How do I work out a common rafter length?

The rafter is the hypotenuse of a right triangle whose base is the horizontal run and whose height is the rise. Its length over the run is the square root of run squared plus rise squared. For a 12-foot run at a 6/12 pitch the rise is 6 feet, so the rafter is √(144 + 36) ≈ 13.4 feet before the overhang.

Is the run the same as the span?

No. The span is the full distance the roof crosses; the run of a common rafter is the horizontal distance from the wall to the ridge, which for a symmetrical gable is half the span less half the ridge thickness. Enter the run, not the span, and this tool gives the sloping length over that run.

Does this size the rafter or tell me the maximum span?

No — it gives a cut length only, the same way a stair tool gives a stringer length. Whether a rafter of a given size can span a distance under snow, wind and its own weight is a structural question answered by span tables and an engineer, and it is deliberately not something this tool outputs.

How do I add the overhang and the ridge cut?

Enter the horizontal overhang and the tool extends the rafter along the slope by the same multiplier. It does not deduct for half the ridge thickness or add for the plumb and seat cuts — those are small adjustments you mark out on the stock once the overall length is known.

Rafter length calculator

Recorrido (horizontal)

Del muro a la cumbrera, no el claro completo.

Pendiente (altura por 12)

/12

O lea el ángulo abajo, en el resultado.

Alero (horizontal)

Medido en horizontal, desde la línea de la solera hacia afuera. Escriba 0 si no hay.

— ft

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This is geometry — the length to cut a rafter over the run and pitch you enter, plus the overhang along the slope. It is not a span rating and does not say what size the rafter must be or what load it will carry; that belongs to your plans, a span book, or an engineer. Mark the ridge and seat cuts on the stock before you saw.

Enter the horizontal run and the roof pitch and the tool returns the sloping rafter length over that run, then extends it by your overhang along the same slope. Pitch is the rise per 12 of run, so the rise is simply the run scaled by pitch over twelve.

Ejemplo resuelto

A 12-foot run at a 6/12 pitch rises 6 feet, so the rafter body is √(12² + 6²) ≈ 13.42 ft at about 26.6° from horizontal. A 1-foot horizontal overhang stretches along the slope to 1 × (13.42 ÷ 12) ≈ 1.12 ft, so the whole rafter is roughly 14.54 ft before the cuts.

In metric: a 3.6 m run at 6/12 rises 1.8 m, so the body is √(3.6² + 1.8²) ≈ 4.025 m. A 300 mm level overhang is 0.3 × 1.118 = 0.335 m along the slope, and the whole rafter is about 4,360 mm before the cuts.

En la obra

A shed roof, buying stock. 10 ft of run at 4/12: the rise is 10 × 4 ÷ 12 = 3.33 ft, the body √(10² + 3.33²) = 10.54 ft, and a 1 ft overhang adds 1 × 1.054 = 1.05 ft. Total 11.6 ft, which reads on the tape as 11′ 7 1/8″. You buy 12 ft stock — the yard sells in 2 ft steps (0.3 m steps in metric), and the plumb cut at the ridge and the tail cut both come out of the same stick, so a 12 with 5 inches to spare is right, not wasteful.

A gable with a real ridge. 28 ft span, 1½ in ridge board, 8/12, 16 in overhang. Run is half the span less half the ridge: 14 ft − ¾ in = 13.94 ft. Rise 13.94 × 8 ÷ 12 = 9.29 ft; body √(13.94² + 9.29²) = 16.75 ft. Overhang 1.333 ft × 1.202 = 1.60 ft. Total 18.35 ft — over an 18, so 20 ft stock, with 1.65 ft coming off each. Forget the ridge deduction and the rafter is ⅞ in long at the peak on both sides: the ridge board does not fit and the plates spread until it does.

Checking a cut against the square. The angle line on the result is the plumb cut: 26.6° for a 6/12, 33.7° for an 8/12. Set the saw to that and the ridge cut lands plumb; the seat cut at the birdsmouth is the same angle from the other edge. If the speed square's pitch scale and this number disagree, the pitch you typed is wrong, not the square.

