Ridge Vent vs Roof Turbines vs Gable Vents

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Ridge Vent

A continuous slot at the peak, covered with a baffled, shingle-over strip. Air Vent ShingleVent II (12 in) and GAF Cobra Rigid Vent 3 both publish 18 sq in NFA per linear foot. Narrower 7–9 in versions drop to about 16. Exhaust is spread along the whole ridge, which is what the 40–50% high / within-3-ft language in R806.2 is describing.

Wins: even wash along the sheathing, low profile, no spinning parts, easy to hit the high-vent location rule. Losses: you need a ridge (hips-only roofs are awkward), you must cut the sheathing slot correctly, and snow-prone roofs want an externally baffled unit that is actually rated for weather. Ridge vent with stuffed soffits is just a slot in the cap.

Roof Turbines (Whirlybirds)

A 12-inch turbine is a point exhaust. NFA is whatever the manufacturer stamped — often in the 50–80 sq in range, not the 113 sq in of a 12-inch circle. They move more air when the wind blows and sit there when it does not. Winter moisture does not wait for a breeze.

Wins: cheap, obvious, works on hips where a ridge slot is ugly or absent, no long cap detail. Losses: point source (dead bays between turbines), moving parts that seize, easy to over-exhaust a house with no intake, and they fight a ridge vent if both are installed. Count NFA, then space them so you actually cover the attic, still paired with soffit intake.

Gable Vents

Louvered triangles or rectangles in the gable wall. Face size is a lie; the NFA after screen and louvers is the number that matters, and it is often roughly half the hole. They provide cross-flow on a simple gable when the wind cooperates, and they sit well below the ridge — which can fail the “within 3 ft of the ridge” clause if you were counting on them for the 1:300 exception.

Wins: cheap retrofit on an existing gable, no roof cutting, some net free area you can actually see. Losses: they short-circuit a ridge vent, they do little for a hip roof, they do not wash the underside of the sheathing the way a ridge does, and a pair of pretty gables can still lose a 1:150 calc once you read the stamp.

Do Not Mix Two Exhaust Types

Ridge + gable, ridge + turbines, turbines + gable: air takes the shortest path between two openings. The rest of the attic stagnates. Pick one exhaust family, size it to the exhaust NFA from the calculator, and put equal or greater NFA in the soffits. If you are reroofing, that is the moment to pull the extra gable louvers or the random box vents and commit to a ridge — then confirm intake is open, not painted shut.

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