By The Roofing Brief Team. Last reviewed: August 2026.
The intake vs exhaust ventilation ratio is the split of a roof’s total net free area between low intake vents and high exhaust vents, and the widely quoted 60/40 version of it is a recommendation from the Home Ventilating Institute, not a building code requirement. Every major vent manufacturer we checked publishes a different number: 50/50, with intake allowed to run higher. The ratio matters because a system can hit its total net free area target exactly and still fail, and the failure is physical, not paperwork.
What the intake vs exhaust ventilation ratio actually measures
The ratio compares intake net free area at the eaves against exhaust net free area at or near the ridge, both in square inches. It is not a comparison of vent counts, vent lengths, or CFM. A 32 foot ridge vent and forty 8 inch soffit slots are only comparable once each is converted to net free area using the manufacturer’s published rating.
Net free area is the open, unobstructed area air can actually pass through after louvers, screens, and baffles subtract from the gross opening. For how net free area is defined and how per-product ratings are published, see our net free area calculation guide. This article covers only the balance question: how that area should be divided between the two halves.
Where the 60/40 rule comes from and why it is not code
The 60/40 figure traces to the Home Ventilating Institute, which advises that “60 percent of the vent net free area is provided by intake vents placed at the under-eave area and 40 percent of the vent net free area is provided by exhaust vents on the roof, at the ridge or high in the gable area.” That is an industry recommendation from a trade body. No section of the International Residential Code states a 60/40 ratio.
What the IRC does contain is narrower and often misquoted. Section R806.2 sets the baseline vent area at 1/150 of the vented space, and the exception that reduces it to 1/300 requires that “not less than 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the attic or rafter space,” with those upper vents “not more than 3 feet below the ridge or highest point of the space, measured vertically, with the balance of the required ventilation provided by eave or cornice vents.” Read plainly: the code caps exhaust at 50 percent of the required area and leaves 50 to 60 percent for the eaves, but only as a condition of taking the reduced area. For the sizing math behind that reduction, see our breakdown of the 1/300 attic ventilation rule.
Here is what four named sources actually publish, which is the part most articles skip.
| Source | Published guidance on the split | Is it code? |
|---|---|---|
| Home Ventilating Institute, static attic ventilation guidance | 60 percent intake at the under-eave area, 40 percent exhaust at the roof | No. Industry recommendation |
| IRC Section R806.2, exception (2021 and 2015 text identical) | 40 to 50 percent of required area in the upper portion, balance at eave or cornice vents | Yes, but only as a condition for the 1/300 reduction |
| Air Vent, Principles of Attic Ventilation | “The net free area of intake venting should be equal to or greater than the net free area of exhaust venting.” Half high, half low | No. Manufacturer guidance |
| GAF, Cobra Exhaust Vent for Roof Ridge data sheet RESCB137 (2025) | “The amount of exhaust ventilation must never exceed the amount of intake ventilation at the soffits or eaves” | No. Manufacturer guidance |
| Owens Corning, VentSure rigid roll application instructions | Equal at soffit and ridge. Where balance cannot be achieved, “always provide more than 50 percent of the total required ventilation at the soffit” | No. Manufacturer guidance |
| Lomanco, OmniRoll OR-20 installation instructions | “50% Intake and 50% Exhaust is a balanced system.” Install only one type of exhaust | No. Manufacturer guidance |
Not one of those manufacturer documents prints “60/40.” They print 50/50 with a floor under intake. The practical effect is close to identical, because both formulations forbid the same thing: exhaust net free area larger than intake net free area. Treat 60/40 as a safety margin on a 50/50 target, not as a code number a jurisdiction will cite.
What balanced means in square inches
Balanced means the intake side, measured in square inches of net free area, equals or exceeds the exhaust side. Percentages are a shorthand for that comparison and nothing more. A roof described as “60/40” is simply one where intake square inches are 1.5 times exhaust square inches.
Air Vent’s technical booklet states the consequence of getting it wrong in one line: “Without that balance, the area of effective ventilation is limited to the lesser of the two vent areas. For example, if 75 percent of the venting is high and 25 percent low, ventilation is limited to the air moving through the lower vents.” The smaller half sets the flow. Adding exhaust to a starved system adds nothing to airflow, because there is no additional air to move.
What happens when exhaust net free area exceeds intake
When exhaust exceeds intake, the exhaust vents cannot draw enough make-up air through the soffits, so they pull it from whatever opening is closest. In practice that is another roof vent, the exhaust vent itself, or the ceiling below. Air follows the path of least resistance, and the soffit is rarely the easiest path once the eaves are undersized or blocked.
Three failure paths follow, and they can occur together:
- Short circuit at the roof plane. A ridge vent or a second exhaust vent starts acting as an intake. Air enters high and leaves high a few feet away, so the lower attic and the deck near the eaves see almost no airflow. Lomanco’s OmniRoll instructions describe the mechanism directly: exhaust vents “pull air from the easiest Intake source,” which can “make one of these vents act as intake instead of pulling air from the soffit vents.”
