufacturers abbreviate it NFA or NFVA (net free ventilation area). The two terms mean the same thing. Codes and product literature both express it in square inches, which is why every ventilation calculation eventually multiplies square feet by 144.
This page is about the rating side of the equation. The 1/300 and 1/150 code ratios that produce your required total, and the split between intake and exhaust, are handled separately in our attic ventilation guide.
Why a vent’s opening is not its net free area
Every static vent puts something in front of the hole. Insect screen, a louver blade, a rain baffle, a nonwoven weather filter, and the frame itself all take area away. Section R806.1 of the International Residential Code requires ventilation openings to have a least dimension of 1/16 inch minimum and 1/4 inch maximum, or to be covered with corrosion-resistant wire cloth screening or hardware cloth within those same limits. That screen is mandatory, and it costs you area.
The Building America Solution Center notes that most screened vents have roughly 60 percent free area. That is a planning heuristic, not a product spec. The actual loss varies enormously by design, which is the whole reason a published rating matters.
The same manufacturer, ratios from 60 percent to 101 percent
Owens Corning publishes both the roof opening size and the net free vent area for each VentSure static vent in its product catalog. Dividing one by the other shows how loosely the published number tracks the physical hole. The opening areas below are our arithmetic from Owens Corning’s published dimensions; the net free vent area figures are Owens Corning’s own declared values.
| VentSure static vent | Roof opening (published) | Geometric area of opening (calculated) | Net free vent area (declared) | Declared NFA as share of opening |
|---|---|---|---|---|
| Metal Dome with Screen | 15 inch diameter | 176.7 sq in | 144 sq in | about 81% |
| Metal Square Hood, High Stack | 8 by 8 inches | 64 sq in | 51 sq in | about 80% |
| Metal Slant Back with Exterior Louver | 8 inch diameter | 50.3 sq in | 51 sq in | about 101% |
| Plastic Slant Back with Screen | 9 by 9 inches | 81 sq in | 55 sq in | about 68% |
| Low Profile Slant Back with Exterior Louver | 11 by 11 inches | 121 sq in | 72 sq in | about 60% |
Two vents in that list are both rated 51 square inches. One sits over a 64 square inch square cut, the other over a 50.3 square inch round cut. The louvered slant back’s declared figure is essentially the raw throat area of an 8 inch hole, so it carries no visible deduction at all. The low profile unit, covering an opening more than twice as large, is declared at 72 square inches. Nothing on the shelf tells you that.
Lomanco’s 750 Slant Back is the same story from another brand: an 8 inch roof hole, a 23 by 27.25 inch flashing footprint, and 50 square inches of published NFA. About 8 percent of the metal you are looking at is doing ventilation work.
How manufacturers publish net free area
Ratings come in two incompatible units, and mixing them is the most common estimating error on a ventilation takeoff. Unit vents are rated per vent. Continuous products are rated per linear foot, so the rating only becomes a total once you commit to a run length.
- Per unit, in square inches: box vents, slant backs, dome vents, turbines, gable louvers, undereave rectangles. One number, one vent.
- Per linear foot, in square inches per foot: shingle-over ridge vent, rolled ridge vent, continuous soffit vent, drip edge vents. Multiply by installed feet.
- Per section: some catalogs list a 4 foot ridge section at 72 square inches rather than 18 square inches per foot. Same product, same rating, different presentation.
The install method can change the rating on a single product line. GAF’s data sheet for the Cobra Exhaust Vent for Roof Ridge lists 16.9 square inches per linear foot for the hand-nailable version and 14.1 square inches per linear foot for the nail-gunnable version. Same brand, same product family, a 17 percent difference in delivered area.
