Apply IRC attic ventilation minimum net free area requirements (1/150 vs. 1/300 rule) for a re-roof or attic remodel

Steps

1. Calculate the baseline minimum net free ventilating area as 1/150 of the vented attic/rafter space area unless a code-recognized exception applies.
2. Check whether the project qualifies for the reduced 1/300 ratio exception, which under the current IRC requires specific conditions to be met together (such as a vapor retarder on the warm-in-winter ceiling side in colder climate zones, or a defined split of ventilation area between upper and lower locations) — confirm the exact current-edition conditions rather than assuming 1/300 always applies with a simple balanced vent split.
3. If claiming the balanced-ventilation path, verify the required portion of ventilating area (per the current code language) is located in the upper portion of the attic/rafter space, with upper vents positioned within the specified distance below the ridge, and the remaining area in the lower portion of the space.
4. Confirm net free area (NFA) values used in the calculation come from the actual vent product's tested NFA rating, not its physical opening size, since manufacturers publish NFA per unit that is smaller than the raw cutout.
5. Check for local amendments to R806 (some jurisdictions/climate zones modify the ventilation ratio or unvented-attic-assembly allowances) before finalizing the ventilation plan.