- Weather ingestion. A vent designed to move air out is not detailed to keep rain and snow out when air moves in. Lomanco states that “improper intake will usually lead to snow or rain infiltration into the exhaust vents.” Paul Scelsi of Air Vent, writing in Professional Roofing, described the same outcome for ridge vents running short on intake: excess exhaust “could pull in weather.”
- Attic depressurization. With intake short, the attic can sit at a slight negative pressure relative to the house, and make-up air is then drawn through ceiling penetrations: recessed light housings, top plate gaps, plumbing chases, and the attic hatch. In winter that carries indoor humidity into a cold attic, which is the opposite of what the system exists to do. Scelsi noted that an oversized power attic fan can force the system to draw air from living spaces and may cause premature motor burnout.
None of these show up in a total net free area calculation. A roof can pass the total and fail all three.
Worked example: an attic that passes total net free area and fails on balance
This is a 1,800 square foot attic on a simple gable, vented with a continuous ridge vent, two gable louvers left in place from the original build, and continuous soffit venting. Net free area figures are the published ratings: 16.9 square inches per linear foot for the hand-nailable GAF Cobra Exhaust Vent for Roof Ridge, and Air Vent’s net free area table for the louvers and soffit venting.
| Component | Quantity | Published NFA | Total sq in | Role |
|---|---|---|---|---|
| Cobra Exhaust Vent for Roof Ridge, hand-nailable | 32 linear ft | 16.9 sq in per lin ft | 540.8 | Exhaust |
| Rectangular gable louvers, 12 in x 18 in | 2 | 82 sq in each | 164.0 | Exhaust |
| Continuous soffit vent | 40 linear ft | 9 sq in per lin ft | 360.0 | Intake |
| Total installed | 1,064.8 |
At 1/300, this attic requires 864 square inches of net free area (1,800 divided by 300, times 144). The installed total is 1,064.8 square inches, roughly 23 percent above requirement. On a total-area check it passes comfortably, and that is exactly how these roofs get signed off.
The balance tells a different story. Exhaust is 704.8 square inches and intake is 360, so the split is 66 percent exhaust and 34 percent intake. Against the 60/40 recommendation it is inverted. Against IRC R806.2, the upper portion may hold between 346 and 432 square inches of the 864 required, and this roof has 704.8 up high. The distribution condition fails, so the 1/300 reduction is not available and the applicable baseline reverts to 1/150, or 1,728 square inches. The installed 1,064.8 now falls about 38 percent short. One balance error moves the roof from 23 percent over to 38 percent under.
Airflow is worse than either number suggests, because the effective ventilation is limited to the smaller half: 360 square inches, about 42 percent of what the 1/300 target assumed. Meanwhile the ridge vent and the gable louvers are competing, and the louvers are the easier air source, so the top of the attic short circuits while the eaves stay still.
The correction is sequential, and it starts by removing exhaust rather than adding it:
- Seal the two gable louvers. Exhaust drops from 704.8 to 540.8 square inches and the roof is back to one exhaust type. Owens Corning’s VentSure instructions give this step as a flat directive: “Close off all gable and other roof vent openings.”
- Measure the real intake before buying anything. Confirm the 40 linear feet of soffit vent is open, not painted shut, and not buried under insulation.
- Add intake until it exceeds exhaust. Intake is 180.8 square inches short. Four 16 in x 8 in undereave vents at 56 square inches each add 224, bringing intake to 584 square inches.
- Install eave baffles so blown insulation cannot close the path you just paid for. IRC R806.3 requires at least a 1 inch space between insulation and roof sheathing at the vent.
- Recheck the split. Total is now 1,124.8 square inches, with 48 percent up high and 52 percent low. That sits inside the code’s 40 to 50 percent upper window and satisfies every manufacturer rule in the table above.
Why mixing exhaust types is the fastest way to break the ratio
Most balance failures are not sizing errors. They are accumulation: a ridge vent added during a reroof while the original gable louvers or box vents stay in place, or a powered fan installed later to fix a hot upstairs. Each addition raises exhaust net free area without touching intake, so the ratio drifts the wrong way and a second short circuit opens at the same time.
| Combination on one attic | What happens | Published guidance |
|---|---|---|
| Ridge vent plus gable louvers | The ridge draws make-up air from the gable louvers instead of the soffits. The lower attic and eave area see little movement | Owens Corning VentSure: close off all gable and other roof vent openings |
| Ridge vent plus powered attic fan | The fan pulls make-up air down through the ridge vent, reversing it into an intake, and can draw conditioned air out of the house | Air Vent, Professional Roofing: excess exhaust can force a power fan to draw from living spaces and may shorten motor life |
| Box vents or turbines at two different heights | The higher vent pulls from the lower vent rather than from the soffit | Lomanco: install all exhaust ventilation at the same height within a common attic area |
| Any two exhaust types together | One of the two reverses and becomes an intake, leaving part of the attic unventilated | Lomanco: install only one type of exhaust ventilation within a common attic area |
The pairings that create the most trouble in the field are covered in detail in our comparisons of soffit vents vs gable vents and ridge vents vs power attic fans. The rule that resolves all of them is the same: pick one exhaust type, then size intake to match or beat it.
