Slope is
defined for roofs as equal to the number of inches the roof rises ("rise") for
every foot of horizontal travel ("run".) You may measure slope from a ladder at
a gable, or from the roof surface itself. All you would need is a 3' level and a framing square (or a yardstick will suffice).
With these, measure the rise over the three foot length of the level, then divide by 3 to get
the rise per foot as follows:
Numbers "1 2 3 " are
shown to indicate "proper end up" for the yardstick
Hold the yardstick tight to the end of the level (the end is square so that the level and yardstick
are at right angles)
Hold the level so that it's "dead level";
adjust the yardstick until it touches the roof
Read the yardstick (see arrow) to find the inches of slope in 3 feet (the length of the level)
Divide by three to get slope in inches per foot
(the level is 3 feet long, you measured the rise per 36 inches. To get rise per
foot, the reading must be divided by 3)
For instance, if you measured 15 inches with the yardstick, then the slope is 15 ÷ 3 = 5 inches
per foot - a typical value - also termed "5 on 12" or "5/12".
Other Methods / Comments:
Use a special engineer's level with a
built-in slope gauge
Use a level of a different length. The example was easy, because the the length of the level
(in feet) was an integer - 3. If you use a different length, then divide the yardstick measurement
by the length of the level in feet. For instance, for a 24-inch level (2 feet), divide by 2;
for an 18-inch level (1.5 feet), divide by 1.5.
The longer the level, the greater the chance
of accurate measurement. A good aluminum level is light, accurate,
inexpensive, and easy to manage
A
yardstick is easier to use than a tape - its rigidity will make it a little
easier to "fumble around with two loose sticks" while you're adjusting both
tools
