180-Degree Spirals
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.
Figure 1.
A typical 180 degree spiral
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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.
Figure 2.
Code approved 30-degree tread
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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".
Figure 3.
A 180-degree spiral that will allowsame stepping
area as code approved treads.jpg)
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 4.
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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.
Figure 5.
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