Headroom is the stair check people skip and the one that hurts most to fail — because it fails at the floor opening, where the fix is framing. The rule is simple: 6 feet 8 inches (80 in) of clearance above every tread nosing. The failure is specific: it happens at the exact point where the stair passes the upper floor's edge.
The geometry of the failure
Walk up any stair through an opening and watch the ceiling above your head: clearance shrinks as you approach the upper floor, reaching its minimum at the last tread before the opening's edge — the tread where the floor header is still above you. The code's 80 inches must exist there. The practical consequence: the opening must be long enough that the stair has already climbed past head-height before the header arrives.
The check before framing
- From your layout (rise & run), count treads from the bottom until cumulative rise exceeds 80 inches.
- Those treads — all of them — must fit inside the floor opening's length (each consuming one tread depth).
- Required opening = treads-to-clear × tread depth. The headroom calculator runs this in one step.
- Add the field margin: joists, headers, and ductwork routinely claim an inch or two.
Why ducts eat the margin
The 80-inch line is measured to the lowest obstruction — which on real jobs is rarely the header: HVAC trunk lines, plumbing stacks, and recessed light housings crowd stair openings. The inspection-stage discovery (after drywall) costs a bulkhead or a duct relocation; the framing-stage discovery costs a longer opening. Snap a plumb line at the opening edge and look up with a flashlight before ordering stair parts.
The fixes, cheapest first
| Fix | Cost | When it works |
|---|---|---|
| Steepen to the 7¾ riser cap | Free (relayout) | Short deficits (~2–4 in) — shorter treads shrink the requirement |
| Move the top riser into the floor | Minor framing | The top step becomes the upper floor — buys one tread of opening |
| Lengthen the opening | Major framing | Deficits over ~6 in; headers and joists relocate |
| The "duck" | Free | Inspection fails. Everyone ducks. Do not. |
Run the check while the decision is still free — headroom is the rare stair problem that gets more expensive every week you wait.