Published net free area by vent type
Every figure below is manufacturer-declared or third-party-evaluated, taken from literature published by the named company. Ridge and soffit products are listed per linear foot; static and intake units are listed per vent. Use this to compare products on a common basis before you price anything.
| Exhaust product | Type | Declared net free area | Published source |
|---|---|---|---|
| Owens Corning VentSure 4-Foot Strip, 12 inch | Shingle-over ridge | 20 sq in per linear foot | Owens Corning VentSure catalog |
| GAF Cobra Rigid Vent 3 | Shingle-over ridge | 18 sq in per linear foot | GAF data sheet RESCB173 |
| Air Vent ShingleVent II (SHFV) | Shingle-over ridge | 18 sq in per linear foot | ICC-ES ESR-2071 (third-party) |
| Lomanco Omni OR-4 | Shingle-over ridge | 18 sq in per linear foot (72 sq in per 4 ft section) | Lomanco product catalog |
| Owens Corning VentSure 4-Foot Strip, 10 inch and 8 inch | Shingle-over ridge | 18 sq in per linear foot | Owens Corning VentSure catalog |
| GAF Cobra roll vent, hand-nailable | Rolled ridge | 16.9 sq in per linear foot | GAF data sheet RESCB137 |
| Air Vent ShingleVent II-7 and II-9 | Shingle-over ridge | 16 sq in per linear foot | ICC-ES ESR-2071 (third-party) |
| GAF Cobra roll vent, nail-gunnable | Rolled ridge | 14.1 sq in per linear foot | GAF data sheet RESCB137 |
| Owens Corning VentSure Rigid Roll | Rolled ridge | 12.5 sq in per linear foot | Owens Corning VentSure catalog |
| Lomanco Model 135 static | Box vent, per unit | 144 sq in per vent | Lomanco product catalog |
| Owens Corning VentSure Metal Dome | Dome vent, per unit | 144 sq in per vent | Owens Corning VentSure catalog |
| O’Hagin Fire and Ice flat low profile | Low profile, per unit | 98.75 sq in per vent (1/4 inch mesh) | O’Hagin compliance bulletin |
| Owens Corning VentSure Low Profile Slant Back | Low profile, per unit | 72 sq in per vent | Owens Corning VentSure catalog |
| Lomanco 600-S | Box vent, per unit | 60 sq in per vent | Lomanco product catalog |
| Owens Corning VentSure Plastic Slant Back | Slant back, per unit | 55 sq in per vent | Owens Corning VentSure catalog |
| Lomanco 750 Slant Back | Slant back, per unit | 50 sq in per vent | Lomanco published ventilation guidance |
| Intake product | Type | Declared net free area | Published source |
|---|---|---|---|
| Owens Corning VentSure Aluminum Undereave, 8 by 16 | Undereave, per unit | 38.12 sq in per vent | Owens Corning VentSure catalog |
| Owens Corning VentSure 8-foot Continuous Soffit Vent | Continuous soffit | 37.47 sq in per 8 ft section (about 4.7 sq in per linear foot) | Owens Corning VentSure catalog |
| Lomanco Deck-Air DA-4 | Deck intake, per 4 ft unit | 36 sq in per unit (9 sq in per linear foot) | Lomanco published ventilation guidance |
| Owens Corning VentSure Aluminum Undereave, 6 by 16 | Undereave, per unit | 27.23 sq in per vent | Owens Corning VentSure catalog |
| Lomanco SV-10 continuous soffit | Continuous soffit | 63 sq in per 10 ft section (6.3 sq in per linear foot) | Lomanco product catalog |
| Owens Corning VentSure Aluminum Undereave, 4 by 16 | Undereave, per unit | 16.34 sq in per vent | Owens Corning VentSure catalog |
| Owens Corning VentSure round mini soffit, 4 inch | Soffit plug, per unit | 2.43 sq in per vent | Owens Corning VentSure catalog |
How to convert a required net free area into a vent count
Once you have a required total in square inches, the product-selection arithmetic is four steps. Round every division up, never down, because a partial vent delivers no area and a rounded-down count fails inspection.
- Take the required total net free area in square inches. If your figure is in square feet, multiply by 144.
- Split it between intake and exhaust according to the balance rule you are designing to, then work each side separately.
- Divide each side’s requirement by the product’s declared NFA. Use square inches per vent for unit products and square inches per linear foot for continuous products.
- Round up to whole vents or whole sections, then re-check that installed intake meets or exceeds installed exhaust.
