In a given stair run (see Figure 2-38), the number of treads will be one less than the number of risers. A
10 1/2-inch tread will be correct for the following example, and the total run would be calculated as
follows:
Number of treads = 12
Total run = 10 1/2 inches x 12 treads = 126 inches or 10 feet 6 inches
The stairs will have--
13 risers each, 7 1/4 inches high.
12 treads each, 10 1/2 inches wide.
A total run of 10 feet and 6 inches.
Figure 2-38. Stair run
2-19. Stairway Frames. To frame simple, straight, string stairs--
a. Take a narrow piece of straight stock, called a story pole, and mark on it the distance from
the lower-floor to the upper-floor level. This is the lower-room height, plus the thickness of the floor
joists and the rough and finished flooring. It is also the total rise of the stairs. Keep in mind that a
flight of stairs forms a right triangle. The rise is the height of the triangle, the run is the base, and the
length of the stringers is the hypotenuse.
b. Set dividers at 7 inches, the average distance from one step to another.
c. Step off this distance on the story pole.
d. Adjust the divider span slightly if this distance will not divide evenly into the length of the
story pole. Step off this distance again.
EN5156
2-28
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