Gambrel Roof Calculator
Calculate gambrel roof dimensions, angles, rafter lengths, and roof area for barn-style roofing
Calculate Gambrel Roof
Building Dimensions
Length along the ridge of the building
Width from wall to wall (perpendicular to ridge)
Roof Pitch Angles
Gentle upper slope angle (typically 15-30°)
Steep lower slope angle (typically 45-70°)
Overhangs (Optional)
Extension beyond the wall at the eaves
Extension beyond the wall at the gable ends
Gambrel Roof Results
Please Complete Required Fields
- ⚠️Please enter a valid building length
- ⚠️Please enter a valid building width
- ⚠️Please enter at least one roof pitch angle
Example Calculation
Traditional Barn Gambrel Roof
Building: 12m × 8m (40ft × 26ft)
Lower Pitch: 60° (steep barn style)
Upper Pitch: 30° (moderate slope)
Method: Two-pitch with custom angles
Calculation Steps
1. Half building width: 8m ÷ 2 = 4m
2. Estimate run distribution: x₁ = 1.5m, x₂ = 2.5m
3. Calculate rises: y₁ = 1.5 × tan(60°) = 2.6m, y₂ = 2.5 × tan(30°) = 1.44m
4. Total height: H = 2.6 + 1.44 = 4.04m
5. Rafter lengths: R₁ = √(2.6² + 1.5²) = 3.0m, R₂ = √(1.44² + 2.5²) = 2.89m
6. Total roof area: 2 × (12 × 3.0 + 12 × 2.89) = 141.4m²
7. Attic volume: 2 × 12 × (1.5×2.6/2 + 2.5×1.44/2 + 2.5×2.6) = 114.7m³
Gambrel Roof Guide
Half-Circle Method
Lower angle = Upper angle + 45°
Traditional proportions
Two-Pitch Method
Custom upper and lower angles
Flexible design options
Angle Guidelines
Upper: 15-30°, Lower: 45-70°
Optimal performance range
Benefits
30-40% more attic space
Compared to gable roofs
Design Tips
Lower section should be steeper than upper for proper drainage
Consider local snow loads when selecting angles
Plan for ventilation in the large attic space
Use structural engineering for spans over 24 feet
Consider dormer windows for natural light
Professional Note: Gambrel roofs require careful engineering at the bend point for structural integrity.
Understanding Gambrel Roofs
What is a Gambrel Roof?
A gambrel roof is a two-slope roof design where the lower section has a steeper pitch than the upper section. This design maximizes attic space while maintaining structural efficiency, commonly seen on barns and Colonial-style homes.
Advantages of Gambrel Roofs
- •Maximum Attic Space: 30-40% more space than gable roofs
- •Efficient Water Drainage: Steep lower section sheds water quickly
- •Material Efficiency: Less roof area than hip roofs
- •Classic Aesthetics: Traditional barn and colonial appearance
Calculation Methods
Two-Pitch Method:
x₁ + x₂ = W/2
y₁ + y₂ = H
tan(θ) = y₁/x₁
tan(φ) = y₂/x₂
Half-Circle Method:
θ - φ = 45°
H = W/2
Design Rule: Lower pitch should be at least 15° steeper than upper pitch for proper gambrel proportions and functionality.
Construction Considerations
Structural Engineering
The bend point where upper and lower rafters meet requires careful engineering to handle load transitions and prevent structural failure.
Material Selection
Different sections may require different materials based on their slopes. Consider membrane roofing for low-pitch upper sections.
Ventilation Planning
Large attic spaces require proper ventilation planning. Consider ridge vents, soffit vents, and possibly gable vents for optimal air circulation.