To understand the typical roof slope in the US, you must first understand how a roof's slope is measured. The first number in the two-number pitch description is the number of inches that rise for every inch of roof run, or length, which is the second number.
For example, a 4-12 roof rises 4 inches for every 12 inches of roof run, or at an angle of 1843 degrees. A roof in the 12-12 category is a perfect 45-degree angle.
National Standard and Trends
Throughout the US, 4-12 roofs are the most commonplace and are considered the national standard. However southern roofs are commonly built in the 3-12 pitch range, or at 1404 degree angle.
If a home's roof is in the 2-12 to 4-12 range, it is considered a low slope, while homes in the 4-12 to 21-12 category are high-sloped. Mansard roofs, which are built for aesthetic effect, have extremely steep roof slopes in the 21-12 to 60-12 range.
To get an idea of how steep this is, a person cannot safely walk on a roof with a pitch higher than 8-12.
The slope of a roof is dependent upon the style of the roof itself. In the US, roofs are most often constructed as gable, hip or Mansard.
A gable roof is a symmetrical roof that looks like an upturned, open book lying atop the frame of the house. Gable roofs can be any slope but are common in the 3-12 category.
Hip roofs are low- to high-sloped roofs that are shaped like a trapezoid pyramid with four connecting points on each corner that meet at the top, or the ridge of the house. A Mansard roof, which is found on more traditionally styled homes, is a hip roof with extremely sloped sides that rise to meet a flat roof.
Less expensive to build than steep roofs, a low-pitch roof is nevertheless more problematic in winter climates because of snow and ice accumulation. If you live in areas with harsh winters, consider a roof in the 4-12 and up range.
The higher the pitch, the less weather-related accumulation. If you already own a low-pitch home and ice and snow are common in your area, add an ice and snow shield to prevent ice damming.