Slope (or Pitch) is the number of inches the roof rises ("rise") for every foot
of horizontal travel ("run"). Measure the slope from a ladder at a gable end, or on the roof surface itself.
A 3' level and a framing square (or a yardstick) is essential.
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:
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 "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 example, if you measured 18 inches with the yardstick, then the slope is 18 ÷ 3 = 6 inches
per foot - a typical term woul be "6 on 12" or "6/12".
Using a level with a built-in slope gauge can be helpful.
If you are using a level of a different length, then divide the yardstick measurement
by the length of the level in feet. For instance, for a 48 inch level (4 feet), divide by
4; for an 18-inch level (1.5 feet), divide by 1.5.
The longer the level, the greater the chance of a more accurate measurement.
A yardstick is easier to use than a tape measure because its rigidity will make it easier to make adjustments of both tools at the same time.