Headroom is the vertical clearance above a stair, measured from the sloped line that connects the tread nosings up to the ceiling or floor opening above. It protects a tall person from striking their head, and it is one of the most commonly failed inspection items because it is set by framing decisions made long before the stair is built.
The IRC requires at least 80.0 inches (6 feet 8 inches) of headroom at every point along the stair, not just at the top and bottom. Inspectors run a measuring stick along the nosing line to check. Spiral stairs have a slightly lower minimum of 78.0 inches.
The pinch point is usually the floor opening at the top of the stair. If the opening is too short in the direction of travel, the edge of the upper floor cuts into the clearance of someone still climbing. Fixing it after framing is expensive, so headroom should be checked at the design stage.
Lengthen the floor opening, shift the stair's start point, or in extreme cases reduce the stair's pitch so it reaches the opening sooner. The straight stair calculator draws the headroom line against your selected code so you can see clearance before you frame.
Headroom is taken vertically from a line drawn along the tread nosings — the same sloped plane your foot travels — up to the nearest obstruction, usually the underside of the floor or stair above. It must hold at every point on the flight, so the controlling spot is wherever the ceiling or floor edge comes closest to the nosing line, not the average clearance.
The clearance at the top of the stair depends on how long the floor opening is in the direction of travel. As a climber ascends, the upper-floor edge approaches their head; the opening must extend far enough horizontally that an 80-inch vertical clearance is preserved at the last point before they reach the upper floor. The steeper the stair and the thicker the upper floor assembly, the longer that opening has to be. This is exactly the calculation the stair opening calculator performs.
If headroom comes up short, the cheapest fixes are made on paper before framing: lengthen the floor opening in the direction of travel, shift the stair's start point so the climber clears the edge sooner, or make the stair steeper (fewer, taller risers within code) so it descends away from the opening faster. After framing, the options shrink to cutting back the floor opening or relocating the stair — both expensive. Headroom is the clearest case for checking the design before any lumber is cut.
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