Worked example. An 1,800 square foot attic at the 1/300 ratio needs 6 square feet of net free area, which is 864 square inches. Split evenly, that is 432 square inches of exhaust and 432 square inches of intake. Here is what 432 square inches of exhaust actually costs in each product.
| Exhaust product | Declared NFA | Arithmetic | What you install |
|---|---|---|---|
| Owens Corning VentSure 4-Foot Strip, 12 inch | 20 sq in per ft | 432 / 20 = 21.6 | 22 linear feet of ridge |
| GAF Cobra Rigid Vent 3 | 18 sq in per ft | 432 / 18 = 24.0 | 24 linear feet of ridge |
| Air Vent ShingleVent II | 18 sq in per ft | 432 / 18 = 24.0 | 24 linear feet of ridge |
| GAF Cobra roll vent, hand-nailable | 16.9 sq in per ft | 432 / 16.9 = 25.6 | 26 linear feet of ridge |
| Air Vent ShingleVent II-7 | 16 sq in per ft | 432 / 16 = 27.0 | 27 linear feet of ridge |
| GAF Cobra roll vent, nail-gunnable | 14.1 sq in per ft | 432 / 14.1 = 30.6 | 31 linear feet of ridge |
| Owens Corning VentSure Rigid Roll | 12.5 sq in per ft | 432 / 12.5 = 34.6 | 35 linear feet of ridge |
| Owens Corning VentSure Plastic Slant Back | 55 sq in per vent | 432 / 55 = 7.9 | 8 vents |
| Lomanco 750 Slant Back | 50 sq in per vent | 432 / 50 = 8.6 | 9 vents |
| Lomanco Model 135 or VentSure Metal Dome | 144 sq in per vent | 432 / 144 = 3.0 | 3 vents |
The spread is the point. The same requirement is satisfied by 22 feet of one ridge vent or 35 feet of another. On a house with a 28 foot ridge, the first product finishes the job and the second one cannot, no matter how many rolls you buy.
Intake is usually the harder side, because soffit products carry small per-foot numbers. The same 432 square inches breaks down like this.
| Intake product | Declared NFA | Arithmetic | What you install |
|---|---|---|---|
| Lomanco Deck-Air DA-4 | 36 sq in per 4 ft unit | 432 / 36 = 12.0 | 12 units, 48 linear feet |
| Owens Corning Undereave, 8 by 16 | 38.12 sq in per vent | 432 / 38.12 = 11.3 | 12 vents |
| Lomanco SV-10 continuous soffit | 63 sq in per 10 ft section | 432 / 63 = 6.9 | 7 sections, 70 linear feet |
| Owens Corning 8-foot Continuous Soffit Vent | 37.47 sq in per 8 ft section | 432 / 37.47 = 11.5 | 12 sections, 96 linear feet |
| Owens Corning Undereave, 4 by 16 | 16.34 sq in per vent | 432 / 16.34 = 26.4 | 27 vents |
Ninety six linear feet of continuous soffit vent is more eave than some two-story houses have. That constraint, not the exhaust selection, is what usually forces a design toward deck intake, larger undereave units, or a rethink of the intake strategy covered in soffit vent vs gable vent.
Who actually verifies a net free area rating?
No model building code requires a manufacturer to prove a published net free area figure. The International Residential Code sets the required ratios and the screen opening limits in R806.1, not a measurement protocol. Verification is voluntary, and it runs through ICC Evaluation Service, whose Acceptance Criteria for Attic Vents (AC132) defines how NFVA is determined and reported for products that carry an evaluation report.
AC132 is stricter than a tape measure. As quoted in O’Hagin’s published compliance bulletin, the criteria state that for vents using corrosion-resistant metal mesh with openings of 1/16 inch minimum and 1/4 inch maximum, “the ventilation area reported in the evaluation report shall be the NFVA determined in accordance with Section 4.1, reduced by 10 percent to address the effects of clogging.” A rated product carrying an evaluation report has already given back a tenth of its calculated area for real-world debris. A product with a self-declared number has not.
ICC-ES report ESR-2071, reissued in June 2026, covers Air Vent’s ShingleVent II family and states the net free ventilation area for each profile: 18 square inches per lineal foot for the 13.8 inch wide SHFV, and 16 square inches per lineal foot for the 10 inch SHFV7 and the 11.3 inch SHFV9. The report also requires that “each panel must be marked with the venting area it provides.” That marking is checkable on the jobsite. A catalog figure is not.
The practical takeaway: an evaluation report number, a Florida Product Approval, or a Miami-Dade Notice of Acceptance means a third party looked at the number. Its absence does not mean the figure is wrong. It means nobody outside the company checked it.
