Stair soffit
The finished underside of a staircase or its landing.
The soffit is the finished underside of a flight of stairs or a landing — the sloping surface you see when you stand beneath the staircase. On a timber stair it is the boarding or plaster closing in the underside of the stringers; on a concrete stair it is the underside of the waist slab. Example: an enclosed under-stair cupboard is formed by lining the soffit and the outer stringer, while an open-plan stair may leave the soffit exposed as a feature. The soffit angle is the same pitch as the flight, so it sets the diagonal headroom for anyone passing below — keep at least the 6 ft 8 in (2032 mm) clearance the IRC requires. Soffit finish is cosmetic, but it often hides services and fire protection.
Related terms
Stair calculators
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Written by the Stairs Calc editorial team. Methodology and code references: see our methodology.
Built and maintained by builders, drafters and engineers who plan stairs for a living — every code limit is transcribed from the published standard and cited to its exact section.
Last reviewed 2026-06-20 against IRC 2021/2024
Stairs Calc gives accurate geometry and checks it against published building-code limits, but results are estimates for planning. Codes are adopted and amended locally and change over time. Always confirm dimensions against your local adopted code and a licensed professional before you build.