By The Roofing Brief Team. Last reviewed: October 2026.
A gable vent is a louvered opening set into the triangular gable end wall near the peak that lets attic air in or out on wind pressure, and it is sized by net free area (NFA): at least 1/150 of the attic floor, or 1/300 when the 2021 IRC R806.2 exception is met. Gable vents are wind driven: air enters the vent on the windward gable and pushes hot, stale air out the vent on the opposite gable. A pair acts as intake and exhaust at the same time, which is why Gibraltar Building Products tells owners to close gable vents whenever a ridge vent is installed.
Gable vent quick answers
| Question | Short answer | Source |
|---|---|---|
| What is a gable vent? | A louvered wall vent in the gable end, near the ridge, venting the attic behind it | Lomanco gable vent line |
| Intake or exhaust? | Both: the windward vent takes air in, the leeward vent exhausts, and the roles flip with the wind | Wind driven cross flow, see How does a gable vent work below |
| What size gable vent do I need? | Total NFA of 1/150 of attic floor area, or 1/300 when 40 to 50% sits in upper vents within 3 ft of the ridge | 2021 IRC R806.2 |
| NFA of a 12 x 18 in gable louver | 79 sq in (Lomanco 150 Series) to 90 sq in (Lomanco 200 Series) | Lomanco gable vent catalog |
| Gable vents with soffit vents? | Yes, when the gables are the only exhaust; close them if you add a ridge vent | Gibraltar Building Products |
What is a gable vent?
A gable vent is a louvered opening mounted high in the vertical gable wall, the flat triangular section of siding under the two roof slopes of a gable roof. It vents the attic behind the wall rather than the roof deck itself, supplying the cross ventilation IRC R806.1 asks of each enclosed attic space. Because it sits on a vertical surface under the rake overhang, it needs no shingle cut and no roof penetration, unlike a ridge or box vent.
What a gable vent looks like comes down to four shapes: rectangular, square, round, or a triangle cut to the roof pitch. ALV Company, for example, sells aluminum triangle gable vents from a 30 in base at 8/12 pitch to a 96 in base at 4/12, with J-channel flanges recommended for vinyl, aluminum or steel siding and a standard nailing flange for brick, stucco or fiber cement (ALV triangle gable vent listing, October 2026). Behind the visible louvers sits an insect screen; IRC R806.1 limits vent openings to a least dimension of 1/16 in to 1/4 in, or requires corrosion resistant wire cloth with openings in that range. Decorative gable vents look the same from the street but are backed by solid board, so they have zero NFA.
How does a gable vent work?
A gable vent works on wind pressure, not the stack effect that drives ridge and soffit systems. When wind hits the gable wall, it forces outside air in through that vent, raises the pressure inside the attic, and pushes warm air out the vent on the far gable. Which vent is intake and which is exhaust flips with the wind direction, which is why the Gable Vent Exhaust System chart in Lomanco’s catalog counts gable louvers in matched pairs, one per gable.
The rule that follows: a gable pair ventilates well only when wind blows across the gables, and moves little air when wind runs parallel to the ridge or the air is still. A balanced roof ventilation system with soffit intake and ridge exhaust runs on the temperature difference instead, so it keeps working on still days, which is why IRC R806.2 ties the reduced 1/300 ratio to upper vents within 3 ft of the ridge plus low vents for the balance.
Types of gable vents
Gable vents fall into three groups: static louvered vents, powered gable fans, and decorative closed units.
| Type | How it moves air | Best use |
|---|---|---|
| Static louvered vent | Passive, wind driven cross flow between two gable ends | Simple gable roofs with soffit intake and no other roof exhaust |
| Powered gable fan | Electric or solar fan pulls attic air out through the gable | Hot attics needing forced exhaust, paired with dedicated intake |
| Decorative gable vent | None, backed by solid board | Aesthetic gable accent only, no ventilation value |
Static vents come in aluminum, vinyl, and wood; Lomanco builds its 100, 200, 300, 900 and A-Series gable louvers entirely of heavy gauge aluminum, and its 900 Series adjusts to roof pitches from 2-1/2/12 to 9/12 (Lomanco). Powered gable fans mount behind an existing gable opening: the Lomanco Add-A-Vent 1800 is designed to mount behind an existing gable louver on 16 in or 24 in on-center framing, so no shingles are cut and no roof penetration is made.
