Roof vents are installed to allow air to circulate in your attic space. Good air circulation will minimize moisture build up in your attic during winter and prevent heat build up in summer. The number of roof vents needed will vary depending on the size of your attic.
WHY ROOF VENTS?
Air can leak from your house into the attic, and can bring water vapor with it. If your roof deck is cool enough to be at or below dew point, then condensation will form on the underside of the roof. Air movement from outside through the attic, helps remove moisture by diverting it out of the attic before it condenses.
ATTIC AIR CIRCULATION
Roof vents allow air to exhaust from your attic space. But keep in mind, attic air circulation is accomplished with both intake and exhaust vents.