Build a Spiral Staircase
Are you planning a new addition to your home or business and realize that you need a more convenient way to access the new area—but you don’t have room for a conventional stairway? Perhaps you’re adding a workout room on a lower level and need a way to get to that area from the master bedroom. Have you built a deck and need quick access to another level?
Consider the advantages of a spiral staircase. It takes less space than a conventional stair and can be a beautiful conversation piece as well.
Although a spiral staircase cannot be used as the main access to an area of your home (per building code requirements), it can be a convenient way to get from one level to another, and, it will take half the space.
You can build your own spiral staircase with a little bit of welding skill and about $400.00 worth of steel, even at current higher steel prices. I have built 18 spiral stairways in the last 15 years, all with some wooden components (treads and handrails). This article explains how to do so.
I recently removed a second-story deck from my house because it was rotting and was in a bad location, in full view of traffic, on a busy street in front of our house. Access to the deck was through a door in the kitchen/dining area. My wife and I decided that a new steel spiral stairway should take the place of the deck to provide a fire escape and to add a beautiful accent to our home.
Planning and Design
Every spiral staircase is built around a center column that rests on a single point at its base (See Fig. 1). I dug an 18-inch hole in the ground below the kitchen door 4 feet deep—below the frost line in our Minnesota climate. I filled a cardboard tube with concrete to a height 3 inches above grade and level with the top of the basement block wall. I used a trowel to smooth and level the concrete.
Now we had a good foundation to build on. I needed just one measurement to get started: the total rise.
The total height from the concrete foundation to the top of the top step was exactly 9 feet or 108 inches. I recorded that figure and chose to make the stairway turn to the left (counterclockwise) when walking up.
But first I had to consider a few building code requirements in order to lay out the stairway.
The national building code requires that each tread must be at least 26 inches wide. Each tread must be at least 7-1/2 inches deep at a point 12 inches from its narrow end.
There must be a minimum of 6 feet, 6 inches of headroom at each step. The handrail must be 34 to 36 inches above each tread nose. The stair railing balusters must not allow a 4-inch ball to pass through.
Additionally, the upper platform must extend 7 inches beyond a door that may open out onto it, measured when the door is fully open.
The planning and layout for this exterior stairway will apply to any spiral staircase installed anywhere to any height and at any diameter.
Because of the applicable building codes, a stairway that is about 9 feet tall works well when it needs to make approximately one full turn through its vertical travel. That means that the first step at the bottom is directly below the last step at the top. Someone using the stairway is facing the same direction at the top and at the bottom steps. This combination of a 360-degree turn over 9 feet of vertical travel also allows the proper headroom required by the building code.
To determine the number of steps needed, we divide the 108 inches of vertical height by 7. A 7-inch rise per step is ideal. 108 inches ÷ 7 inches = 15.43 steps. Since we cannot have an uneven number of steps, we will choose 15 steps. The top deck will be the 15th step, so we will make 14 steps plus the deck. Then 108 inches ÷ 15 steps = 7.2 inches. We’ve now determined that each rise (the vertical distance from one step to another) is 7.2 inches.
The depth of each tread (the run) at the narrow end will be 4 inches to allow it to fit a 4-inch center column with room for welds on the top and sides.
Step run (the horizontal depth of each step at the wide outside end) can be determined by first choosing the outside diameter of the stairway. The building code requires a minimum of 5 feet, 2 inches because of the tread width minimum of 26 inches. If we add the width of two 26-inch steps, a center column and a handrail, we find that a 62-inch diameter is a minimum for any spiral staircase. I chose a 70-inch diameter to make the stairway easier to walk. That made the treads each 32 inches wide instead of the 26-inch minimum.
For purposes of sizing the treads at the outside end, let’s consider the outside diameter of the stairway without the balusters and handrail. The center column and two treads make a total diameter of 68 inches.
The circumference of the stairway is the diameter in inches multiplied by 3.1416. So, 68 inches x 3.1416 = 210 inches (rounded to the nearest inch). Since we will make 14 steps, 210÷14=15 inches. The steps need to overlap by one inch to accommodate the newel posts that tie them together and support the handrail and balusters. So we will make the treads 16-inch deep (the horizontal measurement, front to back) at the outside end. The newel posts are welded to the heel of one tread and the nose of the tread above it. (See Fig.2 above).
So far we have the following information: The stairway total rise is 108 inches (7.2-inch per step). The stairway will turn left on the way up. The stairway has 15 steps (14 steps plus the top deck). The stairway will make one full 360-degree turn through its travel. The outside diameter is 70 inches, including the newel posts and handrail. The handrail will be 36 inches high.
The Top Deck
The top deck, which, for planning purposes is the 15th step, needs to be at least as wide as the top step, but can be wider. I chose to make mine 42 inches wide to make ample room for the screen door to open easily and for easy access to the first step. The deck must extend 7 inches beyond the screen door when the door is wide open. My door is 36 inches wide, so I made the top deck 44 inches deep (from the house to the outer edge of the deck). I made the guard rails on the upper deck 36 inches high with baluster spacing just under 4 inches.
I used a 3-1/2-inch schedule 40 pipe for the center column, which measures 4 inches outside. I cut it 146 inches long. That allowed 108 inches for 15 steps, 36 inches for the top guard railing, and 2 inches extra for the sake of appearance.
Then, I capped the top end with a 1/8-inch thick round plate and ground the weld smooth. The bottom end got a square 1/2-inch plate measuring 12 by 12 inches, welded all around. I punched 7/16-inch holes in each corner of the plate to receive 3/8-inch concrete anchor bolts to attach it to the foundation.
In order to make the layout easier, I laid the column horizontally into a simple jig that I made many years ago for just this purpose. A couple of sawhorses will work just as well. I then laid out soapstone marks for each step approximately 7.2 inches apart, beginning at both ends and adjusting the spacing of the marks in between for equally spaced steps. As discussed earlier, the top mark is 108 inches from the bottom end of the column and is the attaching point for the top deck.