Place a piece of masking tape at the 12 inch mark on the level.
Climb a ladder and carefully walk to the top of your roof.
Balance and hold the level at the peak of the roof. Place the taped mark so it makes contact with the roof's surface at its highest point.
Place the metal straight edge so it is perpendicular to the end of the level. Move the straight edge straight down so it meets the roof surface while maintaining contact with the end of the level, which is 12 inches from the peak. Keep your finger on the point where the straight edge meets the level after you have formed this triangle with the roof.
Measure the straight edge from your mark to the bottom edge. Alternatively, if it is a metal ruler, simply read the measurement. This figure completes your determination of roof pitch. For example, if the measurement of the straight edge is 3 inches, your roof pitch is 3/12.