Rafter Length Calculator
Calculate exact rafter length from roof pitch, building width, and overhang — uses the Pythagorean theorem.
Results update instantly as you type.
Interactive Roof Cross-Section
Shows your roof pitch, run, rise, and rafter length.
How Do You Calculate Rafter Length?
To calculate rafter length, use the Pythagorean theorem: Rafter = √(Run² + Rise²) + Overhang. The run is half the building width. The rise equals (pitch ÷ 12) × run.
Rafter Length = √(Run² + Rise²) + Overhang
What Does Roof Pitch Mean?
Roof pitch describes the slope as a ratio of vertical rise to horizontal run. A 6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal distance. Higher pitch numbers mean steeper roofs.
| Pitch | Angle | Multiplier | Common Use |
|---|---|---|---|
| 2/12 | 9.5° | 1.014 | Flat roof, commercial |
| 4/12 | 18.4° | 1.054 | Ranch, low-profile homes |
| 6/12 | 26.6° | 1.118 | Standard residential |
| 8/12 | 33.7° | 1.202 | Colonial, Cape Cod |
| 10/12 | 39.8° | 1.302 | Steep, A-frame |
| 12/12 | 45.0° | 1.414 | Very steep, chalet |
Frequently Asked Questions
How long should rafters be for a 24-foot wide building?
For a 24 ft wide building with 6/12 pitch and 12″ overhang, each rafter is approximately 14 ft 5 in. Run = 12 ft, Rise = 6 ft, Rafter = √(144+36) + 1 = 14.42 ft.
What is the most common roof pitch for houses?
4/12 to 9/12 pitches are most common for residential houses. A 6/12 pitch is considered standard and works well in most climates, providing good water runoff and attic space.
How much overhang should a rafter have?
Standard rafter overhang is 12 to 24 inches. 12″ is common for most homes. 18–24″ provides better rain protection. Overhangs beyond 24″ may need additional support.
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Accurate rafter calculations — get the right cut length before you climb the ladder.
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