A jack rafter is a unique rafter because it is cut off before it can reach the full length from the plate to the ridge. A jack rafter is a type of discontinued common rafter. Jack rafters are very similar to common rafters since they share the same plane, spacing, pitch, and length.
Because of the fact that they are equally spaced and their positions, each jack rafter is (x) times longer then the first one, (x) being the position of the rafter from the first one. In other words the 5th rafter would be 5 times as long as the first. Also, since the jack rafter has the same rise as a common rafter the same method can be used with a common rafter to find out the top and bottom cuts.
Finding the Length of a Jack Rafter
You can find the length of a jack rafter fairly easy. You have to multiply the value that is in the tables by the position number of the rafter. Once you have this answer you will want to subtract half of the diagonal thickness of the hip.