Technically, a soffit is the underside of any element of a building. The word originates with the Latin suffigere, meaning "to fix underneath." So we have ceiling soffits, stair soffits, and even cornice soffits. The most common soffit, however, is found in the area under the eaves on the exterior of the house. The soffit extends from the side of the house to the edge of the eave and closes the space beneath the eave.

The dimensions of an eave soffit depend on the structure of the roof. A roof with a wide overhang will have a correspondingly wide soffit, sometimes up to three feet (0.9 m) wide. A roof with very little overhang will have a narrow soffit, sometimes as little as three inches (7.6 cm).

The soffit is more vulnerable to weather damage than any other part of the house. Wet material rots, and a soffit can be repeatedly soaked by water from torn shingles, damaged or rusted flashing, ice dams, or poorly functioning gutters. Squirrels and birds can also damage a soffit, since they like to nest in the space in the space behind soffits if they can find a point of entry. Bees, hornets, and wasps do not actually damage a soffit, but when they build a nest behind a soffit, they present a problem for the homeowners.

Fortunately, it is quite easy to repair a damaged soffit. Remove the shingle molding and the fascia. Next, remove the damaged or rotted soffit. If a rafter is rotted or otherwise damaged, remove and replace that section. Replace the damaged soffit, making sure that all surfaces of the new soffit have been treated with a sealer to protect the material from water damage. Replace the fascia board and the molding. Paint or seal the surfaces.

An unventilated, overheated attic can be improved with ridge vents and soffit vents. Because heat rises, the hottest air is exhausted through the ridge vents and cooler air is pulled in through the soffit vents. If air circulation is poor, an attic fan can be used to increase air movement.

Improving ventilation with soffit vents keeps the attic drier, because condensation will not form inside the roof. This reduces damage to the rafters and roof. A cooler attic also means reduced use of the air conditioner in the rest of the house.