Rafter Length Calculator With Pitch & Overhang

How to Calculate Rafter Length (The Pythagorean Way)

A rafter is the hypotenuse of a right triangle: the run (half your building’s span, plus overhang) is one leg, the rise (run × pitch ÷ 12) is the other. Rafter length = √(run² + rise²), which is exactly what the pitch multiplier shortcut does in one step: (half-span + overhang) × multiplier. For a 24-ft-wide building at 6/12 with a 12″ overhang, that is 13 ft × 1.118 = 14’6″ — so you buy 16-ft stock. The calculator handles the math and the lumber rounding for you.

The classic mistakes are forgetting the overhang, and measuring rise from the wrong reference. Measure along the rafter, not vertically, and remember the ridge board steals half its thickness from each rafter (a 1.5″/2 = 3/4″ shortening at the plumb cut — small, but visible in the fit).

Rafter Length Chart for Common Spans

Lengths are ridge-to-tail along the slope. Note how fast steep pitches consume lumber — a 12/12 rafter is ~27% longer than a 4/12 on the same building.

What Size Lumber Do Rafters Need?

Length is geometry; depth is engineering. Rafter dimension (2×6 through 2×12) comes from span tables based on your snow load, spacing, lumber species, and whether the ceiling is attached. In Minnesota’s 35+ psf snow country, typical stick-framed roofs at 16″ on center need 2×8 for modest runs and 2×10/2×12 as spans grow. This is exactly where a permit reviewer earns their fee — bring your span, spacing, and load, and use the stamped tables rather than guessing. For manufactured alternatives, see our roof truss calculator — trusses replace rafter math entirely on most new builds.

Birdsmouth, Plumb Cut, and Tail: The Three Cuts

Every common rafter gets three cuts. The plumb cut at the ridge matches the roof angle (from our pitch calculator: 6/12 = 26.6°). The birdsmouth notches over the wall plate — code limits the seat cut so you never remove more than a third of the rafter depth. The tail cut shapes the overhang end, square or plumb depending on your fascia detail. Lay out the first rafter, test-fit it both ends, then use it as the pattern for the rest — production framers cut entire roofs from one proven template.

Rafters, Sheathing, and Everything Above

Once frames are up, the quantities cascade: sheathing from our sheathing calculator, then underlayment, ice barrier, and shingles from the roofing calculator. Planning a whole structure, it is worth running all four tools with the same dimensions — material lists that agree with each other are how projects stay on budget.

Rafter Calculator FAQs

How long is a rafter for a 24-foot-wide building?

At 6/12 pitch with a 12″ overhang: 14’6″ — buy 16-ft boards.

What spacing do rafters use?

16″ on center is standard for stick framing; 24″ appears with engineered lumber or lighter loads.

How much overhang should a roof have?

12–24 inches is typical in Minnesota — enough to shed water past the siding and shade summer sun without stressing the tails.

Rafters or trusses — which is cheaper?

Trusses, almost always, for spans over ~20 ft. Rafters win when you want vaulted space or are matching existing framing.

Do I need a permit to replace rafters?

Yes — structural roof framing requires permits and inspection in virtually every Minnesota jurisdiction.

Framing question turning into a roofing project? Owl Roofing handles the whole stack from structure to shingles — get a free consult. 🦉