Roof Ventilation and Gable Vents, Explained

A healthy attic pulls cool, dry air in low at the soffits and lets warm, moist air out high at the ridge or gable vents, in balanced amounts. The residential code New Jersey uses calls for at least 1 square foot of net free vent area per 150 square feet of attic, or 1 per 300 when the vents are split properly between high and low.

Why an attic needs to breathe

In summer, an unvented attic bakes, which heats the rooms below and cooks shingles from underneath. In a New Jersey winter, warm, moist air from the house rises into the attic. Without airflow it condenses on the roof deck and nails, and warm spots on the roof melt snow that refreezes at the eaves as ice dams. Good ventilation keeps the attic closer to outdoor temperature and dry.

The two halves: intake and exhaust

• Intake is low, usually vented soffit panels under the overhang, or an edge vent at the eave.
• Exhaust is high: a ridge vent along the peak, box (static) vents, powered fans or gable vents.

GAF's balanced ventilation rule is simple: the air coming in at the soffits should match the air going out near the ridge. More exhaust than intake makes the system less effective.

What the code requires

New Jersey's one- and two-family dwelling subcode is the 2024 International Residential Code, NJ edition. Section R806.2 sets the minimum net free ventilating area at 1/150 of the vented attic area. It drops to 1/300 when 40 to 50 percent of the vent area is in the upper part of the attic (within 3 feet of the ridge) and the rest is in the bottom third. In Climate Zones 6 to 8 the 1/300 option also needs a vapor retarder. For a 1,500 square foot attic, 1/300 works out to 5 square feet of net free area, split between high and low.

Where gable vents fit

Gable vents are the louvered openings high on the end walls of a house. Plenty of older homes rely on gable vents as their main exhaust. They work, but they ventilate the ends of the attic better than the middle.

When a ridge vent is added during a roof replacement, gable vents can cause trouble: GAF advises against mixing different types of exhaust vents on the same attic, because one exhaust can pull air from the other instead of from the soffits. Depending on the house, the fix may be to close the gable vents from the inside and keep the louvers outside for looks. GAF also says powered attic fans should never be mixed with ridge vents or other non-powered exhaust.

Signs your ventilation isn't working

• Frost or dampness on the underside of the roof deck in winter
• Rusty nails, dark stains or mildew in the attic
• Repeated ice dams at the eaves
• Upstairs rooms that stay hot well into the evening
• Soffit vents painted over, blocked by insulation or never cut open

A roof replacement is the easiest time to correct ventilation, since the ridge is open and soffits are within reach. All In Roofing checks intake and exhaust as part of every replacement.