R314.2 Spiral stairs. Spiral stairways are permitted, provided the minimum width shall be 26 inches (660 mm) with each tread having a 7 1/2 inch (190 mm) minimum tread width at 12 inches (305 mm) from the narrow edge. All treads shall be identical, and the rise shall be no more than9 1/2 inches (241 mm). A minimum headroom of 6 feet, 6 inches (1982 mm) shall be provided. - Michigan Residential Code, 2000
Now, can you believe they're requiring a minimum tread width of 7-1/2 inches at 12 inches from the narrow edge? I can remember when it was 7-1/4 inches! That, my friends, is a bold move. Bold with a capital mother-fuckin' B, mother fuckers!