Building your own wooden spiral stairs can be a fun and exciting building experience. You should be an experienced builder before attempting to build wooden spiral stairs. It will require math, saws, precise measurements, and most of all time and patience. If you don't have that kind of experience, you can buy a kit for spiral stairs and it can be finished with nothing more than a drill in just three hours. But, if you prefer to do it all yourself, this article will detail how to build a wooden spiral staircase that will last for decades.
Step 1 - Measure Your Project
The most important part of stair building is the measuring. It is the one thing that you must absolutely get right in order for things to work. So be careful, and go slowly. First you need to measure the floor to floor measurement. This is assuming you already have an opening ready for the stairs to be installed. If not, you need to go ahead and cut the opening in order to make this measurement. For the sake of this article, we will assume that the floor to floor height is 9 feet.
In that case, to determine the number of steps needed, we divide the 120 inches (10 feet) of vertical height by 7. A 7-inch rise per step is ideal. 120-inches ÷ 7 inches = 17.14 steps. Since we cannot have an uneven number of steps, we will round down to 17 steps. The upper level will be the 17th step, so you will construct 16 steps. Then 120-inches ÷ 17 steps = 7.05 inches. Using this calculation, you have found that each rise (the vertical distance from one step to another) is 7.05-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. 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. You can determine the circumference by taking the diameter of the stairs and multiplying it by 3.1416. So, 68 inches x 3.1416 = 210-inches (rounded to the nearest inch). Since we will make 17 steps, 210 ÷ 17 = 12.35-inches. To allow for a 1-inch overlap, we will make the treads 13.5-inches deep (the horizontal measurement, front to back) at the outside end.
Step 2 - Start with the Center Pole
Now that you have your measurements, it's time to begin assembling the stairs. We're going to build a wood spiral stair with a metal center pole and metal tread bases. You can later cover the metal pole with wood veneers if you choose, and each tread base will be covered with a wood tread.
Using a 3 1/2-inch schedule 40 pipe for the center column, which measures 4 inches outside, cut it 158-inches long. That allows 120 inches for 17 steps, 36 inches for the top guard railing, and 2 inches extra for the sake of appearance. If you do not want the center pole to be a part of the upper railing, just remove the additional 36 inches. Before installing the center pole, go ahead and mark the pole where you are going to install each of the treads, every 7.05 inches. Then, you can cap the top end with a 1/8-inch thick round plate and ground the weld smooth. The bottom end needs to have a square 1/2 inch plate measuring 12 by 12 inches, welded all around. Then punch 7/16 inch holes in each corner of the plate to receive 3/8 inch concrete anchor bolts to attach it to the foundation.
Step 3 - Construct Treads
For the tread base construction, you can either purchase pre-made metal tread bases, or you can make your own using a 1/8 inch steel safety floor plate (also called diamond tread). Tack the treads onto the center column at your marks, being sure to get them perpendicular to the column in both directions. Advance the treads according to which way your stair is turning. Each tread needs to overlap the preceding tread by 1 inch so you can add your newel posts. Newel posts are the posts that connect one tread to the other. For your stairs, you can choose any type of material for newels and balusters, from metal to wood.
Once all of your tread bases and newels are attached, go back and weld all of the bases to the center column. This is also a good time to add your wood tread that goes over your metal base. Like our calculations in step 1, the treads should be cut 4 inches in the interior width, 13.5 inches in the exterior width, and 32 inches long. You can just notch the wood in the area that you have your newel post installed.
Step 4 - Finish Up
Now that you have both the treads and newels installed, all that is left is to add the balusters and handrail. Like the newel posts, you can choose any type of construction for balusters, but you want to choose something that will compliment your newel post choice. Go ahead and screw in 3 balusters per tread, so that there is never an opening wider than 4 inches, as per most building codes. After the balusters are installed, you can attach the handrail to the newel posts and balusters. The handrail needs to be purchased pre-formed so that it can curve along the stairs. After the handrail is installed, you are finished and free to enjoy your new spiral stair.