What size gable vent do you need? Net free area and the 1/300 rule
Size a gable vent by net free area (NFA), the open square inches left after louvers and insect screen block part of the opening, not by the frame size. In Lomanco’s catalog a 12 x 18 in hole (216 sq in gross) rates 79 sq in NFA in the 150 Series and 90 sq in in the 200 Series, so only about 37% to 42% of the hole is usable airflow. Our guide to the net free area calculation shows how to turn a rating into a vent count.
The 2021 IRC (section R806.2) sets the target: total net free ventilating area of at least 1/150 of the vented attic area, reduced to 1/300 under the exception, where 40 to 50 percent of the required area sits in upper ventilators no more than 3 feet below the ridge, measured vertically, with the balance low in the attic, and, in Climate Zones 6, 7 and 8, a Class I or II vapor retarder sits on the warm side of the ceiling. Gable vents mounted at the peak count as those upper ventilators. Split the total roughly 50/50 between low intake and high exhaust, and check which IRC edition and local amendments your jurisdiction enforces.
Gable vent sizing: worked example
Worked example for a 1,500 square foot attic:
- Divide attic area by 300: 1,500 / 300 = 5 square feet of total NFA, which is 720 square inches (at the 1/150 default it doubles to 1,440 square inches).
- Split 50/50: about 360 square inches of intake at the soffits and 360 square inches of exhaust high in the attic.
- If gable vents are the only exhaust, the two gables must supply that full 360 square inches between them, so 180 square inches per end.
- Pick from the maker’s NFA column: a pair of Lomanco 200 Series 18 x 24 in louvers gives 2 x 190 = 380 square inches, which clears 360; a pair of 12 x 18 in units gives only 2 x 90 = 180 square inches.
That shortfall is the usual reason gable only ventilation falls short on bigger attics: a 12 x 18 in pair covers roughly a 750 sq ft attic at 1/300 (180 sq in of exhaust is half of 360 sq in total NFA, and 360 / 144 x 300 = 750 sq ft). Lomanco’s own sizing chart is more generous and lists a 12 x 18 pair for a 1,000 sq ft attic, so where the chart and the code math disagree, size to the code math your inspector will check.
Gable vent sizes and NFA by model
NFA ratings below come from Lomanco’s gable vent catalog; other brands of the same nominal size rate differently.
| Model (hole size) | Overall size | NFA (sq in) | Source |
|---|---|---|---|
| Lomanco 100/200 Series 8 x 8 in | 11 x 11 in | 22 | Lomanco catalog |
| Lomanco 100/200 Series 12 x 12 in | 15 x 15 in | 56 | Lomanco catalog |
| Lomanco 150 Series 12 x 18 in | 14 x 20 in | 79 | Lomanco catalog |
| Lomanco 100/200 Series 12 x 18 in | 15 x 21 in | 90 | Lomanco catalog |
| Lomanco 100/200 Series 14 x 24 in | 17 x 27 in | 146 | Lomanco catalog |
| Lomanco 100/200 Series 18 x 24 in | 21 x 27 in | 190 | Lomanco catalog |
| Lomanco 100/200 Series 24 x 30 in | 27 x 33 in | 325 | Lomanco catalog |
| Lomanco 308-4 triangle (8 ft base, 4/12) | 16 in center height | 242 | Lomanco catalog |
| Lomanco 310-5 triangle (10 ft base, 5/12) | 25 in center height | 523 | Lomanco catalog |
Gable vent decision table
| If your attic is | Gable exhaust you need at 1/300 | Lomanco pair that meets it |
|---|---|---|
| Up to 1,000 sq ft, simple gable roof, soffit intake | 240 sq in total (120 per gable) | 14 x 24 in (146 each) or larger; Lomanco’s chart also lists a 12 x 18 pair here, though 2 x 90 = 180 sq in falls short of 240 |
| 1,001 to 1,500 sq ft | Up to 360 sq in total | 18 x 24 in (190 each); 14 x 24 is not recommended past 1,000 sq ft on Lomanco’s chart |
| 1,501 to 2,500 sq ft | Up to 600 sq in total | 24 x 30 in (325 each), the largest rectangular size Lomanco charts to 2,500 sq ft |
| Over 2,500 sq ft, hip roof, or several roof planes | Over 600 sq in, with dead zones the cross flow misses | Move to soffit to ridge venting or add a powered gable fan with dedicated intake |
| Already has a ridge vent with soffit intake | None from the gables | Close the gable vents from inside (Gibraltar guidance) |
Gable vent vs ridge vent
A gable vent exhausts through the end wall on wind pressure; a ridge vent exhausts along the roof peak on rising heat, which is the arrangement the IRC R806.2 exception describes when it puts upper vents within 3 feet of the ridge and the balance low at the soffit vents.
