Measure the distance between the outside walls at their top, where the rafters meet the wall. This is the overall horizontal run of your roof, including both faces. Multiply the feet by 12 to get inches and add any remainder in inches to get a total number of inches. Divide this number by two, then divide by 12. So, if your walls are 10 feet apart, you have 120 inches, divided by two equals 60, the number of inches run for one face of the roof. Sixty divided by 12 equals five, the number of feet of horizontal run.
Measure from the center of the top of the wall to the peak of the roof. This number represents the overall rise of the roof. Divide the feet by 12 to get inches and add any remainder in inches to get a total number of inches in rise. If your roof rises 1 foot, 8 inches, that is an overall rise of 20 inches.
Divide the product of Step 2 by the product of Step 1 to get a total number of inches in rise for every foot. In the example expressed in steps 1 and 2, divide 20 inches by 5 for a total of 4 inches rise per foot. Express it in any of the three forms, such as 4/12; 4-in-12, or 4-to-12.