A 5 ft diameter spiral staircase climbing 9 ft floor to floor needs 12 risers of 9 in: 11 treads plus the top landing. Turning a full 360°, each tread covers 32.7°, which makes it 8 in wide at the walkline and 16 ⅞ in at the outer edge, and leaves 8 ft 1½ in of headroom where the stair passes over itself. All of that meets the spiral stair rules in IRC R311.7.10.1.
How the spiral staircase is calculated
A spiral stair is a stack of identical pie-shaped treads around a center column, and the code treats it as its own kind of stair. Its risers may be up to 9½ in, not the 7¾ in of an ordinary stair, so the riser count is the floor-to-floor height divided by 9½ in and rounded up. As with any stair, the upper floor is the last step, so there is one tread fewer than risers.
Risers = height ÷ 9½ in, rounded up. Angle per tread = rotation ÷ treads. Tread at the walkline = 2 × walkline radius × sin(angle ÷ 2). Headroom = (360° ÷ angle per tread) × riser − tread thickness.
For the default: 108 ÷ 9.5 = 11.4, so 12 risers of 9 in and 11 treads. 360° ÷ 11 = 32.7° per tread. The walkline sits 12 in out from the inside of the turn, so with a 4½ in column its radius is 2¼ + 12 = 14¼ in, and 2 × 14.25 × sin(16.4°) = 8.03 in. Eleven treads make one full turn, so the tread directly overhead is 11 × 9 = 99 in higher; less its 1½ in thickness that leaves 97½ in, 8 ft 1½ in.
The rotation here is measured from the first riser to the edge of the top landing. Kit makers sometimes count the landing as a tread; check which way your kit's drawings count before ordering.
Spiral staircase code requirements
IRC R311.7.10.1 sets five numbers:
| Rule | Limit |
|---|---|
| Clear width at and below the handrail | 26 in min |
| Tread depth at the walkline | 6¾ in min |
| Rise per tread | 9½ in max |
| Headroom | 6 ft 6 in min |
| Walkline radius | 24½ in max |
The treads must be identical. The walkline is defined in R311.7.4 as 12 in from the inside of the turn, measured from the edge of the clear width, which on a spiral is the face of the column.
What diameter spiral staircase do I need?
Clear width is measured inside the handrail, from the column to the rail. For 26 in clear with a 4½ in column and a rail that takes 1½ in off the tread, the stair has to be at least 2 × (26 + 2¼ + 1½) = 59½ in across, so 5 ft is the smallest diameter that passes. A 4 ft stair gives only 20¼ in and fails, however steep or shallow you make it. A 5 ft 6 in stair gives 29¼ in and feels noticeably roomier when carrying things.
How much rotation?
Rotation trades tread width against headroom. With the default 5 ft stair and 9 ft rise:
| Rotation | Angle per tread | Tread at walkline | Headroom | Result |
|---|---|---|---|---|
| 270° | 24.5° | 6 ¹⁄₁₆ in | 10 ft 10½ in | Treads too narrow |
| 360° | 32.7° | 8 in | 8 ft 1½ in | Passes |
| 450° | 40.9° | 9 ¹⁵⁄₁₆ in | 6 ft 5 ¹¹⁄₁₆ in | Headroom too low |
With 11 treads, anything from about 302° to 448° passes both rules. Adding a riser helps: 13 risers of 8 ⁵⁄₁₆ in at 360° give 30° treads, 7 ⅜ in at the walkline, and still 8 ft 2 ³⁄₁₆ in of headroom.
In metric, a 1.52 m stair with a 2.74 m rise and an 11.5 cm column gives 12 risers of 22.8 cm, 20.4 cm treads at the walkline and 66.5 cm of clear width, just over the 66 cm the code asks for.
Planning the opening
The top landing sits at the floor above and fills one tread position; the floor opening is framed around the stair's diameter plus whatever clearance the kit calls for. The opening's header is a structural beam when it cuts floor joists, which the beam span calculator sizes from the IRC tables, and the walls around it come from the framing calculator.
Other stair types
A spiral saves floor space but is hard to carry furniture up. Compare a conventional stair on the stair calculator, or a compact L or U stair with the stair landing calculator. Wood stairs with cut stringers are laid out on the stair stringer calculator.