The space underneath the end of an overhanging roof of a house and the wall is called the soffit. Holes are usually cut in the soffit to provide venting for the attic. The holes can be rectangular or circular, and a circular soffit vent refers to the cover of a circular vent hole. A circular soffit vent can be made of metal, plastic, or wood, and is usually louvered or screened to prevent the entrance of birds or small animals into the attic.
If an attic has proper ventilation, hot moist air rises and is exhausted by an attic vent placed on the roof or in the gable while cool air is drawn in through the soffit vents to replace it. The amount of air that circulates is determined by the sizes of the vents. For adequate ventilation, there should be a 50:50 ratio between the amount of air drawn in through the soffit to the amount exhausted by the roof or gable vent.
A typical circular soffit vent is smaller than a rectangular one so it can fit in a smaller space, but this means more are required to provide adequate circulation. The maximum diameter of a circular soffit vent is 8 inches (20 cm), which provides a venting area of 25 square inches (0.02 square meters). The largest rectangular vents are 8 inches x 22 inches (20 cm x 56 cm) for an area of 176 square inches (0.11 square meters). Both fit between pairs of rafters, but, since a circular soffit vent is less obtrusive, it is often easier to incorporate into the exterior design motif of the house. A circular soffit vent can be as small as 1 inch (2.5 cm) in diameter and placed in a corner or near the outer edge of the soffit where it all but disappears.
A circular soffit vent array is not difficult to install. A typical strategy is to make evenly-spaced marks on the soffit and drill or cut a circular hole centered on each mark. The flange can be used around the underside of the vent cap as a guide for drawing the outline of the hole. Holes up to 3 inches (8 cm) can be cut with a drill fitted with a hole saw and larger ones with a jig saw. The vent is installed by fitting the flange into the hole, pushing the vent flush with the soffit, and screwing it in place with galvanized or other weather-resistant screws.