To determine the actual length of any common rafter you need to know H.A.P. to calculate the actual rise (arz).  . The H.A.P. Can be determined by setting or holding your framing square at the appropriate roof slope on you rafter stock. Drawing a plumb line and then measuring it. Take a value that's approx. 2/3 of the total measurement, this will be your H.A.P. Dimension. If we are using 2"x8" as your rafter stock we can determine the H.A.P. Is 6", just like in the diagram above.