The roof deck has been a cause of some confusion with some readers. This is the point that is located at the "top" of the roof wall plate in a conventionally framed house. This plate is also commonly called the rafter plate.

In a home that would be using part of the attic space for living space this would be located at the top of the plywood deck that would be used as the floor.

This height can be determined by a specification on the drawing, code or some other person in charge. It can also be determined by calculation. The calculation would be if the propose roof slope is an 8-12 and the span is 32', the run will be 16'. Since the height is determined by the roof slope, (8) multiplied by the run (16'), you would have rz = (8 * 16) which equals 128". You have to remember that the run is an incremental value of how many 8" to travel vertically, so your answer will always be in inches!