Which unit of measure is always equal to 12 inches on a common rafter?

The unit of measure that is always equal to 12 inches on a common rafter is the unit of run. In roofing terminology, the run refers specifically to the horizontal distance over which the rafter spans, typically measured from the center of the ridge down to the outside edge of the wall plate. For conventional framing, the unit of run is standardized at 12 inches, which makes it a basis for calculating slopes and pitch in roofing design.

This standardization allows for easier calculations when determining the overall length of rafters as well as the angle of the roof slope. Understanding this measurement is crucial for any construction work involving roofs, as it directly influences the design and stability of the roof framework.