What do the IRC attic ventilation requirements say that people usually miss?

Each separate space counts on its own

Section R806.1 requires ventilation for each separate enclosed attic and each enclosed rafter space. An attic over a garage divided from the house attic by a fire wall is its own space. So is every rafter bay over a vaulted or cathedral ceiling where drywall is fastened to the rafters. Each one needs its own intake and exhaust, which is why vaulted rooms so often have ventilation problems.

The 1/300 option has a ceiling, not just a floor

Section R806.2 allows the smaller 1/300 ratio when at least 40 percent and no more than 50 percent of the required vent area is in the upper part of the attic, no more than 3 feet below the ridge measured vertically. Put too much area up high, and the exception no longer applies. That is a design rule that keeps exhaust from overpowering intake.

Opening sizes

Openings must be at least 1/16 inch and no more than 1/4 inch in their least dimension. Anything larger needs corrosion resistant screen, hardware cloth or perforated material with openings in that range. That keeps out wasps, birds and squirrels while staying open enough not to clog with pollen and dust.

Insulation clearance

Section R806.3 requires at least a 1 inch air space between insulation and the sheathing at the vent locations, and insulation cannot block the airflow. Baffles are the usual way to meet it.

Installation follows the manufacturer

Section R806.4 points to the vent maker's instructions, so slot widths and nailing schedules become code requirements. Georgia enforces the 2024 IRC with state amendments as of January 1, 2026. More in roof ventilation basics and roofing permits in Georgia.