How to design spiral stairs in three parts
This article shows you how to design spiral stairs to fit detailed floor plans. We offer this information for homeowners or anyone who is not experienced with spiral stairs design. Our post Spiral stair terms defines spiral stair parts and concepts and may be of help.
We present these instructions in three parts.
Part 1 deals with how to determine the number of treads, rises, and rise height. There are some simple calculations (Yuk!) but we give you the answers.
Part 2 discusses the top landing, total turn of the treads, and other basic concepts used to design spiral stairs.
Part 3 works uses the information of Parts 1 and 2 to develop a layout for three specific floor plans.
Below, Figure 1 shows floor plans of the three proposed installations. We present each as if you were above the stair, looking down on either the top or bottom floors.
When you design spiral stairs properly, the final layout must provide comfortable access to the top and bottom floors. Access is a path to the spiral that clears any nearby obstructions (see spiral stair access).
Floor height, codes and spiral stair diameter, important elements to design spiral stairs
First determine the height from the lower to upper floor. Measure this from the finished floor downstairs up to the top of the finished floor upstairs.
Will building codes govern the installation? If so, the spiral stair will need a diameter of at least 5″-0″ or a bit larger. If not, smaller diameters are available, though a larger spiral may be selected for comfort.
In Table 1 the floor height, code requirement and diameter are listed for each of the three design examples.
The spiral staircase height will influence the number of required rises (and give us the height of each rise). The number of rises determines the number of treads. The number of treads affects how far the spiral will turn when you climb it.
Detailed rise and tread calculations required to design spiral stairs
We have provided calculations only for those who might be interested in the detail. If you are not interested, skip down to Table 3. This contains a summary of rises and treads for various ranges in floor heights.
First, find the number of rises. Divide the floor height by the maximum code-approved rise of 9-1/2″. If the result is a whole number, this will be the number of rises. If the result is not a whole number, round up one, regardless of the size of the remainder.
For instance, divide the height of the spiral stair in Example 1 by 9-1/2″: 104.5″ divided by 9.5″ = 11. So this spiral stair will have 11 rises. Spirals with heights just slightly higher would need one more rise. For instance, if the height were only 1.00″ higher (105.5″ / 9.5″ = 11.11) the spiral would require 12 rises.
Second, find the rise height. Divide the stair height by the number of rises. For the spiral in Example 1, the rise height is 104.5″ / 11, or 9.50″.
Third, determine the number of treads. This will be the number of rises less one. (There is always one less tread than rises in any stair.)
Fourth, figure the spiral stair’s total turn when you climb it. We will use a rather typical tread rotation of 30 degrees*. So, if you have 11 treads, the total turn will be 330 degrees. * You can substitute any suitable tread angle. But check tread requirements for width and depth if your spiral stair has to be code approved.
Table 2 summarizes these calculations for all three spiral stair design examples.
Table 3 lists the number of rises and treads for different ranges of floor heights.