Gambrel Roof Calculator – Rafter Lengths & Angles
Calculate gambrel roof rafter lengths, angles, and roof area. Free online gambrel roof truss calculator for barn-style roofs.
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Gambrel Roof Calculator Formula
Rafter Length = √(Rise² + Run²) | Angle = arctan(Rise ÷ Run)
How to Calculate Gambrel Roof
- 1. Enter the building span (total width)
- 2. Specify the lower slope as rise/run (e.g., 12/12)
- 3. Specify the upper slope as rise/run (e.g., 5/12)
- 4. Calculate each rafter: √(rise² + run²)
- 5. Calculate each angle: arctan(rise/run) × 180/π
- 6. Total roof area = 2 × (lower rafter + upper rafter) × building length
Example Calculation
Let's say you're calculating a 24ft span gambrel roof with 12/12 lower slope and 5/12 upper slope, 40ft long building.
- span: 24
- lowerRise: 12
- lowerRun: 12
- upperRise: 5
- upperRun: 12
- buildingLength: 40
Lower rafter = 16.97 per side (12ft run), Upper rafter = 13.00 per side, Lower angle = 45°, Upper angle = 22.62°
Tips
- A gambrel roof has two slopes on each side — a steeper lower slope and a shallower upper slope
- Common lower slopes range from 6/12 to 12/12 (27° to 45°)
- Common upper slopes range from 4/12 to 6/12 (18° to 27°)
- The gambrel design maximizes attic or loft space
- Always consult local building codes for snow load requirements
Frequently Asked Questions
What is a gambrel roof?
A gambrel roof is a symmetrical two-sided roof with two slopes on each side. The upper slope is shallow and the lower slope is steep, creating extra space in the attic or loft area.
What is the best pitch for a gambrel roof?
The most common gambrel roof pitches are a 12/12 lower slope (45°) with a 5/12 or 6/12 upper slope (22-27°). This combination provides good headroom and classic barn-style appearance.
How do you calculate gambrel roof rafters?
Each rafter length equals the square root of (rise squared + run squared). For a 12/12 lower pitch with a 12ft run, the lower rafter = √(144 + 144) = 16.97 ft.