After shape, the most striking feature is pitch, the angle that roofs make
with the horizontal. Both gable and hipped roofs have low to steep pitches.
Mansard and gambrel roofs always have steep sided slopes. Flat roofs, of
course, have no pitch! 
Houses in the 1800-1900s generally had steep roofs, although the base was rather narrow. In the 1980s, steep roofs began to reappear on much wider houses, making the roofs appear more massive than in the 19th century.
Medium slopes range from 30 to 45 degrees.
Low slopes have a pitch of less than 30 degrees.
The
pitch of roofs decreased from the 1920s to 1950s, culminating in the ranch
style.
