Spiral stair code
The IRC sets four limits for spiral stairs in R311.7.10.1 (R318.7.11.1 in 2024): at least 26 in clear width at and below the handrail, treads at least 6-3/4 in deep at the walkline (walkline radius at most 24-1/2 in), risers no taller than 9-1/2 in, and 6 ft 6 in of headroom[1][2]. Spiral stairs are also exempt from the 4 in open-riser limit.
| Rule | US IRC | Canada NBC 2020 |
|---|---|---|
| Clear width | 26 in (660 mm) | 660 mm between handrails |
| Tread | 6-3/4 in at the walkline | 190 mm, 300 mm from the narrow handrail |
| Riser | 9-1/2 in max | 240 mm max |
| Headroom | 6 ft 6 in | 1,980 mm |
| Handrails | per the stair rules | both sides; outer one 1,070 mm high |
Canada's numbers come from NBC 9.8.4.7, which also bars spiral stairs as a required exit[3].
Spiral stair calculator: degrees per tread and the walkline
A spiral stair calculator splits the total turn between the treads: a 360° stair with 11 treads turns 32.7° per tread. A tread's depth grows with distance from the centre, so the code measures it on the walkline. The calculator puts the walkline 12 in from the edge of the centre column (300 mm in Canada) and reports the depth there and at the outer edge.
- More turn (405° or 450°) gives deeper treads and more headroom for the same height.
- Less turn (270°) makes a steeper, tighter stair; check the walkline depth.
Headroom on a spiral stair
Headroom on a spiral comes from the tread one full turn above you: treads per 360° times the riser height, less the tread thickness. With 11 treads per turn and 9″ risers that is 8′ 1-1/2″, over the 6 ft 6 in minimum. A larger stairwell opening or more treads per turn adds headroom.
Choosing the diameter
The clear width is measured from the column to the handrail, so the diameter must be more than twice 26 in plus the column: a 60 in stair with a 4 in column leaves 28 in from the column to the edge before the handrail. Size the floor opening to the stair maker's drawing. For a straight stair compare the stair calculator, and see the full stair code for handrails and guards; the baluster spacing calculator covers the guard around the opening.
Frequently asked questions
How many steps are in a spiral staircase?
Rise ÷ 9-1/2 in, rounded up, for the risers; one fewer treads. A 9 ft rise takes 12 risers and 11 treads.
What is the minimum diameter for a spiral staircase?
Enough for 26 in of clear width from the column to the handrail under the IRC: with a 4 in column the diameter must be at least 56 in plus the handrail.
How many degrees per step on a spiral staircase?
The total turn divided by the number of treads: 360° over 11 treads is 32.7° each.
What is the headroom code for spiral stairs?
6 ft 6 in under the IRC and 1,980 mm under Canada's NBC.
Sources
- 2021 International Residential Code, Chapter 3: Building Planning (R301.5, R311.7 stairways, R312 guards)
- 2024 International Residential Code, Chapter 3 (R318.7 stairways, R320 handrails, R321 guards)
- National Building Code of Canada 2020, Division B, Part 9, Section 9.8 Stairs, ramps, handrails and guards