Rafter Length Calculator
Find common rafter length, roof angle, pitch, and total sloped length including a horizontal eave overhang.
About the rafter length calculator
A common rafter forms the sloped side of a right triangle. The horizontal run is the distance from the outside wall line to the ridge centerline, and the rise is the vertical distance over that run. This rafter length calculator applies the Pythagorean theorem to those measurements: the rafter length to the wall is the square root of run squared plus rise squared. The result is the theoretical line length before allowances for ridge boards, birdsmouth details, and end cuts. Roof angle is calculated with the inverse tangent of rise divided by run. Pitch is also shown as inches of rise per 12 inches of horizontal run, a familiar framing convention. For example, six feet of rise over twelve feet of run is a 6-in-12 pitch and an angle of about 26.57 degrees. Pitch and angle describe the same slope in different forms, but saw settings and plan notes may use different references, so verify the intended convention. An eave overhang extends the same roof slope beyond the wall. Enter its horizontal projection rather than a measurement taken along the rafter. The calculator extends the rise-to-run ratio over that projection and adds the resulting sloped segment to the wall-to-ridge length. Fascia details, rafter tails, and plumb cuts can alter the final stock length. Real framing requires more than the geometric line. The ridge board or ridge beam thickness can shorten the upper end, while the birdsmouth seat and heel determine how the member bears on the wall. Building codes often limit notch depth, and lumber crowns, defects, treatment, species, grade, snow load, wind load, and spacing affect structural suitability. Never use length alone to size a roof member. Measure from consistent reference points and keep all inputs in feet. Convert inches to decimal feet before entering them; for example, six inches equals 0.5 feet. Round material purchases up, allow extra length for layout and cuts, and test the first rafter before cutting the remainder. The calculator provides dependable triangle geometry, while approved drawings and local code requirements should govern the final structure.
Rafter length examples
These examples show common roof slopes using horizontal run and vertical rise.
| Roof geometry | Calculated result | Framing note |
|---|---|---|
| 12 ft run, 6 ft rise, 1 ft overhang | 13.42 ft to wall, 14.53 ft total, 26.57 degrees | A 6-in-12 roof pitch. |
| 15 ft run, 5 ft rise, no overhang | 15.81 ft total, 18.43 degrees | A 4-in-12 roof pitch. |
| 10 ft run, 10 ft rise, 2 ft overhang | 14.14 ft to wall, 16.97 ft total, 45 degrees | A steep 12-in-12 roof pitch. |
How to calculate rafter length
- Measure the horizontal run from the supporting wall to the ridge centerline.
- Enter the vertical rise over that run.
- Enter the horizontal projection of any planned eave overhang.
- Select Calculate rafter to see line lengths, angle, and pitch.
- Apply ridge, birdsmouth, tail, and cutting allowances from the approved framing plan.
Rafter length calculator FAQ
Is roof run half of the building span?
For a symmetrical gable roof, run is usually half the outside wall span. Shed roofs and offset ridges require the actual horizontal distance for the rafter being calculated.
What does a 6-in-12 pitch mean?
It means the roof rises 6 inches for every 12 inches of horizontal run. This ratio corresponds to an angle of approximately 26.57 degrees.
Is the overhang measured along the rafter?
No, this calculator expects the horizontal projection beyond the wall. It converts that projection to sloped length using the same roof pitch.
Does the result include a ridge deduction?
No, the result is the theoretical line from wall to ridge centerline. Adjust for ridge thickness and the specified upper plumb cut during layout.
Can the result determine lumber size?
No, member size depends on span, spacing, loads, species, grade, and local code. Use engineered tables, approved plans, or a qualified structural professional.