This installment of Code Corner is part two in a series looking at stairway requirements of the 2006 International Residential Code. This installment will focus on the different requirements of winder treads and spiral staircases. Stairways are considered a means of egress in the home, so proper stairway dimensions are essential to occupant safety.
Winder treads are tapered, causing the staircase to curve or change direction abruptly. A staircase may consist of both typical stair treads and winder stair treads. As a rule, a spiral staircase has all its treads attached to a center pole.
While both the winder-type stair and the spiral stair have a curved direction of travel, the requirement for each is substantially different.
Winder stair treads are treated similar to the typical stair tread in terms of rise and run: 7-3/4 inches is the maximum tread rise. However, the nature of the winder staircase says that its shape will be narrower on the inside edge, leading to a pie-shaped tread. This is where additional requirements come into play, separating the winder tread from a typical straight stair tread.
Minimum tread depth requirements for a winder stair tread. Guardrail has been omitted for clarity.
First and foremost, a winder tread cannot come to a point narrower than 6 inches at the inside edge. This is different from the common practice in the past of forming a 90-degree turn in a staircase by use of two 45-degree angle treads that came to a point on the inside corner. In addition to the
6-inch inside edge requirement, there is a requirement for a minimum 10-inch-tread depth 12 inches from the inside edge. Since the most common path of travel up and down a staircase is confined to the inside half of the tread, the code requires that same minimum 10-inch tread depth one foot from the inside since most people are not walking on the inside 12 inches of the stairway. This is a compromise between safety and functionality in a curved staircase.
Minimum tread depth for a winder stair at the narrowest point, 6 inches. Guard has been
eliminated for clarity.
Spiral staircases fall under totally different code requirements. The minimum width of a spiral staircase is 26 inches, 10 inches less than a typical or winder staircase. Spiral stairs have no minimum tread depth at the narrow end and a minimum of 7-1/2 inches of tread depth at a point 12 inches from the inside edge. The maximum rise is 9-1/2 inches, much greater than the typical staircase. All treads within a spiral stair must be identical.
Even headroom requirements are less restrictive for spiral staircases. The minimum head room requirement is reduced to 6 feet 6 inches rather than 6 feet 8 inches. Spiral staircases can offer obvious advantages in terms of fitting a stairway into a tight space. Although a spiral staircase is more awkward to use, there are no restrictions placed on its location and use in the code.
Robert Moss, Wisdom & Assoc., Kenai, Alaska, is an ASHI Certified Member, who joined ASHI in 2002. Visit his company Web site at www.wisdomandassociates.com.