I'm in the process of repairing steps leading into my basement apartment. The threads are put together with 2x10 pressure treated lumber and the riser height is 6 3/16" high. There's 2 stringers spaced 36 1/4" apart. Do I need to add a 3rd stringer in the middle or will 2 suffice? I'm not sure what the max span distance for 2x10 board, but the old steps seemed to have held up pretty good over the years, no signs of warping etc.