Spiral stairs do not always have to turn almost a full circle as you climb them. Typically spiral treads turn about 30 degrees each, so an 11-tread spiral will turn 330 degrees. Stairs can be designed to turn smaller angles. For instance treads that turn about 16.4 degrees each will make an 11-tread spiral that turns half a circle (Figure 1.) — a 180-degree spiral stair.
You could use fewer treads for a larger angle, say eight treads at 22.5 degrees each (180/8 = 22.5) but this would require a much higher rise between steps, on the order of 12″ where codes allow 9-1/2″ at most. Twelve inches would be an uncomfortable step up for most people. To see for yourself stack books on the floor up to 12″ high and try stepping up onto them.
IMPORTANT: All the spiral stairs in this article are shown to clarify considerations for 180 degree spirals. Arbitrary diameters and stairwell sizes are used, but do not depend on these dimensions for a stair to fit a specific floor layout or to work in other ways. Instead consult your stair supplier to be sure you have a design that will fit your floor plans.
A 180-degree spiral can be used to access a second floor loft edge or to fit a rectangular stairwell were a large conventional stair has been removed. The stairwell must be wide enough to accommodate a 180-degree spiral that allows secure footing on each tread. If the treads are too narrow walking on the spiral is difficult, particularly when you go down. As you can see in Figure 1. you could easily scrape the heels of your feet, it would be easier to go down backwards, as if climbing down a ladder.
How much foot room do other spiral stairs allow: Consider a typical spiral that meets national building codes (Figure 2a.). This stair has a diameter of about 5′ with treads that turn 30 degrees each. This allows a stepping area 14-3/4″ long, 7-1/2″ wide at the small end, 15″ at the other. Figure 2b. shows how the stepping area is determined.
Larger stepping areas for 180-degree spiral stairs can be achieved by increasing the spiral diameter so that the narrow angle treads spread wider near the handrail. Figure 3. shows a 9-ft. diameter spiral. The stepping area is 23″ long, and 7-1/2″ to 14″ wide, as comfortable as the 30-degree spiral.
Figures 4. and 5. show 180-degree spiral stairs with diameters of 7′ and 5′-8″ respectively. The 5′-8″ diameter was chosen to possibly fit into a 36″-wide stairwell, where a conventional straight stair has been removed.
The 7’ stair’s stepping area is 11-1/2″ long with widths of 7-1/2″ up to 10-3/4″. This is tight, but possibly doable.
The 5′-8″ diameter provides a stepping area 12″ long and widths of 5-3/4″ to 9″, extremely tight and not a good stair for high traffic use.
This information comes from the spiral stairs planning blogs on the spiralstaircase.com website: