Box steps are different than regular steps in that they use only two "solid" stringers (the sloping board that supports stair treads). Regular steps have "cut" stringers, which means the shape of the stair pattern is cut out of the support board. Not having to cut out the stringer makes box steps a less intimidating option for most do-it-yourselfers. There are limitations, however to the width of a typical box step system for decks, because the tread (usually two 2-inch by 6-inch lumber) is spanning the entire distance. Contact your local permit office for maximum spans in your area.
Measure total stair rise (TSR) in inches. This is the vertical measurement from the finished top surface to the finished bottom surface.
Divide the TSR by 7.5 (an average rise per tread) and round to the nearest whole number. This result is the number of steps you need.
Divide TSR by the result in Step 2 to get the precise riser height (RH) and so that each step is equal. Be sure to convert from decimal to fractions. Rounding to the nearest 1/8th inch is typical.
Locate the RH on one leg of the framing square and 10 inches (the step depth) on the other leg.
Place the framing square onto a 2-inch by 12-inch pressure-treated board so that both points align with the board's edge.
Trace the step location (10 inches) onto the lumber and then slide the framing square along the lumber edge so that the RH point aligns with the step mark of the previous tracing.
Repeat Step 6 for as many steps as you require.
For the bottom step, slide the square down and draw the RH and a line perpendicular to the RH to the back of the stringer.
Cut along the two lines in Step 8 with your circular saw and discard the waste.
Draw a line perpendicular to the back of the top step line down through the back of the stringer and cut this line with your circular saw. Discard the waste material.
Repeat Steps 4 through 10 for the other stringer. Be sure to make your marks on the correct side of the lumber. When both stringers are marked correctly, the markings will line up exactly if the boards are placed face-to-face. (That is to say, if one board were marked with wet paint instead of a pencil and the other board were placed over it, the pattern would be transferred.)
Align all the staircase angles (speciality hardware available at most hardware stores or lumber yards) at each step marking made in the previous step so that they form a ledge and nail them in place. The side of the angle resting against the stringer should face downward.
Nail two 2-inch by 6-inch boards to the staircase angle ledge. Repeat on the opposite stringer (be sure to nail to the same step's ledge).
Repeat Step 2 for all your step treads.
Place a 2-inch by 8-inch pressure-treated lumber board so that its face is against the back of a step (forming an "L") and its top is firmly against the bottom of the step above, and nail in place. This forms a riser.
Repeat Step 4 for all step treads.
Attach completed box steps to deck using slopeable stair stringer hangers (speciality hardware available at most hardware stores or lumber yards).
Consult your local permit office to determine the maximum allowable span for the step tread material you are using and be sure not to build wider than that. Use a cut stringer if necessary.