Be careful in the attic. It gets very hot. Walk only on joists or solid boards, never on insulation, and keep your hands off wiring. Wear a mask and gloves, and bring a flashlight.

What the code calls for

The International Residential Code (IRC, section R806) generally requires one square foot of net free vent area for every 150 square feet of attic floor. The ratio drops to 1:300 when low intake vents at the eaves and high exhaust vents provide the airflow, and a vapor retarder is installed on the warm side of the ceiling. Confirm the version your local building department enforces.

Step 1: Check the intake vents at the eaves

From outside, look at the soffit vents. Make sure they aren't painted over, screened over with debris, or covered.
From inside, check that insulation isn't blocking the soffits. Baffles (also called rafter vents) keep insulation back from the vent.

Step 2: Check the exhaust vents

Look for ridge vents, gable vents, or roof vents, and confirm none are blocked by insulation or debris.

Step 3: Check bath fans and dryer vents

Exhaust fans and dryer lines should vent to the outside, not into the attic. Moist air dumped into the attic is a common cause of mold on the roof deck.

Step 4: Look for signs of trouble

Dark staining or wet spots on the underside of the roof sheathing
A musty smell, or visible mold on framing or sheathing
Insulation that is matted, damp, or sagging
Rust on nails or metal fasteners on the roof deck

When to call a professional

Call a licensed roofer or HVAC contractor if you find wet or moldy sheathing, the vents are too few or too small for the attic, or you need new vents cut into the roof. They can calculate the net free area and recommend a fix.

Sources

This guide is based on the following sources. Requirements change, so check the originals and your local building department before you start.

IRC 2024 Section R806: Roof Ventilation Requirements (summary of code text)

This guide is general maintenance information, not a substitute for a licensed contractor's inspection. Follow the manufacturer's instructions and your local code requirements.