The baluster material can determine the quality and strength of the spiral stair design. On a conventional straight stair the baluster is not part of the structural strength of the stair. That is not true with a spiral stairs. The treads are cantilevered off of the center column and the stair balusters tie the outside structure of the stair together. Each main baluster is connected to the back corner of a lower tread and the front corner of an upper stair tread. SalterSpiralStair.com only offer its stairs with 1 ¾” x 1 ¾” pin top balusters not 1 ¼” x 1 ¼”.