What is a rafter?

The plan shows horizontal run, not slope. A rafter follows the pitch, so its true length is the run times the line length factor. On a 6:12 roof that factor is about 1.118, so a 15 ft run cuts at roughly 16 ft 9 in before the overhang.

Divide the roof plane length by the on-center spacing in feet and add one for the starting rafter. A 40 ft plane at 24 in OC gives 40 / 2 + 1 = 21 rafters per slope. Double it for both slopes of a gable.

They are the sloped structural members that carry the roof deck, running from the ridge down to the eave. In a stick-framed roof they are the field-built alternative to manufactured trusses, cut on site to the pitch. Each one bears at the ridge at the top and notches over the wall's top plate at the bottom.

In US light-frame work rafters are dimensional lumber, most commonly 2x8, 2x10, or 2x12, with the size driven by the span, the roof load, and the species and grade in the span tables. The size is called out on the roof framing plan and the wall section, so read it there rather than assuming a default. The length you order is the sloped length plus the overhang, rounded up to the next standard stock length.

Run is the horizontal distance the rafter covers in plan, measured from the wall to the ridge. On a gable roof it is half the building width. Run is not the rafter's length: multiply it by the pitch's line length factor to get the true sloped length the lumber has to cover.

Trusses are faster and engineered off-site for clear spans. Stick-framed rafters cost more labor but give a usable attic and handle odd geometry without a truss package. Span, attic use, and schedule decide it.