Why installers are told to err toward more intake, never more exhaust
Surplus intake is close to harmless. Surplus exhaust is not. Extra intake area simply does not admit extra air once the exhaust side is saturated, and on a windward wall the surplus tends to leave through the leeward eave, so the attic sits at neutral or slightly positive pressure. Extra exhaust, by contrast, has to find make-up air somewhere, and every available source is a defect.
Scelsi’s guidance in the Asphalt Roofing Manufacturers Association’s ventilation article, revised September 2022, puts the asymmetry plainly: where balance cannot be achieved it is better to have more intake than exhaust, because most attics lack proper intake, which he identifies as the leading cause of ventilation callbacks. That is also why 60/40 became the contractor shorthand. It builds a 10 point cushion into a 50/50 rule on the side where the error is cheap.
Intake is also the half that degrades over time. Soffit vents get painted, stuffed with blown insulation, blocked by a new bathroom fan run, or covered when a soffit is re-clad. Exhaust rarely shrinks. A roof commissioned at exactly 50/50 can drift to 40/60 within a decade without anyone touching the ridge, which is the practical argument for starting with an intake surplus rather than a tie.
How to check your own intake vs exhaust ratio
Checking the ratio takes a tape measure, the vent manufacturers’ published net free area figures, and about an hour. Do it in this order, because the answer usually changes at step 4.
- Measure the attic floor area and calculate the required total net free area at 1/300 and at 1/150 so you know both targets.
- Inventory every exhaust opening. Ridge vent length, box vents, turbines, gable louvers, powered fans. Count anything that is currently open, including vents someone assumed were abandoned.
- Convert each to square inches using the specific product’s published rating, not a generic figure. Ridge vent ratings vary widely: GAF publishes 16.9 square inches per linear foot for the hand-nailable Cobra Exhaust Vent and 14.1 for the nail-gunnable version of the same product.
- Inventory intake the same way, then walk the attic and subtract anything blocked. Insulation over the top plate is the usual culprit, and blocked intake counts as zero regardless of what is on the spec sheet.
- Compare the two totals. If exhaust is larger, remove exhaust first, then add intake. Adding exhaust to a system already short on intake makes the short circuit worse.
- Verify against the code condition if you are relying on 1/300: upper vents must supply 40 to 50 percent of the required area and sit within 3 feet of the ridge, measured vertically.
For the wider system context, including how the two halves work as a loop, see our roof ventilation overview.
Frequently asked questions
Is the 60/40 intake to exhaust ratio required by building code?
No. The 60/40 split is a recommendation from the Home Ventilating Institute and a contractor rule of thumb. IRC Section R806.2 contains no 60/40 requirement. Its only distribution rule appears in the exception allowing 1/300 vent area, which requires 40 to 50 percent of the required area in the upper portion of the attic, with the balance at eave or cornice vents.
What is the correct intake to exhaust ventilation ratio?
Target intake net free area equal to or greater than exhaust net free area. Air Vent, GAF, Owens Corning, and Lomanco all publish 50/50 as the balanced condition, with intake permitted to run higher. The 60/40 version adds a margin on the intake side. Any split where exhaust square inches exceed intake square inches is the one to avoid.
What happens if you have more exhaust than intake?
The exhaust vents cannot draw enough make-up air from the soffits, so they pull it from the nearest opening instead. That is often another roof vent, the exhaust vent itself, or ceiling penetrations below. The result is a short circuit that leaves the lower attic unventilated, possible rain or snow entry through exhaust vents, and an attic that can depressurize relative to the house.
Can you have too much intake ventilation?
Practically, no. Extra intake area does not admit more air than the exhaust side can move, and surplus intake tends to vent out the leeward eave, leaving the attic at neutral or slightly positive pressure. Manufacturers treat intake surplus as acceptable and intake shortfall as a defect, which is why guidance always errs toward more intake.
Can you mix a ridge vent with gable vents or a powered fan?
Manufacturers advise against it. Lomanco instructs installers to use only one type of exhaust ventilation within a common attic area, because exhaust vents pull from the easiest intake source and one vent will reverse into an intake. Owens Corning directs installers to close off all gable and other roof vent openings when a ridge vent is used.
How do I calculate my intake to exhaust ratio?
Convert every vent to square inches of net free area using its published rating, total the exhaust openings and the intake openings separately, then divide each by the combined total. A 32 foot ridge vent at 16.9 square inches per linear foot gives 540.8 square inches of exhaust, which needs at least 540.8 square inches of open intake to match it.
Does a roof pass code if it meets total net free area but not the split?
Not if it relies on the 1/300 reduction. IRC R806.2 makes that reduction conditional on 40 to 50 percent of the required area sitting in the upper portion of the attic. Fail the distribution condition and the applicable baseline returns to 1/150, which doubles the required area and usually turns a passing total into a failing one.