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Birdsmouth Cut Planning Calculator — result sheet
Estimate a rafter seat-cut length from roof pitch, rafter depth, and a user-selected cut-depth fraction.
Inputs used
Results
Visual chart
Breakdown
Calculation steps
Returned data table
Formula and methodology
Formula: roof angle=atan(rise/run); cut depth=rafter depth×fraction; seat-cut length=cut depth/cos(roof angle).
A birdsmouth has several geometric and structural constraints. This planning model turns a chosen cut depth into the length along the rafter and reports the roof angle, without claiming that the result is an approved structural detail.
This result follows the calculator's declared inputs, precision, validation boundaries, and model limits.
Input contract
- Roof rise — minimum 1.0E-6; maximum 1000
- Roof run — minimum 1.0E-6; maximum 1000
- Rafter depth — length units; minimum 0.001; maximum 1000
- Cut depth fraction — minimum 0.05; maximum 0.33
Worked example
| Input | Value |
|---|---|
| Roof rise | 6 |
| Roof run | 12 |
| Rafter depth | 9.25 |
| Cut depth fraction | 0.25 |
Roof angle ≈26.565°, cut depth=2.3125, seat-cut length≈2.586.
Assumptions and limits
- Rise and run use the same length unit and describe the roof slope.
- The cut fraction is a planning input; local structural limits and member capacity are not inferred.
- The calculator does not size the rafter, verify bearing, or approve a cut for construction.
Calculator note
Source and methodology
Use the official WorldCalculate methodology policy for the source, formula, precision, and boundary standards behind this calculator.
Planning estimate, not financial, medical, legal, or professional advice. © WorldCalculate — reuse with attribution. Built and curated by Hassan ALRowaie.
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