Because of the geometry of a spiral staircase, all the model codes in the United States have basically the same requirements for spiral stairs. These rules are considerably different than those for straight stair systems.
Spiral code requirements are:
Maximum rise: 9-1/2″ (this is higher than the maximum rise for a straight stair because you have to go down a spiral more steeply to gain sufficient headroom under the top landing).
Minimum headroom: 78″.
Minimum horizontal width of tread: 7-1/2″ at a point 12″ out from the center column.
Minimum width of clear passageway: 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 putting your package together 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 find out the exact requirements before installation begins.