Your roof

Pitch (rise per 12)

/ 12

Run

Outside of the wall to the center of the ridge (half the span).

ft in (6 1/2 ok)

Rafter board

Seat cut (birdsmouth)

Overhang (level, past the wall)

Ridge board thickness

Results

Common rafter · line length to ridge center

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Ridge cut to the wall line

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Ridge cut to the tail cut

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Plumb cut

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ridge & tail

Seat cut angle

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level cut

HAP

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mark on the wall

Heel

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House tip (not code)

The rafter, 2D (broken to fit)

The math, number by number

Angle
Factor (line per foot of run)
Line length
Half the ridge, on the rafter
Tail on the rafter
Ridge cut to the tail cut
HAP
Left of the depth above the notch

How to use it

Pick the pitch, enter the run (outside of the wall to the center of the ridge, half the span), then the rafter board, the seat cut (3½″ on a 2×4 wall, 5½″ on a 2×6), the overhang and the ridge board thickness. Fractions are fine: 6 1/2 or 6-1/2.

Line length runs along the rafter from the wall line (plumb over the outside of the wall) to the center of the ridge. The plumb cut at the ridge comes back half the ridge board, so ridge cut to the wall line is the line length minus that, and ridge cut to the tail cut adds the overhang on the rafter. That last one is the length you cut the board to.
HAP (height above plate) is the number you mark on the wall, measured plumb.
The notch check follows IRC 2018: with an overhang, at least 3½″ of the rafter has to stay above the birdsmouth, measured square to the edge, and the overhang stays at 24″ or less (R802.7.1.1). Flush, the end notch can't pass ¼ of the depth (R802.7.1).

Why square and not plumb? Measured plumb, the leftover wood always reads bigger, so it can pass a notch the code figure fails. The HAP stays a framing number; the check uses what is left measured square to the edge.

What it does not do: size the rafter for span, load or snow. That comes from the rafter span tables (IRC 2018 R802.4.1) or the engineer. Check which code edition your jurisdiction uses.

En español

Pa' los que cortan rafters a mano. Le metes el pitch, el run, la tabla y el asiento, y te da el largo del rafter al centro del ridge, el largo del corte del ridge a la línea de la pared (ya con el medio ridge descontado) y al corte de la cola, que es con el que cortas la tabla, el HAP pa' marcar en la pared y el check de la muesca del birdsmouth. Cada número trae su cuenta abajo, pa' que la revises antes de cortar.

El run va de la cara de afuera de la pared al centro del ridge: la mitad del claro.
El HAP es el que se marca en la pared, a plomo. Pa'l código no se usa ése: se usa lo que queda del canto medido perpendicular, porque a plomo sale más grande y te puede engañar.
La muesca se revisa contra el IRC 2018: con overhang, del canto tienen que quedar 3½″ o más, medidos perpendicular, y el overhang no pasa de 24″ (R802.7.1.1). A ras, la muesca del extremo no pasa de ¼ del depth (R802.7.1).

Lo que no hace: dimensionar el rafter por claro, carga o nieve. Eso sale de las tablas de rafters (R802.4.1) o del ingeniero. Y checa qué edición del código usa tu ciudad.

Hips, valleys and jacks, a cut list that shows where every line comes from and a step-by-step cut sheet, on your phone and off-line on the jobsite.

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