All stairways must measure at least 36 inches across. This minimum width must remain constant upward to at least the minimum head allowance of six feet and eight inches. This height is determined by measuring straight upward from the diagonal line formed by the nosing, or leading edge, of all the individual stairs. Handrails cannot infringe on this width by more than 4 1/2 inches. Individual risers must have a height no greater than 7 3/4 inches and a depth of at least 10 inches. If individual steps within a single staircase vary in depth, there can be no more than a 3/8-inch difference among them. At bends in a staircase, the pie-shaped or "winder" risers must be at least 10 inches deep at a point 12 inches from their narrower side.
Handrails must be continuous for the full length of a staircase, from the point directly above the first riser to a point directly above the final riser. Handrails may attach to walls or be anchored with newels (posts). Any handrails affixed to walls must have a clearance of at least 1 1/2 inches from the wall. A volute, the curved extension of a handrail, is permissible on the lowest point of the handrail. Circular handrails must have a diameter of at least 1 1/4 inches. Non-circular handrails must have a perimeter of at least 4 inches at any cross-section and a width of at least 2 1/4 inches. Handrails with a perimeter larger than 6 1/2 inches must have a graspable "finger recess" indentation on both sides of the design.
Stairs that reach a rise of at least 30 inches above the floor must have a guard at least 36 inches tall on any sides not already protected by a wall. These requirements apply to risers as well as landings. Any openings in guards must not permit the passage of a 4 3/8-inch sphere between balusters. As an exception, at the triangle formed between two steps and the bottom rail of a guard, the space must not permit a 6-inch sphere to pass.
- spiral staircase image by Alison Bowden from Fotolia.com