Econo grate Spiral Stair treads are available in steel, aluminum or stainless steel and can be used on welded, standard kits or heavy duty model stairs. This step works great for exterior applications, and as the name states, it will not break the bank. ¼" x 3" slots fanning off the center of the column not only offer excellent water drainage, but traction, and a contemporary touch to the spiral staircase.