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.
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.
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
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.
©Precision Pine 2006
Precision Pine, Inc.,
7322 Hodges Ferry Road,