Pitch, degrees, and percent slope
Roof pitch is traditionally expressed as the rise in inches per 12 inches of horizontal run. A 6/12 pitch means the roof rises 6 inches for every 12 inches of run. The equivalent angle in degrees is the arctangent of the rise divided by the run. Percentage slope is the rise divided by the run times 100.
Low-slope roofs (below 2/12) require membrane or built-up roofing. Standard asphalt shingles are suitable for pitches from 4/12 to 12/12. Steeper pitches above 12/12 need additional fasteners and may require special steep-slope products.
Angle = arctan(Rise / Run)
A 6/12 pitch equals arctan(6/12) which is approximately 26.6 degrees.
Slope factor = sqrt(1 + (Rise/Run)^2)
Multiplied by the horizontal footprint area to get the actual roof surface area.