Question

What is a "jack" rafter, what is a "roof tree," and what would you call the shortest distance between two opposite common rafter seats?

Answer

A "jack" rafter is the rafter that spans the distance from a wall plate to a hip or from a valley to a ridge. "Roof tree" is an old term for the ridge of a roof. The shortest distance between two opposite common rafter seats is the "span" of the roof. Rafter seats are the flat cuts near to the rafter tail made so that a sloped rafter can sit neatly upon and be secured to the top wall plate.