stairs steeper, the riser should not exceed 8 inches. To make the stairs steeper, increase the rise and
shorten the run (see Figure 2-37).
Figure 2-37. Stair design
2-17. Stair Design. Use the following rules when designing stairways:
Rule 1. The sum of 2 risers and 1 tread should equal 25 inches.
Rule 2. The sum of 1 riser and 1 tread should equal 17 to 19 inches.
Rule 3. The height of the riser, multiplied by the width of the tread, should equal approximately 75
inches. According to rule 1, a riser of 7 1/2 inches would require a tread of 10 inches. A 6 1/2-inch
riser would require a 12-inch tread.
2-18. Stairway Calculations. To calculate the number and size of risers and treads for a given run,
first divide the total rise by 7. If the total rise for a stairway is 7 feet 10 inches or 94 inches, the answer
will be 13.43. Since there must be a whole number for risers, select the one closest to 13.43 (13) and
divide it into the total rise.
94 inches divided by 13 = 7.23 or 7 l/4 inches
Number of riser = 13
Riser height = 7 1/4 inches