What went wrong?

Quick referencefor rafters

Rafter multiplier: √(1 + (pitch ÷ 12)²) — slope length per foot of run
By pitch: 3/12 = 1.031 · 4/12 = 1.054 · 5/12 = 1.083 · 6/12 = 1.118 · 8/12 = 1.202 · 10/12 = 1.302 · 12/12 = 1.414
Rise per foot of run: the pitch number, in inches — 6/12 rises 6 in per foot · in metric, pitch ÷ 12 × 1,000 mm per metre — 6/12 rises 500 mm
Run of a gable: span ÷ 2, less half the ridge thickness
Plumb-cut angle: 4/12 = 18.4° · 6/12 = 26.6° · 8/12 = 33.7° · 12/12 = 45°
Stock: round the total up to the next 2 ft length (the next 0.3 m in metric) — the cuts come out of the spare

Waste & overagewhat to add, and why

Order rafter stock a size longer than the cut length — the plumb cut at the ridge and the bird's mouth at the plate both need a few inches beyond the line length, and stock comes in 2 ft steps.
Add one or two rafters for the pair that has a bad crown; a roof with 20 rafters usually gets 22 on the truck.

Mistakes to catchbefore you order

Typing the span as the run — the run is half the span on a gable, less half the ridge thickness.
Forgetting the overhang is horizontal in this tool: a 12 in overhang typed as a sloped length is short by the pitch multiplier.
Cutting the first rafter from the tool's number without checking it against the actual ridge height on the wall — the plates are rarely dead level.

What an inspector looks forthe building inspector, at the roof framing call

On this part of the job

Rafter size and spacing against the span table or the engineer's design, with the span measured horizontally rather than along the slope.
The ridge and the connection at the top: ridge beam or ridge board, and which one it is changes everything below it.
Ties and collar ties — that they are there, at the height shown, and fixed with the fasteners the detail calls for. A cut rafter roof without its ties will spread.
Bearing at the plate, the birdsmouth cut, and how much rafter is left above the seat cut.
Hips, valleys and any rafter that has been cut to make an opening — those usually need to be doubled or hung.

Have this ready

The roof plan and section, with the pitch and the rafter schedule.
Your pitch confirmed on the building, not assumed from the drawing.
The truss layout and hanger schedule instead, if the roof is trussed — a truss may not be cut or altered without the supplier's engineering.

Every figure an inspector measures against comes from the code your jurisdiction has adopted, the stamped drawing, or the manufacturer's printed instructions — so none of them are printed here. Look the number up in your own book, type it into the tool, and the arithmetic is yours.

La fórmula

rise = run × (pitch ÷ 12) · rafter = √(run² + rise²) × (1 + overhang ÷ run)

The body of the rafter is Pythagoras over the run and rise. Because the overhang sits on the same slope, it grows by the same run-to-rafter ratio, so multiplying the run length by that ratio and adding it gives the full cut length.

Preguntas

How do I work out a common rafter length?: The rafter is the hypotenuse of a right triangle whose base is the horizontal run and whose height is the rise. Its length over the run is the square root of run squared plus rise squared. For a 12-foot run at a 6/12 pitch the rise is 6 feet, so the rafter is √(144 + 36) ≈ 13.4 feet before the overhang.
Is the run the same as the span?: No. The span is the full distance the roof crosses; the run of a common rafter is the horizontal distance from the wall to the ridge, which for a symmetrical gable is half the span less half the ridge thickness. Enter the run, not the span, and this tool gives the sloping length over that run.
Does this size the rafter or tell me the maximum span?: No — it gives a cut length only, the same way a stair tool gives a stringer length. Whether a rafter of a given size can span a distance under snow, wind and its own weight is a structural question answered by span tables and an engineer, and it is deliberately not something this tool outputs.
How do I add the overhang and the ridge cut?: Enter the horizontal overhang and the tool extends the rafter along the slope by the same multiplier. It does not deduct for half the ridge thickness or add for the plumb and seat cuts — those are small adjustments you mark out on the stock once the overall length is known.

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