Almost every carpenter has made this mistake once in their early career: you calculate a perfect 7.5-inch riser height, cut all your stringer notches, install the stringers, nail on 1.5-inch thick 2×6 treads—and suddenly find the bottom step is 9 inches high while the top step is only 6 inches high!
The Correction Formula
Bottom Stringer Cut = Riser Height − Tread Material ThicknessBecause the bottom tread sits on top of the bottom stringer notch, you must trim the thickness of that tread off the bottom seating foot of the stringer.