Kentucky Roof Code · Ventilation
Attic ventilation code in Kentucky starts at one square foot of net free vent area for every 150 square feet of attic. The widely quoted 1/300 figure is an exception, not the rule — and in Kentucky only one of the two ways to earn it is actually available.
What is the baseline attic ventilation requirement?
One to 150. Section R806.2 of the residential code sets the minimum net free ventilating area at 1/150 of the area of the vented space.
Two words in that sentence do the work. “Net free” means the open area after the louver blades, insect screen and baffle are subtracted — not the size of the hole cut in the sheathing, and not the outside dimensions of the vent. Manufacturers publish net free area per unit; use that figure, not the vent’s footprint. And “the vented space” is the attic or rafter space being ventilated, measured separately for each enclosed space.
Section R806.1 is the one that catches finished rooms. Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters must each have cross ventilation, for each separate space. A cathedral ceiling, a bonus room over a garage and a dormer knee-wall cavity are each their own vented space with their own requirement — which is why a house can have a well-vented main attic and a rotting roof deck over one room.
When does the 1/300 exception apply?
Only when one of two listed conditions is met. One requires Climate Zone 6, 7 or 8. The other requires a specific split between high and low vents.
The first condition is a Class I or II vapor retarder installed on the warm-in-winter side of the ceiling, in Climate Zones 6, 7 and 8. Table N1101.7, which the code uses to assign climate zones by state and county, lists Kentucky as Zone 4A for all counties. So that route is not available anywhere in the Commonwealth. In Kentucky, the vapor-retarder path to 1/300 does not exist.
The second condition is the one Kentucky roofs actually use: 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, positioned 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. Where wall or roof framing conflicts with placing the upper ventilators, the code permits installation more than 3 feet below the ridge.
In Kentucky there is exactly one route to the 1/300 allowance: 40 to 50 percent of the vent area up high within three feet of the ridge, and the balance at the eaves. Miss the split and the requirement is 1/150.
What does that look like in numbers?
Worked from the code, for a 1,500-square-foot vented attic. At 1/150 the requirement is 10 square feet of net free area; at 1/300 it is 5.
Ten square feet is 1,440 square inches of net free area. Five square feet is 720 square inches — of which 40 to 50 percent, or 288 to 360 square inches, must sit in the upper portion within three feet of the ridge, and the remaining 360 to 432 square inches must come from eave or cornice vents. These are arithmetic from the code ratios applied to a stated attic size, not measurements of any particular house; the real number depends on the attic area your own roof encloses.
The practical consequence is that a roof with ridge vent and no functioning soffit intake does not qualify for the exception at all, because the balance has to come from the eaves. It is held to 1/150, and it almost never meets it.
| Requirement | Code figure | Section |
|---|---|---|
| Baseline net free ventilating area | 1/150 of the vented space | R806.2 |
| Reduced area, with a condition met | 1/300 of the vented space | R806.2 Exception |
| Vapor-retarder route to 1/300 | Climate Zones 6, 7 and 8 only — Kentucky is Zone 4A | R806.2 Exception; Table N1101.7 |
| Upper ventilator share for the 1/300 route | Not less than 40% and not more than 50% | R806.2 Exception |
| Upper ventilator position | Not more than 3 feet below the ridge or highest point | R806.2 Exception |
| Vent opening least dimension | 1/16 inch minimum, 1/4 inch maximum | R806.1 |
| Clearance between insulation and sheathing | Not less than 1 inch | R806.3 |
Why do openings have a maximum size?
To keep weather and pests out. R806.1 sets vent openings at a least dimension of 1/16 inch minimum and 1/4 inch maximum, and requires larger openings to be screened.
Where an opening’s least dimension exceeds 1/4 inch, the code requires corrosion-resistant wire cloth, hardware cloth or similar material with openings in that same 1/16-to-1/4-inch range. “Corrosion-resistant” is the word that fails first in practice — plain steel screen behind a gable louver rusts out and the opening reverts to unscreened. The section also requires that openings be protected against the entrance of rain or snow, and that required ventilation openings open directly to outside air. A soffit vent that opens into a vented-and-then-boxed porch ceiling does not.
What makes a correctly sized vent stop working?
Insulation. R806.3 requires not less than a 1-inch space between the insulation and the roof sheathing, and states insulation shall not block the free flow of air.
This is the most common finding on a Central Kentucky attic that was air-sealed or re-insulated without baffles: the blown insulation is pushed out over the top plate, fills the eave, and the soffit intake measured on paper is zero in fact. Net free area the air cannot reach is not net free area. Our note on soffit vents and intake ventilation covers how that failure reads from the ground and from inside.
The code also recognises unvented assemblies at R806.5, permitted where every one of a list of conditions is met — which is a different design, not a fallback for a vented attic that fell short.
Which code book applies to the job?
Kentucky enforces its own statewide edition of the residential code, published by the Department of Housing, Buildings and Construction.
The authority is the Kentucky Residential Code as published by the Department of Housing, Buildings and Construction, and the International Code Council publishes the adoption record for the state. Which edition governs a particular permit is the first question on any code argument, and our note on which rulebook applies in Lexington covers how that is settled locally.
What I look at first
Working practice, not a diagnosis of any particular roof: on a Central Kentucky attic complaint — a stain with no leak, frost on nail points in January, shingles aging unevenly on one slope — the ventilation measurement comes before any talk of a roof covering, because a covering installed over a ventilation defect fails the same way the last one did. I count the vented spaces first, not the vents: each enclosed rafter space counts separately under R806.1, and a house with a cathedral ceiling and a bonus room has three requirements, not one. Then I check whether intake exists at all, because the 1/300 allowance depends on the eaves and almost every shortfall I find is on the intake side.
Is the attic ventilation requirement in Kentucky 1/150 or 1/300?
The baseline in R806.2 is 1/150 of the vented space. 1/300 is an exception that applies only when one of the listed conditions is met.
Can a Kentucky house use the vapor-retarder route to 1/300?
That condition applies in Climate Zones 6, 7 and 8. Table N1101.7 lists Kentucky as Zone 4A for all counties, so that route is not available here.
Does a ridge vent alone satisfy the code?
Not for the 1/300 exception, which requires the balance of the ventilating area to come from eave or cornice vents. Without intake, the attic is held to the 1/150 baseline.
Last updated: October 27, 2026
By Central Roof Repair · Call or text (859) 310-1209.
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