When building a house, roof pitch is an indication of the steepness of the roof that is designed for your house.
The roof pitch is the vertical rise divided by the horizontal span.
Your roof is considered pitched (very steep) if its pitch factor is greater than 3.215 in 12, or equivalently if its angle from horizontal is greater than 15 degrees. The main purpose of a pitched roof is to shed rain. Typically, the pitch of roofs is greater in areas where there is more precipitation. Houses in areas of low rainfall frequently have roofs of low pitch while those in areas of high rainfall and snow have steep roofs.
Example: The pitch of a shed roof with an 4 ft rise (above wall height) by a 12 ft span (between exterior supporting walls) will be 1/3. The pitch of a gable roof with a 3 ft rise by a 18 ft span will be 1/6. However, the slopes of each roof will be the same (1/3).
In the United States, slope is typically given in inches per 1 foot or as a ratio of inches per 12 inches; and commonly referred to with units of “pitch” (e.g., for a slope of 1/3, “4 pitch(es)” is 4 inches of rise over 1 foot of run; “4:12 pitch” is 4 inches of rise over 12 inches of run).