The size of your roof vent and attic space will determine how many roof vents you need. There is a basic formula you can use to calculate this.
Let’s start by looking at different types of roof vents.
Ridge vents run along the apex of your roof – they are long and narrow. Gable vents attach to the gables rather than the roof itself and go through the siding - they come in a number of shapes, from squares to rectangles to triangles. Under-eave, or soffit, vents are installed on the underside of the roof that hangs over the siding—the eaves. There are also box- or dome-shaped vents that go through the roof itself. Finally, you have gable and roof fans, which actively pull the hot air out of the attic.
Let’s get calculating.
As a general rule, your roof needs 1 square foot of vent area for every 150 square feet of attic space. If your attic is 450 square feet, you need roof vents equaling 3 square feet. They should be divided equally between inlet and outlet vents and can be in any combination that will work with your roof design.