Soffit

In roofing, a soffit is the horizontal (or sometimes sloping) underside of the eaves, spanning from the back of the fascia to the face of the wall. More broadly in architecture, "soffit" means the exposed underside of any overhead element, such as an arch, beam or stair.

Purpose

• It closes off the gap between rafters at the overhang, keeping out birds, insects, rodents and weather.
• It gives the eaves a finished appearance.
• It is commonly the location for intake vents that supply outside air to the roof space in a ventilated (cold) roof.

Boxed and open eaves

• Boxed (closed) eaves: the overhang is enclosed with a soffit, sometimes on short horizontal lookouts that make a flat underside.
• Sloped soffit: the lining is fixed directly to the underside of the rafters, following the roof pitch.
• Open eaves: no soffit; rafter tails are left exposed, a common feature of Craftsman and many traditional styles. Blocking or bird-stop pieces close the gaps above the wall.

Materials

Soffits have been made of timber boards, plywood, asbestos cement (in older buildings, which should be checked before disturbance), aluminium, vinyl, uPVC, fibre-cement and steel. Perforated aluminium and vinyl panels are common in North America; uPVC soffit boards are typical in UK house building.

Ventilation role

Where soffits provide intake ventilation, the vents must not be blocked by insulation inside the roof. US code requires at least 1 in (25 mm) of space between insulation and roof sheathing at eave vents, and baffles or eaves trays are used to maintain the air path. Ventilating soffit panels are rated by net free vent area so designers can confirm the intake meets code ratios.