Common rafter basics
A common rafter runs from the wall plate to the ridge. Its length follows from the run (half the span) and the rise (run × pitch ÷ 12).
rafter = √(run² + rise²) + tail
Remember to add the overhang tail and subtract half the ridge board thickness when cutting. For birdsmouth notes see rafter length and birdsmouth, for size see rafter size and for the comparison read truss vs rafter.