Ridge Vent Calculator

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.