With standard floor heights, spiral stairs do not always have to turn a full circle as you climb them (typically treads turn 30 degrees). They can be designed to turn approximately half a circle as shown in Figure 1, a so-called 180-degree spiral. These treads turn about 16.4 degrees each.
A typical 180 degree spiral
This type of spiral comes in handy for narrow, rectangular stairwell openings, say where a conventional straight stair has been removed.
However, one area of concern with a 180-degree spiral is that the diameter must be large enough to provide secure footing, particularly when going down the stair. If the treads are too narrow, going down the spiral would be difficult (easy to scrape your heels).
Consider a typical code-approved 30-degree tread (Figure 2). Codes require spiral treads to have a minimum stepping width of 7-1/2″. A 30-degree tread is about 15″ wide at the perimeter, 16-1/2″ beyond the 7-1/2″ width. The 180-degree spiral should allow a comparable area.
Code approved 30-degree tread
Figure 3 shows a 9-ft. diameter 180-degree spiral. The maximum tread width is 15″, the 7-1/2″ minimum is 25-1/2″ in from the perimeter. The required stairwell would be approximately 112″ by 60″.
A 7′-0″ diameter 180-degree spiral to fit a smaller stairwell is shown in Figure 4. The stepping area is smaller than that allowed by the 30 degree code tread. If you plan for a spiral this size you should consult local code officials and/or satisfy yourself that the smaller stepping area will be adequate.
Figure 5 shows a 180 degree spiral that would fit a standard 36″ wide straight stairwell. Note that the stepping area is extremely tight. This would not be a good stair for high traffic.