Stair building code requirements, explained
Most stair code questions come down to one section of the International Residential Code: R311.7. Here's what it actually says, why each limit exists, and where local jurisdictions commonly tighten it. This is a planning reference, not legal advice. Your local amendments always win.
The numbers at a glance
| Requirement | IRC limit | Section |
|---|---|---|
| Riser height (max) | 7¾″ (196 mm) | R311.7.5.1 |
| Tread depth (min) | 10″ (254 mm) | R311.7.5.2 |
| Riser/tread variation (max) | ⅜″ (9.5 mm) | R311.7.5.1-.2 |
| Stair width (min, above handrail) | 36″ (914 mm) | R311.7.1 |
| Headroom (min) | 6′8″ (2032 mm) | R311.7.2 |
| Handrail height | 34″-38″ | R311.7.8.1 |
| Vertical rise per flight (max) | 12′3″ (3734 mm) | R311.7.3 |
| Nosing (where tread < 11″) | ¾″-1¼″ | R311.7.5.3 |
Risers: the 7¾″ ceiling
Riser height is measured vertically between the leading edges of adjacent treads. The 7¾″ maximum exists because fall risk climbs steeply with step height. Every extra ¼″ of riser measurably increases missed-step accidents, especially descending. Open risers are allowed only where the gap between treads is too small to pass a 4″ sphere, a child-safety guard that applies where the drop to the floor below exceeds 30″.
Treads: the 10″ floor
Tread depth is measured horizontally nosing-to-nosing. Ten inches is roughly the minimum that keeps the ball of an adult foot on the tread while descending. The dangerous direction. Note that the older BOCA-derived "7-11" rule (7″ max riser, 11″ min tread) survives in several states and cities as an amendment; if your jurisdiction uses it, the steepest legal stair is meaningfully shallower.
The ⅜″ uniformity rule
The largest riser (or tread) in a flight may not exceed the smallest by more than ⅜″. This is the rule that catches DIY builds: human gait locks onto a rhythm within two or three steps, and a single odd step breaks it. Which is why the odd step is nearly always the one people trip on. It's also why stringer layout obsesses over repeatability, and why forgetting to "drop the stringer" for tread thickness fails inspection: it makes the bottom step taller than all the others.
Width, headroom, and flight height
- 36″ width is measured above the handrail; at and below handrail height, 31½″ (one rail) or 27″ (two rails) is permitted.
- 6′8″ headroom is measured plumb from the sloped nosing line to the ceiling or floor opening above. The advanced calculator mode checks your floor-opening length against this.
- 12′3″ max rise per flight forces a landing on tall runs; landings must be at least as deep as the stair is wide, never under 36″. Sizing a stair around a landing? The L-shaped stair calculator splits the flights and checks the landing rules for you.
Handrails
Required on at least one side of any flight with four or more risers, mounted 34″-38″ above the nosing line, graspable (1¼″-2″ circular cross-section or equivalent), continuous, and returned to the wall or a post at the ends so cuffs and bag straps can't hook the rail's open end.
Where local codes commonly differ
- 7-11 jurisdictions: stricter riser/tread as noted above (parts of the Northeast and Pacific Northwest).
- Deck stairs: same R311.7 rules apply, but some jurisdictions add closed-riser or graspable-rail requirements outdoors.
- Spiral and winder stairs: separate, more permissive geometry rules (R311.7.10) with their own minimums.
- Egress vs convenience stairs: a secondary stair to a basement workshop may be treated differently from the primary egress stair in some jurisdictions.
Inspection-day checklist
- All risers within ⅜″ of each other (measure the bottom and top steps first. That's where inspectors look)
- Tread nosings uniform, ¾″-1¼″ where treads are under 11″
- Handrail 34″-38″, graspable, returned at both ends
- 6′8″ clear over every nosing, including under beams and ducts
- Guards at open sides 36″ min with openings that reject a 4″ sphere