Quick Rafter Length Calculator
Enter your roof measurements below. The calculator returns common rafter length, total rafter with overhang, roof rise, and roof angle.
How this rafter length calculator works
This tool calculates the length of a common roof rafter using basic roof geometry and the Pythagorean theorem. You provide span, pitch, and optional overhang. The calculator then determines the run, rise, and slope length.
If you include ridge board thickness, the calculator reduces run by half of that thickness, which gives a more practical cut length from wall plate to ridge connection.
Formulas used
1) Run
Run is half the total building span:
run = span ÷ 2
2) Adjusted run (optional ridge board)
If ridge board thickness is entered:
adjusted run = run − (ridge thickness ÷ 2)
3) Rise from pitch
Pitch is entered as rise per 12 horizontal units (for example, 6/12):
rise = adjusted run × (pitch ÷ 12)
4) Common rafter length
Using the right-triangle relationship:
rafter length = √(adjusted run² + rise²)
5) Overhang slope length
Overhang is horizontal, so its sloped length is:
overhang slope = √(overhang² + (overhang × pitch/12)²)
6) Total rafter length
total length = common rafter length + overhang slope
Example calculation
- Span: 24 ft
- Pitch: 6/12
- Overhang: 1.5 ft
- Ridge thickness: 0.125 ft (1.5 in)
Run = 12 ft. Adjusted run = 12 − 0.0625 = 11.9375 ft. Rise = 11.9375 × 6/12 = 5.96875 ft. Common rafter length ≈ 13.35 ft. Add overhang slope for final total length.
Why rafter length matters
Correct rafter length affects structural fit, roof line appearance, sheathing alignment, and material cost. Even small errors multiply quickly across a full roof.
- Improves cut accuracy and installation speed
- Reduces waste from mis-cuts
- Helps estimate lumber and sheathing needs
- Supports safer, cleaner framing layout
Common mistakes to avoid
- Using full span as run (run is half span for a centered ridge)
- Mixing units (feet and inches or meters and centimeters) without conversion
- Ignoring ridge board thickness in tighter framing tolerances
- Forgetting that overhang is horizontal input, not sloped length
Quick FAQ
Can I use this for metric roofs?
Yes. Choose meters and keep all distance inputs in meters.
Does this include birdsmouth seat cuts?
No. This gives geometric rafter length. Layout and cut details depend on framing method and code requirements.
What pitch format should I enter?
Enter only the rise value from an X/12 pitch. For 8/12, enter 8.