Georgia Roof Guide

How much ventilation does a ridge vent provide?

Short answer. Most shingle-over ridge vents provide somewhere around 12 to 18 square inches of net free area per linear foot, as printed on the product. On the balanced 1/300 path, each foot of an 18 square inch vent covers the exhaust needs of about 75 square feet of attic floor, and each foot of a 12.5 square inch vent covers about 52.

How the per foot numbers translate

On the 1/300 path with balanced intake and exhaust, the exhaust needed is about 0.24 square inches per square foot of attic floor. Dividing the vent's rating by 0.24 gives the attic area each foot can serve:

So a 1,500 square foot attic needs about 20 feet of an 18 rated vent, or about 29 feet of a 12.5 rated vent. On the 1/150 path, double the footage.

Net free area is not airflow

Net free area is the open area through the vent after its filters and louvers. Actual airflow depends on wind speed, the temperature difference between the attic and outdoors, the height between intake and exhaust, and how much intake is available. A ridge vent with plenty of rated area still moves little air if the soffits are blocked.

What reduces the real number

Read more in ridge vent vs box vents and roof ventilation basics.

In depth

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