How to Install Round Soffit Vents

Calculate the total area, in feet, of the attic space. Measure the length and the width of the space and multiply the two numbers to find the area.

Divide the area by 150 to determine the amount of space that requires venting in total square feet. Every 150 feet of attic space needs 1 square foot of venting.

Divide the total square feet of venting required by 2 as half of the venting is supplied by the ridge vent, and half is supplied by the soffit vents.

Calculate the area of each soffit vent. The formula for calculating the area of a circle is the radius squared times pi. The radius is half of the diameter. A 6-inch vent has a diameter of 6 inches; therefore, the radius is 3 inches. Square the radius by multiplying it by itself. In this example, 3 squared equals 9. Multiply that by pi, or 3.14. The area of a 6-inch round soffit vent is 28.26 inches.

Convert the area of the soffit vents into square feet by dividing the area in square inches by 144. The area of a 6-inch vent in square feet is .196 square feet.

Calculate the total number of soffit vents needed by dividing the soffit vent area in square feet from Step 3 by the area in square feet of a single vent from Step 5. For example, 5 square feet of soffit vent area divided by .196 square feet per vent equals about 26 6-inch round soffit vents.

Mark where each soffit vent is to be placed. Space the vents evenly along the soffit, placing them between joists and rafters. Ideally, the vents should line up vertically with existing or planned roof vents.

Inspect the soffit overhang area in the attic. Carefully cut away any insulation on the soffit overhang that would block the vents using a utility knife. Wear goggles, a dust mask and work gloves.

Cut a hole into the soffit at the mark using a hole saw bit and an electric drill. Choose a hole saw bit size that is 1 inch smaller than the vent, creating a 1/2-inch area for attaching the vent to the soffit.

Apply a thin bead of caulk to the flange or lip area of the soffit vent.

Center the soffit vent over the hole, stopping at the flange. Secure it into place using a hammer and the nails provided with the vent, or galvanized roofing nails. Alternately, attache the vent using 1/2-inch-long number 4 sheet metal screws and an electric drill. Most round soffit vents have four to six pre-drilled holes, indicating nail or screw placement. If the vent does not already have holes, space four to six nails or screws evenly around the vent.