Calculate common rafter length
This calculator uses building span, roof pitch, ridge deduction, and overhang to estimate the length of a common rafter. It also separates the horizontal run and roof rise so the roof geometry is easy to review.
Free construction tool
Free Roof Rafter Calculator
Calculate common rafter length, roof rise, overhang, birdsmouth position, rafter count, and a simple roof framing layout.
Roof framing results
Your common rafter length is —
Enter roof inputs and calculate the framing geometry.
Common rafter length
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Horizontal run
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Roof rise
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Plumb cut angle
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Seat cut angle
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Ridge deduction
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Rafter tail / overhang length
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Estimated number of rafters
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Pairs of rafters
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Estimated rafter material
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Estimated roof slope angle
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This tool provides framing geometry and material estimates only. Verify all structural requirements, loads, connections, and local code requirements with a qualified professional.
Roof framing layout
A simple roof framing reference showing walls, ridge, rafters, span, run, rise, overhang, and birdsmouth position when entered.
Estimated material list
A quick material reference based on the geometry and spacing entered above.
| Item | Quantity | Length | Notes |
|---|
Material quantities are estimates. Waste, splices, blocking, bracing, ties, connectors, and local requirements are not included unless noted.
Roof pitch and rafter layout
Use familiar pitch formats such as 6/12 and 4/12, or switch to degrees. The framing layout visualizes the span, run, rise, ridge board, and overhang from the same inputs.
Birdsmouth and seat cut reference
The tool shows birdsmouth geometry as a reference when a seat depth is entered. The final cut, bearing detail, and remaining rafter section must be verified for local requirements and structural design.
Gable and shed roof framing
The calculator currently supports gable and shed roofs. Hip roofs, valley roofs, jack rafters, and more detailed material estimating are planned for later phases.
Frequently asked questions
How do I calculate rafter length?
Start with the horizontal run, apply the roof pitch to find the rise, then use the right-triangle slope length and add the sloped length of the horizontal overhang. For gable roofs, account for one-half of the ridge board thickness in the run.
What does a 6/12 roof pitch mean?
A 6/12 pitch rises 6 inches vertically for every 12 inches of horizontal run. It is approximately a 26.57° slope.
How far apart should rafters be?
Common nominal spacings are 16 inches or 24 inches on center in imperial framing, and 400 mm or 600 mm in metric work. Actual spacing must be selected for the applicable loads, lumber, sheathing, and local requirements.
Does this calculator design roof trusses?
No. This tool calculates rafter geometry and a basic framing layout. It does not replace engineered truss design, connection design, load analysis, or local code review.
Can I download a roof framing layout?
Yes. Create a free StructaCAD account to open the printable layout and download the estimated cut list. The calculator and basic drawing remain free to use without an account.
Is this calculator suitable for engineered roof design?
No. It is a geometry and estimating reference. Structural member selection, loads, connections, deflection, bracing, and local code requirements require review by a qualified professional.