How to compare two vent products honestly
Comparing NFA properly takes five checks, and four of them are about units rather than performance. Do them before price enters the conversation, because a cheaper vent with a lower rating often costs more once you count how many you need.
- Normalize the unit. Convert everything to square inches per linear foot or square inches per vent. A 72 square inch ridge section and an 18 square inch per foot rating are the same product.
- Check the install version. Hand-nailed and nail-gunned versions of the same roll vent can differ by several square inches per foot, as GAF’s Cobra data sheet shows.
- Ask whether the screen is in the number. A declared figure that matches the geometric area of the roof opening, as with an 8 inch round hole rated at about 51 square inches, is reporting the throat rather than a screen-deducted area.
- Ask for the report number. An ICC-ES ESR, Florida Product Approval, or Miami-Dade NOA means the figure was reviewed against AC132.
- Test it against the roof you have. Divide your requirement by the rating and compare the answer to your actual ridge length and eave length before ordering.
Turbines, power vents and gable louvers complicate this further, because a turbine is often marketed in cubic feet per minute rather than net free area. For a side-by-side on those categories, see roof vent types and ridge vent vs turbine vent.
Frequently asked questions
What does net free area mean on a roof vent?
Net free area is the clear open area of a vent in square inches once screening, louvers, baffles and slot geometry are subtracted. The Building America Solution Center defines net free ventilation area as the clear open area of a vent taking into account the restrictions of the vent screening and the dimensions of the vent slots. It is a rating the manufacturer publishes, not a dimension you measure on the roof.
Is net free area the same as the size of the hole in the roof?
No. Across Owens Corning’s own VentSure static vent line, declared net free vent area runs from about 60 percent of the roof opening area on the low profile slant back to roughly 100 percent on the 8 inch louvered slant back. Two vents in the same catalog can be rated 51 square inches while covering openings of 64 and 50.3 square inches.
How much does a screen reduce net free area?
It depends on the mesh and the design. The Building America Solution Center uses roughly 60 percent free area as a general figure for screened vents. Under ICC-ES Acceptance Criteria AC132, vents with corrosion-resistant metal mesh have their reported net free ventilation area reduced by a further 10 percent to account for clogging, so an evaluated rating is more conservative than a raw calculation.
How many square inches of net free area does a ridge vent provide?
Published shingle-over ridge vent ratings cluster between 12.5 and 20 square inches per linear foot. Owens Corning’s VentSure 4-Foot Strip in the 12 inch width is declared at 20, GAF’s Cobra Rigid Vent 3 and Air Vent’s ShingleVent II at 18, GAF’s hand-nailable Cobra roll at 16.9, and Owens Corning’s VentSure Rigid Roll at 12.5.
Who tests net free area ratings?
There is no mandatory third-party test. Model codes set ventilation ratios and screen opening sizes, not a measurement method. Voluntary evaluation runs through ICC Evaluation Service under Acceptance Criteria AC132, which governs how net free ventilation area is determined and reported. Products with an ICC-ES evaluation report, such as Air Vent’s ShingleVent II under ESR-2071, carry a reviewed figure.
How do you convert required net free area into a number of vents?
Divide the requirement for that side of the system by the product’s declared net free area, then round up. For continuous products the rating is per linear foot, so 432 square inches of exhaust is 24 feet of an 18 square inch per foot ridge vent. For unit products the rating is per vent, so the same 432 square inches is nine Lomanco 750 slant backs at 50 square inches each.
How many roof vents do I need for a 2,000 square foot attic?
Using Lomanco’s own published worked example at the 1/300 ratio, a 2,000 square foot attic needs 6.66 square feet of ventilation, split into 3.33 square feet each for intake and exhaust, which converts to 480 square inches per side. At 50 square inches per Lomanco 750 slant back, that is 9.6 vents, rounded up to ten. Local code may require the 1/150 ratio instead.
What is the difference between NFA and NFVA?
Nothing. NFA (net free area) and NFVA (net free ventilation area) are the same measurement expressed with different abbreviations. Manufacturer catalogs tend to use NFA, while ICC-ES evaluation reports and acceptance criteria use NFVA. Both are reported in square inches, either per vent or per linear foot, and both are compared against the same code-derived requirement.