FREE ROOF FRAMING TOOL
Roof rafter calculator.
EXAMPLE RAFTER LENGTH
LiveTotal length, including the tail. Ridge deduction applied.
Check the highlighted measurement to see your results.
Tap an underlined dimension to edit it.
Birdsmouth / seat cut
- Seat cut · level
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- Heel cut · plumb
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- Height above plate
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- Notch depth · square
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HAP is measured vertically at the outside wall. Timber and notch depths are measured perpendicular to the rafter. Angles are to the rafter edge.
All measurements & quantities
FROM CALCULATION TO WORKSHEET
Take the plan with you.
A dimensioned PDF, a material list and an editable CAD drawing.
One export pass. Every roof you calculate.
Optional PDF, CSV & DXF exports
For common rafters on symmetrical gable and single-slope shed roofs. Geometry and quantity estimates only; member sizing, loads, connections and birdsmouth design require separate verification.
A LITTLE CLARITY
Measure once.
Understand every number.
Use actual dimensions and check a test rafter before cutting the full set.
Do I enter the full span or half?
Enter the full horizontal width between the outside faces of the bearing walls. For a symmetrical gable roof, the calculator halves this width and subtracts half the ridge thickness. For a shed roof, the width is the full run, with no ridge deduction.
What does the rafter length include?
The sloping line length from the ridge face to the outside wall, plus the sloping tail. The overhang you enter is horizontal, so it is converted to a sloping length. Fascia, high-side shed overhangs and cutting allowances are not included.
Length = (run + overhang) × √(1 + (pitch ÷ 12)²)
How do I use the angles and birdsmouth reference?
The roof angle is measured from horizontal. A plumb cut line makes 90° minus that angle with the rafter’s long edge; this is not a universal saw setting. The wall-to-ridge length locates the outside wall reference. Enter the actual timber depth, thickness and horizontal seat length to see the birdsmouth geometry. The enlarged detail shows the vertical heel cut, height above plate (HAP) and perpendicular notch depth. Member sizing, required bearing and permitted notch depth require separate verification.
How are the rafters counted?
A rafter line is placed at each end of the building and at the selected on-center spacing between them. The final bay may be shorter. A gable has two rafters per line; a shed has one. Extra rafters are rounded up separately. Gable ladders, blocking, ties and bracing are excluded.
What is free, and what does the pass unlock?
All calculations, diagrams, quantities, copying and sharing are free, without an account. The US$19 export pass unlocks PDF, CSV and DXF downloads for unlimited calculations in this tool, with no recurring charge. Your key is delivered by Lemon Squeezy and can restore access on another device. It does not include the StructaCAD modeling app or structural engineering services.