| Factor | Gable vent | Ridge vent |
|---|---|---|
| Airflow driver | Wind pressure across the gables | Stack effect, warm air rising to the peak |
| Consistency | Variable, depends on wind direction | Steady whenever the attic is warmer than outside |
| Coverage | Cross section between two ends only | Even exhaust along the entire ridge line |
| Complex roofs | Leaves dead zones on hips and multiple planes | Follows every ridge, few dead pockets |
| Leak risk | Low, mounted on protected vertical wall | Low when installed to spec with a slot cut |
On a simple gable roof with soffit intake and an attic under about 2,500 sq ft, a correctly sized gable pair can meet the 1/300 target using Lomanco’s charted sizes. On hip roofs or homes with several roof planes, gable vents leave warm, moist air trapped in pockets the cross flow never reaches. See our ridge vent installation guide for the exhaust path that avoids those dead zones. For what each vent swap costs, see roof vent replacement cost by type.
Should you seal gable vents if you have a ridge vent?
In most cases yes: Gibraltar Building Products’ ventilation help center says gable vents “should be closed off whenever a ridge vent … is installed,” because the eave or overhang vents should supply the ridge’s intake air. Running both at once short circuits the system: a ridge vent a few feet from an open gable pulls its make up air from that gable instead of drawing fresh air up from the soffits, so the lower attic gets bypassed.
The same Gibraltar Building Products page warns the gable vents then become intake for the ridge vent, which can let weather in through the gable louvers. Use this decision path:
- Gable vents plus soffit vents, no other roof exhaust: keep the gable vents as the exhaust and confirm total NFA meets the 1/300 target (720 sq in for a 1,500 sq ft attic).
- Adding a ridge vent with soffit intake: seal the gable vents from inside the attic with rigid foam board or plywood cut to the opening, and caulk or tape the edges, as Gibraltar Building Products advises closing gables under a ridge vent. Ridge plus soffit is the balanced pair.
- Installing a powered gable fan: give it dedicated intake through the soffits (Maxx Air’s 1,650 CFM model calls for 624 sq in of intake NFA) and keep any ridge vent sealed, or the fan pulls air straight from the ridge and starves the rest of the attic.
- Hip roof or complex roof: gable vents alone leave dead zones, so move to a soffit to ridge system or add forced exhaust.
For the full intake and exhaust math by attic size and climate zone, see our attic ventilation guide; the 1/150 and 1/300 ratios above come from IRC R806.2. For gable vents with soffit vents side by side, see our soffit vent vs gable vent comparison.
