Saving space without giving up comfort
A spiral staircase fits in a circle as small as 4 or 5 feet across, which makes it popular for lofts, small apartments and access to roof decks. The trade-off is steepness and narrow treads near the center. A careful layout keeps the stair safe and pleasant to climb.
How the layout works
- Number of risers: divide the floor-to-floor height by the maximum riser height and round up. The last riser steps onto the landing.
- Riser height = floor-to-floor height ÷ number of risers.
- Rotation per step = total rotation ÷ number of risers, with the landing taking the last position.
- Tread depth at the walkline = 2π × walkline radius × rotation per step ÷ 360.
- Headroom = riser height × steps per full turn − tread thickness (1½ in assumed).
Example
A 9-foot (108 in) floor-to-floor height, a 60-inch diameter, a 4½-inch center pole and a full 360° turn, with risers of 9½ in or less:
- 108 ÷ 9.5 = 11.4, so 12 risers of 9 in
- Rotation per step: 360 ÷ 12 = 30°
- Walkline radius: 2.25 + 12 = 14.25 in, so the tread depth there is 2π × 14.25 × 30 ÷ 360 ≈ 7.5 in
- Headroom: 9 × 12 − 1.5 = 106.5 in
- Clear width: (60 − 4.5) ÷ 2 ≈ 27.75 in
IRC requirements for spiral stairs
| Item | Limit (R311.7.10.1) |
|---|---|
| Clear width | at least 26 in |
| Riser height | at most 9½ in |
| Tread depth at the walkline | at least 6¾ in, 12 in from the narrow edge |
| Headroom | at least 6 ft 6 in (78 in) |
All treads must be identical. Local codes can add requirements, and spiral stairs are restricted in many commercial buildings, so check with your building department.
Comfort beyond the code
The check of two risers plus one tread at the walkline, around 24 to 25½ in (61 to 65 cm), is a classic measure of a comfortable stride. Spiral stairs are usually a bit steeper. If the result is short, a larger diameter or more rotation gives deeper treads; if headroom is tight, increasing the rotation spreads the steps over a longer path.
Plan for the handrail on the outer edge, a landing large enough to step off safely, and the size of furniture you might need to carry up, which is often the real limit of a spiral stair.