180 Degree Spiral Stair
Most spiral stairs with 11 treads turn almost a full circle as you climb them. We are addressing spirals that fit common floor heights of nine feet or so. Building codes set the maximum rise between treads at 9-1/2″. Therefore the spirals in question will require at least 12 rises. (If the spiral had less than 12 rises, the rise height would be greater that 9-1/2″.)
Typically spiral treads turn about 30 degrees each, so an 11-tread spiral will turn 330 degrees.
But you can design them to turn less. For instance, what if the treads turn only 16.4 degrees each? This design would make an 11-tread spiral that turns half a circle. You would have a 180 degree spiral stair (see Figure 1).
You could design a 180 degree spiral staircase with fewer treads and larger angles. Consider a spiral with eight treads that turn 22.5 degrees each. (180 degrees/8 treads = 22.5 degrees.) This stair would have rises on the order of 12”. A 12″ rise would be an uncomfortable step up for most people.
You can use a 180 degree spiral stair against a second floor loft edge, or in some vacated stairwells, if they are wide enough. In either case we must be sure to provide secure footing on each tread. Walking on narrow treads could be difficult, particularly when you go down. As you can see in Figure 1. you could easily scrape the heels of your feet. So it would be easier to go down backwards, as if climbing down a ladder.
Footing on a 180 degree spiral spiral stair
Lets look at how much footing a code-approved spiral stair will provide. Figure 2a shows a stair with a diameter of about 5’, and treads that turn 30 degrees each.
Figure 2b shows the available footing, or stepping area. Codes define this area. Here the stepping area is 14-3/4″ wide, 7-1/2″ deep at the small end (measured 12″ out from the center column), 15″ at the other.
Narrow angle treads spread from the center column going out. Therefore they are deepest at the spiral handrail. A 180 degree spiral stair with a large enough diameter will provide an adequate stepping area. Figure 3 shows a 9-ft. diameter spiral. The stepping area is 23” wide, and 7-1/2″ to 14” deep. This is as comfortable as provided by the 30-degree treads.
(Note, Figure 3 shows a wood overlay that defines the narrow end of the walking area. For a detailed explanation of this overlay go to “Spiral stair landing designs” and see the pads shown in Figure 3 of that post.)
IMPORTANT: We have shown all the 180 degree spiral stairs in this article for purposes of explanation. Moreover we used arbitrary diameters and stairwell sizes. Do not depend on these dimensions for a stair to fit a specific floor layout or to work for you in other ways. Instead consult your stair supplier to be sure you have a design that will fit your floor plans.
How diameter affects a 180 degree spiral stair
Figures 4. and 5. show two examples of a180 degree spiral stair with diameters of 7’ and 5′-8” respectively.
The 7′-0″ stair’s stepping area is 11-1/2” wide with depths of 7-1/2” up to 10-3/4”. This is tight, but possibly doable.
The 5′-8″ diameter was chosen to possibly fit into a 36″-wide stairwell, where a conventional straight stair has been removed. The stepping area for this stair is extremely tight and not good for high traffic use, if any traffic at all.
(If 7-1/2″ is the minimum tread depth the stepping area will be only 8″ wide and 9″ deep at the outer end. If the tread width is 12″, the minimum tread depth is reduced to 5-3/4″ at the small end.)