# Ridge Vent Calculator

Ridge vent calculator per IRC R806. Enter attic area to get required net free area, linear feet of ridge vent, matching soffit intake, and a ridge-length feasibility check.

## What this calculates

A ridge vent only works if it is sized correctly and matched with enough soffit intake to feed it. This ridge vent calculator takes your attic square footage and returns the required net free area under IRC R806, then converts that into linear feet of ridge vent for any product from GAF Cobra to generic shingle-over vents, plus the matching soffit length you need to keep the system balanced.

## Inputs

- **Attic Floor Area** (sq ft) — min 0 — Usually equals the home footprint. For a 1,500 sq ft ranch, enter 1,500.
- **Ventilation Rule (IRC R806)** — options: 1:150 rule (default, no vapor retarder), 1:300 rule (balanced 50/50 or vapor retarder) — 1:300 applies when intake and exhaust are balanced, or a Class I/II vapor retarder is installed.
- **Ridge Vent Product** — options: GAF Cobra Snow Country (20 sq in/ft), GAF Cobra / Air Vent ShingleVent II (18 sq in/ft), Owens Corning VentSure (17 sq in/ft), Lomanco OR-4 / shingle-over (14 sq in/ft), CertainTeed Filtered (12.5 sq in/ft), Custom NFA value (enter below) — Pick a product or use a custom NFA per linear foot.
- **Custom Ridge Vent NFA** (sq in/ft) — min 1, max 30 — Only used when Product = Custom. Leave at 18 for a generic ridge vent.
- **Ridge Length Available** (ft) — min 0 — Total straight ridge length available for continuous vent (not including hips).
- **Soffit Intake NFA** (sq in/ft) — min 1, max 30 — Continuous aluminum soffit = 9 sq in/ft. Perforated vinyl panels = 3-5 sq in/ft.

## Outputs

- **Total Required NFA** (sq in) — Total net free area required by the IRC rule
- **Ridge (Exhaust) NFA** (sq in) — 50% of total, provided by ridge vent
- **Soffit (Intake) NFA** (sq in) — 50% of total, provided by soffit vents
- **Ridge Vent Length Needed** (linear ft) — Continuous ridge vent length to cover exhaust NFA
- **Soffit Vent Length Needed** (linear ft) — Continuous soffit length to cover intake NFA
- **Ridge Length Check** — formatted as text — Whether the available ridge length is enough

## Details

## How the Ridge Vent Calculator Works

The International Residential Code (R806.2) requires **1 square foot of net free area (NFA) for every 150 square feet of attic floor area**. The ratio drops to 1:300 if:

1. Intake and exhaust are balanced 40-50% at each of the upper (ridge) and lower (soffit) portions of the roof, OR
2. A Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.

Most modern homes with continuous soffit intake and continuous ridge exhaust meet the 1:300 threshold.

## Ridge Vent NFA by Product

Ridge vent products publish their net free area per linear foot. Higher NFA means you need less linear feet to hit your target.

| Ridge Vent Product | NFA per Linear Foot |
|---|---|
| GAF Cobra Snow Country | 20 sq in/ft |
| GAF Cobra Exhaust Vent | 18 sq in/ft |
| Air Vent ShingleVent II | 18 sq in/ft |
| Owens Corning VentSure | 17 sq in/ft |
| Lomanco OR-4 Omni Roll | 14 sq in/ft |
| CertainTeed Filtered Ridge Vent | 12.5 sq in/ft |
| Generic shingle-over | 12-14 sq in/ft |

## Example: 1,500 sq ft Attic

- Attic floor area: 1,500 sq ft
- Rule: 1:300 (balanced vents)
- Total NFA: 1,500 / 300 = 5 sq ft = 720 sq in
- Balanced 50/50: 360 sq in ridge exhaust + 360 sq in soffit intake
- GAF Cobra at 18 sq in/ft: 360 / 18 = **20 linear feet of ridge vent**
- Soffit at 9 sq in/ft: 360 / 9 = **40 linear feet of soffit intake**

A typical 1,500 sq ft ranch has 50+ feet of ridge, so 20 ft of continuous Cobra down the ridge is easy to install and leaves capacity for future additions.

## Matching Soffit Intake

The single most common mistake is installing ridge vent without enough soffit intake. If your ridge vent can exhaust 500 cfm but the soffits can only supply 200 cfm, the ridge pulls the other 300 cfm from inside the house, through recessed lights and bath fan penetrations. That wastes heating and cooling energy, and in winter can pull moisture-laden air into the attic where it condenses on the cold roof deck.

