Taking measurements of the slope, or pitch, of a roof requires two ruled levels. The measurements take the form of a ratio that shows the height increase of the roof over a given distance.
Place one ruled level at the roof's peak and measure out 12 inches horizontally. From that point, measure down vertically with the second ruled level, noting the length where it intersects with the roof. The measurement of the slope is always rise (vertical distance) over run (horizontal distance), so a measurement of one horizontal foot and seven vertical inches gives a slope of 7/12, or seven inches of rise for every twelve horizontal inches.