Attic Ventilation Guide: The 1:300 Rule and Why Most Roofs Break It
Why 8 out of 10 roofs are under-vented — and how the 1:300 rule, ridge vent geometry, and balanced intake add 5–10 years of shingle life.
Under-ventilated attics cook shingles from below, causing curling and premature granule loss. IRC requires 1 sq ft of net free vent area per 300 sq ft of attic floor, split evenly between intake and exhaust.
The math, quickly
A 2,000 sq ft attic needs 6.67 sq ft (960 sq in) of net free area. Half at the soffit (intake), half at the ridge (exhaust). Most homes have working exhaust and blocked intake — insulation baffles fix this in an afternoon.
Never mix exhaust types
Ridge vent + gable vent = short-circuited airflow. Ridge vent + power vent = same problem. Pick ONE exhaust type per attic and seal the others.
Signs of failed ventilation
Attic over 130°F on a 90°F day. Rusted nail heads on the underside of the deck (condensation). Curled shingles on south/west slopes at year 10 instead of 20. Ice dams every winter.
Frequently Asked Questions
- Are solar attic fans worth it?
- Rarely. They move less air than passive ridge vent for 10x the cost, and they can depressurize the attic and pull conditioned air from the house.
- Does ridge vent work without soffit intake?
- No. Ridge vent needs intake to work. Without it, it pulls air backward through itself in winter, drawing moisture into the attic.
Add years to your roof — before you spend a dollar on replacement
Estimate how much life you have left, price small repairs, and see whether prevention beats replacement in your situation.