05-11-2013, 03:33 PM
A spiral staircase is typically used because it saves a lot of space/no place else to put stairs. To give the same clearance as a standard doorway, the stairs will need to rise around 6'-8" plus the tread thickness plus bracing, say 7' overall per turn around the spiral. If the rise in each step is to be 8", you will need 21 steps per two times around the spiral, or roughly 34.28 degrees per step radially. The tread dimensions will depend upon the diameter of the spiral, which in turn will depend upon the usage of the stairs. Backstage at a little theater the spiral diameter might be really tiny, like 60" or so. If there is a requirement for two men walking abreast while carrying a load to use the stairway, the diameter will obviously need to be larger. Have fun! Ric