Explanations of spiral stair terms
Spiral stair terms describe the sizes, configurations and placement of individual components of a spiral staircase. Spiral terms are similar to those that relate to straight stair design. But obviously elements of a spiral staircase are different from their straight stair counterparts. This post offers to explain these variations to someone new to spiral stair design.
First we will help familiarize you with individual spiral stair parts and their functions (see Figure 1).
Treads (steps) provide the surfaces to walk on as you climb the stairs.
The landing acts as a bridge from the top tread to the top floor. It provides adequate headroom as you go down the spiral. (See “Spiral Stair Planning: Why Spiral Stair Landings Are Needed”.) In addition, the landing anchors the top of the spiral to the top floor.
The center column acts as the primary support for the entire stair.
The spiral handrail, landing handrail, posts and balusters guard against falls off the stair and landing. Also, the spiral handrail provides a solid handhold.
The top center post supports part of the landing handrail system.
Building codes use spiral stair terms to differentiate the rules from those of straight stairs.
We list definitions of each of these terms below. Moreover, we discuss building code requirements and their consequences in a separate post, “Spiral Stair Building Codes: The Minimum Dimensions of a Spiral Staircase”.
Spiral stair terms regarding treads
The term “nosing” refers to the leading edge of a tread.
Spiral stair treads have the same outline as a slim piece of pie. The narrow end shapes itself to fit the center column. However, once assembled into a spiral stair, only a section of each full tread can be used.
Figure 2 shows an exposed portion of a tread sticking out from under the tread above it. The nosing of the tread above borders the exposed portion of the tread below. Spiral stair terms always refer to this exposed portion to define tread dimensions.
Building codes require that all (exposed) tread surfaces should be the same shape.
Tread width is one of the spiral stair terms that refers to the distance from the center column out to the handrail (see Figure 3). So, larger tread widths provide more shoulder room.
Except for small diameter spiral stairs, the tread area up to 12″ out from the center column is usually not used for a walking surface. People tend to keep closer to the handrail.
The term tread depth means the distance from the tread’s nosing back to the nosing of the tread above. Since this measurement varies, codes call for a minimum depth at 12″ out from the center column.
Vertical spiral stair terms
Stair rise means the height between any two treads. This is how far you step up or down at each tread. Codes require that the rise should be the same between all treads.
Headroom represents the height over spiral stair treads to an obstruction above. Specifically you measure headroom down to a spiral line that follows the tread leading edges (see Figure 4a).
This line parallels the passage of your upper body under the obstruction. The line is 12″ out from the center column. (Tread surfaces inside 12″ are too small for normal use.)
Often obstacles are located over a tread at a point between edges. For example, look at the edge of the landing in the overhead view in Figure 4a.
Figure 4b shows you how to use the line to tell how far you are under the top floor at this point.
Finally, Figure 4c outlines a sample calculation to find the headroom under the landing.