ItsMyRoof
Ventilation
Balanced intake and exhaust, why exhaust without intake short-circuits, and the ventilation ratio the code actually requires.
A vented attic needs air entering low at the eaves and leaving high near the ridge. With exhaust alone, the vent pulls air from the nearest other vent instead of from the soffits and the attic does not ventilate at all. That short-circuit is the most common failure, and it explains a large share of roofs that only last fifteen years.
IRC R806.2 — Attic ventilation — minimum net free vent area
The 1/150 ratio, the 1/300 exception and the two conditions it requires, verified against the code.
Attic ventilation calculator
Required net free area from attic floor area, split into intake and exhaust.
Net free area
Why a 12 by 12 inch vent is nowhere near 144 square inches of ventilation.
Soffit
Blocked soffit vents are the most common reason an attic with plenty of ridge vent still runs hot and damp.
Ice dams
A symptom of heat loss and poor ventilation, not of a bad roof covering.