What is the actual rise of a roof rafter?

Depends on what pitch the roof is. Measure the ceiling joist to a point or to center and then plumb to the rafter and do the math. 3, 4, 5, x 12 pitch rises that much per foot of distance on the ceiling joist. 3x12 pitch, 12 feet to center of joist, 3x12=36. 36 inch rise from the edge of the roof to the center.