Powered gable vent fans: CFM sizing and cost
A powered gable fan mounts behind a gable opening and forces attic air out with an electric or solar motor. Size it by airflow in cubic feet per minute (CFM): Maxx Air’s sizing guidance is attic floor area x 0.7 for the minimum CFM, within a 0.7 to 1.0 CFM per square foot range when soffit intake is adequate (Maxx Air 1,600 CFM gable fan page). Without that intake, Maxx Air notes the fan may pull air from the home’s interior instead of outside.
| Attic floor area | Target fan CFM (0.7 to 1.0 per sq ft) | Source |
|---|---|---|
| 1,000 sq ft | 700 to 1,000 CFM | Maxx Air sizing FAQ |
| 1,500 sq ft | 1,050 to 1,500 CFM | Maxx Air sizing FAQ |
| 1,800 sq ft | 1,250 to 1,800 CFM | Maxx Air sizing FAQ |
| Gable fan model | Rated airflow or motor | Attic size or intake rating | Source |
|---|---|---|---|
| Maxx Air CX2500UPS gable fan | 1,650 CFM, 14 in galvanized blades, thermostat 50 to 120°F | Attics up to 2,500 sq ft; needs 624 sq in intake NFA | Maxx Air |
| Maxx Air Premium gable fan | 1,300 CFM, 14 in blades | Attics up to 1,850 sq ft | Maxx Air |
| Lomanco Add-A-Vent 1800 | 120 V, 3.4 A, 1/14 HP, 915 RPM; on at 100°F, off at 85°F | Mounts behind an existing gable louver; 5 year limited warranty | Lomanco power vents |
On cost, Fixr’s attic fan guide (updated August 26, 2022) puts the national average for an installed attic fan at $225 to $850, an average of $450 for a gable mounted electric fan with a thermostat, and about $75 to install a gable mounted passive vent (Fixr). Add electrician time if no circuit reaches the gable, and carpentry if the opening has to be cut or enlarged to fit the fan shroud.
Because a gable fan drives out summer heat, it can trim attic temperatures and cooling load in hot climates, and Maxx Air’s thermostat range of 50 to 120°F lets you set when it runs. The size of any saving depends on roof color, insulation level and region, as our cool roof energy savings report shows across climate zones.
Frequently asked questions
What is a gable vent?
A gable vent is a louvered opening in the triangular gable end wall, near the peak, that ventilates the attic behind it. It sits on a vertical wall, so it needs no roof penetration, and it is rated by net free area (NFA): a Lomanco 200 Series 12 x 18 in louver rates 90 square inches.
Do gable vents work with a ridge vent?
Not well together. An open gable vent near a ridge vent supplies the ridge with make up air, so the ridge stops pulling from the soffits and the lower attic goes under ventilated. Gibraltar Building Products says gable vents should be closed off whenever a ridge vent is installed, because the gables become intake and can let weather in.
Should I cover my gable vents?
Cover or seal gable vents only when they compete with a ridge vent or a powered fan. If your attic uses gable vents plus soffit vents and has no other roof exhaust, leave them open, as they are the exhaust. If you add a ridge vent, Gibraltar Building Products advises closing the gables; seal them from inside with rigid foam or plywood and sealed edges.
Are gable vents intake or exhaust?
Both, depending on wind direction. A gable pair works on cross flow: the windward vent acts as intake and the leeward vent as exhaust, and the roles flip when the wind shifts. That is different from soffit vents, which are dedicated intake, and ridge vents, which are dedicated exhaust in a balanced system and count as the upper vents within 3 feet of the ridge under IRC R806.2.
What size gable vent do I need?
Size by net free area. 2021 IRC R806.2 requires total NFA of 1/150 of attic floor area, or 1/300 when 40 to 50 percent sits in upper vents within 3 feet of the ridge. A 1,500 square foot attic at 1/300 needs 720 square inches total, so about 360 square inches of gable exhaust, which a pair of 18 x 24 in Lomanco 200 Series louvers (190 each) covers.
How much does a gable vent cost to install?
Fixr’s attic fan cost guide (updated August 2022) puts a gable mounted passive vent at about $75 installed and a gable mounted electric fan with a thermostat at an average of $450. Cutting a new opening or running a new circuit adds carpentry and electrician time.
Do gable vents let rain in?
Rarely under normal conditions. Gable vents sit on a vertical wall under the roof overhang, and the louvers and screen shed most wind driven rain. Water can enter when strong wind blows straight at the gable, or when an open gable vent turns into intake for a ridge vent, a risk Gibraltar Building Products flags. Sealing unused gable vents removes that path.