Decking, Fencing & Timber calculator
Rafter Length Calculator
Calculate common rafter length from horizontal run, roof pitch and overhang; structural sizing remains separate.
Calculate common rafter length from horizontal run, roof pitch and overhang; structural sizing remains separate. This estimate is most useful when the inputs reflect the job as it will actually be built, fitted or coated. Give particular attention to run, pitch and overhang. Rafter length geometry does not determine structural adequacy. Birdsmouth details, ridge thickness, span, loading and timber size must be designed separately.
What you need to enter
The Rafter Length Calculator uses the following project information to calculate the result.
- Horizontal run
- Roof pitch
- Overhang
Calculator result
Rafter length
How the Rafter Length Calculator works
Core method: right-triangle geometry plus overhang. On the Rafter Length Calculator, this relationship is applied specifically to common rafter length from horizontal run, pitch and overhang. Compatible measurements are converted to a common internal unit before calculation, then the result is returned in the selected unit system. The main variable to verify is run, pitch and overhang.
Rafter Length Calculator worked example
Rafter Length Calculator example: For a 3.6 m horizontal run at a 30° roof pitch, the common-rafter line length is about 4.16 m before the overhang is added. A 300 mm overhang along the rafter line would take the planning length to roughly 4.46 m. Before relying on that figure, check run, pitch and overhang; the final decision should also follow the actual product or project specification.
Important calculation assumptions
For common rafter length from horizontal run, pitch and overhang, rafter length geometry does not determine structural adequacy. Birdsmouth details, ridge thickness, span, loading and timber size must be designed separately.
Frequently asked questions
How should I collect the measurements for the Rafter Length Calculator?
Start with the real project rather than nominal dimensions. Measure horizontal run, roof pitch and planned overhang. Use horizontal run—not the sloping rafter length—as the base input. That gives the Rafter Length Calculator the inputs it needs for common rafter length from horizontal run, pitch and overhang.
What should I know about run, pitch and overhang before using the Rafter Length Calculator?
In the Rafter Length Calculator, errors in run, pitch and overhang can materially affect the result. Rafter length geometry does not determine structural adequacy. Birdsmouth details, ridge thickness, span, loading and timber size must be designed separately.
What must I verify after using the Rafter Length Calculator?
Once the Rafter Length Calculator has produced a result, use the geometric length for take-off and cutting layout only. Timber size, birdsmouth details, ridge connection, loads and structural capacity require separate design.