Spiral Stairway Building Codes
The first thing you need
to clarify about spiral stairway building codes is that most of them are
different from most of the other stair
building codes. The maximum stair
riser is 9 1/2 inches and the minimum tread width is 7 - 1/2 inches, but
to obtain this measurement, we need to measure 12 inches from the inside
of the stair tread.
Spiral Stairway Tread Minimum
Now here's the biggest problem with spiral stairways. Most building codes only allow them to be used on accessible areas less than 250 square feet. This code has recently changed and used to be 400 square feet, so make sure that you check with your local building department to verify this information.
2000 International Building Code Book says that spiral stairs can only be used for access areas 250 square feet and less, with no more than five occupants.
Circular stairways are difficult for most people to walk up and down and you might think twice before using these stairs to move any large furniture. It wouldn't be a bad idea to move large furniture to these areas, during construction and working around them. This could save you a lot of frustration, after this area is completed and more difficult to access.
The 2010 California Building Code Book does not require spiral stairways to have closed risers.