Figure 6-15.Unit rise and run.
are cut from solid pieces of dimensional lumber (usually
2 by 12s) and are called cutout, or sawed, stringers.
The first step in stairway layout is to determine the
unit rise and unit run (fig. 6-14, view B). The unit rise
is calculated on the basis of the total rise of the stairway,
and the fact that the customary unit rise for stairs is
The total rise is the vertical distance between the
lower finish-floor level and the upper finish-floor level.
This may be shown in the elevations. However, since
the actual vertical distance as constructed may vary
slightly from that shown in the plans, the distance should
At the time stairs are laid out, only the subflooring
is installed. If both the lower and the upper floors are to
be covered with finish flooring of the same thickness,
the measured vertical distance from the lower subfloor
surface to the upper subfloor surface will be the same as
the eventual distance between the finish floor surfaces.
The distance is, therefore, equal to the total rise of the
stairway. But if you are measuring up from a finish floor,
such as a concrete basement floor, then you must add to
the measured distance the thickness of the upper finish
flooring to get the total rise of the stairway. If the upper
and lower finish floors will be of different thickness,
then you must add the difference in thickness to the
measured distance between subfloor surfaces to get the
rise of the stairway. To measure the vertical distance, use
a straight piece of lumber plumbed in the stair opening
with a spirit level.
Lets assume that the total rise measures 8 feet
11 inches, as shown in figure 6-15. Knowing this, you
can determine the unit rise as follows. First, reduce the
total rise to inches-in this case it comes to 107 inches.
Next, divide the total rise in inches by the average unit
rise, which is 7 inches. The result, disregarding any
fraction, is the number of risers the stairway will
havein this case, 107/7 or 15. Now, divide the total
rise in inches by the number of risers-in this case,
107/15, or nearly 7 1/8 inches. This is the unit rise, as
shown in figure 6-15.
The unit run is calculated on the basis of the unit
rise and a general architects rule that the sum of the unit
run and unit rise should be 17 1/2 inches. Then, by this
rule, the unit run is 17 1/2 inches minus 7 1/8 inches or
10 3/8 inches.
You can now calculate the total run of the stairway.
The total run is the unit run multiplied by the total
number of treads in the stairway. However, the total
number of treads depends upon the manner in which the
upper end of the stairway will be anchored to the header.
In figure 6-16, three methods of anchoring the upper
end of a stairway are shown. In view A, there is a
complete tread at the top of the stairway. This means the
number of complete treads is the same as the number of
Figure 6.16.Method for anchoring upper end of a stairway.