Pinwheel spiral stair step is available in mild steel, stainless steel, and aluminum and can be incorporated in all kit and fully assembled units. As the name states, the 1/4" slots fanning off the center column create "Pinwheel" effect, looking down from step to step. Although this spiral stair step works great for exterior use, offering great water drainage and traction; its also a good option for interior spiral stairs letting natural light through, and adding to the over all appeal of the Spiral Stair unit.