Roof pitch is the measure of a roof's slope or angle of incline. Is there a difference between pitch and slope? Yes...and no. To understand how to properly use these terms, a brief lesson in roof geometry helps.

Roof pitch is expressed as a fraction, such as 1/4, each number representing the coordinates of an angle. That angle is based on a roof's rise (height) and span (width). Pitch is the rise over the span.

Say your house is 38 feet wide and the gable roof has a 1-foot overhang on each side; that makes the roof 's span 40 feet. From the eaves to the peak, it's 10 feet high--that's the rise. Figure 10/40 and reduce that to 1/4. It has a 1/4 pitch.

Okay, then what's its slope? In discussing pitch, you may also hear the term run, which is a horizontal measurement. A roof's slope is the number of inches it rises for every 12 inches of horizontal run. A roof with a "4-in-12 slope" rises 4 inches for every 12 inches of horizontal run. The same roof has a 4/12--or 1/3--pitch. Pitch and slope are two ways of expressing the same measurement.

The word pitch was first used in this manner in the early 17th century, rooted in earlier usage that denoted "the highest point," referring to everything from musical sound to the height a falcon reaches before swooping down to attack its prey.