When you install wood spiral stairs on concrete, be careful. You should never leave untreated wood in direct contact with concrete, because even old concrete retains enough moisture to damage the wood. For this reason most building codes do not allow untreated wood against concrete.
Here are three ways to protect your installation of concrete floor wood spiral stairs. You should ask your building inspector which method he or she would prefer.
Install spiral stairs on concrete using a moisture barrier
First, you could place a moisture barrier under all the wood spiral stair parts that will sit on the concrete. (We will refer to these parts as “floor parts” in the rest of this article.) Figure 1 illustrates a bottom tread and the floor parts involved.
We suggest that you use two layers of either 6-mil polyethylene or building felt. Cut these materials to fit the floor parts. To assure complete coverage, leave a border of about 1″ all around. You can trim this border later.
You will need to pre-drill the concrete to precisely match the fastener holes in the spiral stair parts that will sit on the floor.
Use treated floor parts under wood spiral stairs on concrete
Second, you could ask your manufacturer to supply the lower parts in treated pine. Figure 2 shows how this would work. You could sand the treated parts to be as smooth as the rest of the stair, and stain them as well. Their color may be different but they will be partially hidden down on the floor. Again, precise holes in the concrete are required.
Figure 2 also demonstrates an alternative method that uses 1/4” treated plywood against the concrete. This way you could hide the treated wood altogether with caulk. Your manufacturer will have to take 1/4″ off the height of the bottom tread so the plywood will fit.
Place 3/4″ treated plywood under spiral stairs on concrete
Third, you can install wood spiral stairs on concrete on top of a larger piece of 3/4” treated plywood. The thicker plywood provides more material to fasten down the floor parts. Ask your manufacturer to construct the bottom tread with a rise 3/4″ less than required.
Figure 3 illustrates how this piece might look. Leave at least 1” under the tread nosing for toe clearance. You can extend the plywood behind the bottom tread. This provides easy access to fasten the plywood down after you install the spiral.
Drill 1” diameter holes in the plywood. Once you install the stair, you can drill into the concrete through these holes without being precise. Use fender washers with the concrete fasteners to cover the 1” holes.
Before starting any of these installation methods, consider the type of concrete fastener you will use. There several kinds. These require different tools and different ways to drill into the concrete. Also, select fasteners meant to be used for treated wood.