All building codes in the United States have similar rules for spiral stairs. These are considerably different than rules for straight stair systems. Spiral staircase code requirements are:
Maximum rise, the vertical distance between treads: 9-1/2″ (this is higher than the maximum rise for a straight stair—usually 7-3/4” or so– because you have to go down a spiral more steeply to gain sufficient headroom under the top landing).
Minimum head clearance above any tread: 78″.
Minimum horizontal tread width: Treads must be at least 7-1/2″ at a point 12″ out from the center column.
Minimum width of clear passageway: At least 26″ from the inside of the handrail to the outside of the center column.
Baluster spacing: most codes require a space of no more than 4″ between balusters.
Minimum opening between treads: some codes require no more than 4″.Before manufacturing your spiral stair kit at the plant, Precision Pine will need to know if local building codes will govern the installation of your spiral staircase. If so, we prefer to contact the local inspector either directly or through the customer to assure that the installation will meet local requirements. Regardless of the exact wording of building codes, the local inspector will be the final authority on the interpretation of code requirements and what or when allowances can be made. It is best to include the local inspector before installation begins.