WHAT IS ROOF/WALL "SLOPE" (OR "PITCH")?
The slope—also called pitch—of a roof or angled wall is the ratio of the structure’s rise to its run. See the illustration below.
HOW TO DETERMINE SLOPE of your roof or wall
You will need a level and a tape measure to determine the slope of your roof or wall.
1) Even if your level has a ruler on one side, it's usually easier to measure and mark a line 12 inches from the end of the level at the bottom edge. 12 inches is a typical measurement of run.
2) Use your level to measure the roof run. While holding the tape measure with one hand, place the bottom corner of the level on the roof. Using the level's corner as the pivot point, hold it parallel to the ground (the horizontal vial's bubble is between the two lines marked on the vial).
3) Measure the rise with the tape measure. Keeping the level parallel to the ground, hold the tape measure perpendicular to the level and measure the distance from the roof's surface to the 12-inch mark on the bottom of the level. This measurement is the rise of your roof.
4) The slope, or pitch, of your roof is the rise you just measured compared to the 12-inch run marked on our level. In the example below, the rise is 5 inches and the run is 12 inches. The slope is expressed as the raio of run:pitch. In this example, the roof's slope is expressed as 5:12.