Rule of thumb: soffit intake NFA should equal or slightly exceed ridge exhaust NFA. A 55/45 intake-to-exhaust ratio is ideal; 50/50 is acceptable; more than 50% exhaust is a problem.

## Ridge Length Limits

A continuous ridge vent can only cover the length of ridge that exists. A gable roof with hips has a short ridge line. If your calculator output calls for 40 linear feet of ridge vent but you only have 25 feet of ridge, you need to:

- Use the 25 feet you have for ridge vent
- Supplement with static roof louvers (50 sq in NFA each, install 3 ft below the peak)
- Or use a higher-NFA product like Cobra Snow Country (20 sq in/ft vs 14 sq in/ft) to fit more capacity in less length

## Install Requirements

- Cut a 1-inch slot per side along the ridge with a circular saw set to 1 inch of depth. Stop 6-12 inches from each gable end so wind cannot drive rain under the vent.
- Ridge vent should extend the entire length of the slot, then cap with 12-inch ridge cap shingles.
- The ridge vent must be compatible with the roof pitch. Most products work from 3:12 to 16:12. Low-slope roofs (below 3:12) need specific low-slope ridge vents.
- Nail with roofing nails through the vent, through the underlayment, into the decking. Use the nail length specified on the ridge vent spec sheet; too short and the vent lifts in wind.
- Do not install ridge vent with gable vents active. Block off gable vents from the inside (foam board and sealant) when you install a ridge vent.

## Common Ridge Vent Mistakes

- **Skipping the slot cut.** A ridge vent over uncut decking does nothing. Cut 1 inch of slot on each side of the ridge board.
- **Running the slot all the way to the gable.** Stop 6-12 inches short on each end to prevent wind-driven rain.
- **No soffit baffles.** Batt insulation in the eave space blocks soffit airflow. Install foam or cardboard baffles at every rafter bay.
- **Mixing with gable or turbine vents.** Ridge vents short-circuit when paired with other exhaust types. Pick one exhaust strategy and commit to it.

## Frequently Asked Questions

**Q: How do I calculate how much ridge vent I need?**

A: Start with attic floor area in square feet. Divide by 300 (balanced system or with vapor retarder) or 150 (unbalanced) to get net free area in square feet. Multiply by 144 to convert to square inches. Take half of that for ridge exhaust. Divide by your ridge vent product's NFA per linear foot to get linear feet needed. Example: a 1,500 sq ft attic at 1:300 needs 720 sq in total, 360 sq in exhaust, divided by 18 sq in/ft = 20 linear feet of ridge vent.

**Q: What is the NFA of common ridge vent products?**

A: GAF Cobra Snow Country is 20 sq in/ft, GAF Cobra Exhaust Vent and Air Vent ShingleVent II are 18 sq in/ft, Owens Corning VentSure is 17 sq in/ft, Lomanco OR-4 is 14 sq in/ft, and CertainTeed Filtered Ridge Vent is 12.5 sq in/ft. Always verify on the product cutsheet because manufacturers update specs annually.

**Q: How much soffit intake do I need with ridge vent?**

A: Match or slightly exceed ridge exhaust NFA. For a 1,500 sq ft attic with 360 sq in of ridge exhaust, you need 360 sq in of soffit intake. Continuous aluminum soffit provides 9 sq in of NFA per linear foot, so 360 / 9 = 40 linear feet of soffit. Perforated vinyl soffit panels only provide 3-5 sq in/ft, so you would need 72-120 linear feet of perforated soffit to match.

**Q: Can I install ridge vent with gable vents?**

A: No. Ridge vents and gable vents short-circuit each other. Wind pressurizes one gable vent, which pushes air across the attic and out the other gable, skipping the ridge entirely. Block off gable vents from the inside (1-inch foam board and sealant) before installing ridge vent. Pair ridge vent only with soffit intake.

**Q: What is the minimum roof pitch for ridge vent?**

A: Most ridge vents require a minimum pitch of 3:12 to work correctly. Below 3:12 the ridge vent cannot drive convective airflow and can leak in wind-driven rain. Some manufacturers offer low-slope ridge vents rated for 2:12 roofs. Above 16:12 the vent may need custom flashing. Check the installation instructions for your specific product.

**Q: How long does a ridge vent last?**

A: A properly installed ridge vent lasts as long as the shingles it sits under, typically 25-30 years for asphalt architectural shingles. Plastic ridge vents can fade and become brittle from UV exposure in 15-20 years in high-sun climates. Metal ridge vents (copper, aluminum) last 40+ years but cost 3-5x more. Replace ridge vent whenever you reroof.

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Source: https://vastcalc.com/calculators/construction/ridge-vent
Category: Construction
Last updated: 2026-04-08