Rafter Calculator
Find a common rafter's sloped length from roof run, pitch, and an optional eave for an early check of simple roof framing.
Horizontal distance from wall to ridge center.
E.g. 6 for a 6/12 pitch.
Optional eave overhang, measured horizontally.
Result
14.53 ft
A 12 ft run at a 6/12 pitch needs a rafter about 14.53 ft long
Quick Answer
A rafter calculator finds roof rise from run and pitch, then uses a right triangle for sloped length. With a 12-foot run, 6/12 pitch, and 1-foot level eave, the base rafter is 13.42 feet. The eave adds 1.12 feet along the slope, for a total of 14.53 feet before cuts.
Rafter Length Explained From Run, Pitch And Overhang
This tool finds the sloped line from the wall to the ridge. It can also add an eave. First, pitch sets the rise. A 6/12 roof rises 6 inches for each 12 inches of level run. A 12-foot run then has 6 feet of rise. Run and rise form the two square sides of a right triangle. The long side is the base rafter. If you add a level eave, the tool uses the same roof factor to turn that eave into a sloped length. It then adds the two sloped parts. This math can help check a simple common rafter. It does not size the board and leaves out the ridge gap, birdsmouth, end cuts, roof loads, and odd roof shapes.
The Rafter Formula For Rise, Slope And Eave Overhang
Run is the level span from the wall to the ridge center. Pitch is the rise for each 12 inches of run. Enter the eave as a level span, and the tool scales that span to match the roof slope.
- Rise = run × (pitch ÷ 12).
- Base rafter length = √(run² + rise²).
- Roof factor = base rafter length ÷ run.
- Sloped eave length = level eave × roof factor.
- Total rafter length = base rafter length + sloped eave length.
Rafter Calculator Steps For Run, Pitch And Overhang
Inputs
- Run: the level wall-to-ridge-center span in feet.
- Pitch: the roof rise for every 12 inches of run.
- Overhang: the optional level eave span in feet.
Steps
- Measure the level run, not the roof face.
- Enter the rise number from the X/12 pitch.
- Enter the level eave, or use zero for none.
- Read the rise, base length, and roof factor.
- Use the total as an uncut length check only.
Rafter Example With Pitch And Overhang Fully Shown
12 ft run, 6/12 pitch, and 1 ft level eave.
- Find rise: 12 ft × (6 ÷ 12) = 6 ft.
- Find base rafter length: √(12² + 6²) = √180 = 13.41641 ft.
- Find roof factor: 13.41641 ft ÷ 12 ft = 1.11803.
- Scale the eave: 1 ft × 1.11803 = 1.11803 ft.
- Find total length: 13.41641 ft + 1.11803 ft = 14.53444 ft.
14.53 ft, rounded to two decimal places.
When A Rafter Geometry Estimate Helps With Planning
Use this result to check the main line of a simple common rafter. Check all cuts, loads, board sizes, joints, and local rules on the roof plan.
The result represents centerline geometry before ridge shortening, birdsmouth layout, fascia alignment, or structural design. Lumber species, grade, size, spacing, bearing, connections, and roof loads require separate verification.
For nearby tools, the construction calculators hub lists every material and quantity calculator, and the glossary defines common calculator terms.
Assumptions
- The roof side forms a simple right triangle.
- Pitch uses rise per 12 inches of level run.
- Run ends at the ridge centerline.
- The eave input is a level span.
- All lengths use feet.
Limitations
- Ridge width and rafter shortening are not included.
- Birdsmouth, plumb, seat, and fascia cuts are not added.
- Hip, valley, jack, and compound rafters need other math.
- The result does not select board size or safe span.
- Roof loads, joints, and local code need expert review.
In Practice
A common error is using the sloped roof face as the run. Run must be level from the wall to the ridge center. Use the result as an uncut line check, then follow the roof plan for the final board and cuts.
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Frequently Asked Questions About Rafter Calculations
Does Rafter Run Mean Half Of The Building Width?
Rafter run is often half the width on a plain gable roof with a centered ridge. That shortcut fails when the ridge sits off center or the wall lines differ. Use the true level span from the wall to the ridge center shown on the plan.
How Does A Six In Twelve Pitch Change Rafter Length?
A 6/12 pitch adds 6 inches of rise for each 12 inches of run. With a 12-foot run, rise is 12 × 6 ÷ 12 = 6 feet. The base sloped length is √(12² + 6²) = 13.42 feet after rounding to two decimal places.
Is The Overhang Added As A Straight Foot?
A one-foot level eave adds more than one foot along a sloped rafter. The tool times the eave by the roof factor. At a 6/12 pitch, 1 × 1.11803 = 1.12 feet along the slope after rounding.
Does This Calculator Size The Rafter Lumber?
This rafter calculator finds length geometry only and does not size the lumber for structural use. It does not pick board size, wood type, grade, spacing, or safe span. The American Wood Council notes that rafter size also depends on loads, bend limits, wood values, bearing, and the span tables used by the job.
Can The Result Be Used As A Final Cut Length?
The calculated rafter length is not a final cut length on its own. Ridge width, birdsmouth shape, end cuts, fascia work, and field sizes can change the board length. Check the roof plan and all site marks before you cut.
Sources
Last updated: . Reviewed for accuracy against the formula shown above.