Skirt board (string board)
The trim board against the wall that the treads and risers butt into.
A skirt board — also called a skirting or close string — is the finish board run up the wall side of a staircase, against which the tread and riser ends butt or are housed-in. It hides the rough joint between the steps and the wall and gives a clean sawtooth line that mirrors the open-side nosing return. Example: a 1× pine skirt board is scribed to the wall and runs parallel to the stringer about an inch above the tread nosings, capping the wall finish. On a housed (closed) string the treads are actually trenched into the board so no end-grain shows. It is trim, not structure — the stringer carries the load — but a tidy skirt board is what makes a stair look finished where it meets the wall.
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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.