🏠 Gambrel Roof Calculator
Calculate gambrel roof dimensions and materials
Steeper upper section (e.g., 18/12)
Shallower lower section (e.g., 7/12)
How to Use This Calculator
Enter Building Dimensions
Input the building width and length in feet. These are the overall dimensions of the structure.
Enter Roof Pitches
Input the upper roof pitch (steeper, typically 16-20/12) and lower roof pitch (shallower, typically 5-10/12). Pitch is rise in inches per 12 inches of run.
Enter Overhang
Input the roof overhang in feet. This is the horizontal distance the roof extends beyond the building walls.
Calculate and Review
Click "Calculate Gambrel Roof" to see roof dimensions, rafter lengths, area, and materials needed. Use this for construction planning and material estimation.
Formula
Upper Rafter = √((Upper Width/2)² + Upper Height²)
Lower Rafter = √(Lower Width² + Lower Height²)
Height = Width/2 × tan(Pitch)
Roof Area = 2 × (Upper Rafter + Lower Rafter) × Length
Example: 30 ft wide, 40 ft long, 18/12 upper pitch, 7/12 lower pitch
Step 1: Upper width = 30 / 3 = 10 ft, Lower width = 10 ft per side
Step 2: Upper height = 5 × tan(18/12) = 7.5 ft
Step 3: Lower height = 10 × tan(7/12) = 5.8 ft
Step 4: Upper rafter = √(5² + 7.5²) = 9.0 ft
Step 5: Lower rafter = √(10² + 5.8²) = 11.6 ft
Step 6: Total area = 2 × (9.0 + 11.6) × 40 = 1,648 sq ft
About Gambrel Roof Calculator
The Gambrel Roof Calculator is an essential tool for contractors, carpenters, architects, and builders who need to calculate gambrel roof dimensions. This calculator eliminates guesswork by providing precise calculations for gambrel roofs based on building dimensions, roof pitches, and overhang, helping you estimate materials and plan construction accurately.
When to Use This Calculator
- Roof Construction: Calculate gambrel roof dimensions for construction projects
- Material Estimation: Estimate shingles, underlayment, and roofing materials needed
- Rafter Cutting: Determine rafter lengths for upper and lower sections
- Project Planning: Plan gambrel roof projects with accurate dimensions
- Cost Estimation: Calculate material costs based on roof area
Why Use Our Calculator?
- ✅ Accurate Calculations: Precise gambrel roof calculations using standard geometric formulas
- ✅ Multiple Dimensions: Calculates rafter lengths, heights, areas, and materials
- ✅ Material Estimates: Estimates shingles bundles and squares needed
- ✅ Time Savings: Instant calculations eliminate manual math
- ✅ Easy to Use: Simple input fields for building dimensions and pitches
Understanding Gambrel Roofs
Gambrel Roof Definition: A gambrel roof has two slopes on each side—a steeper upper slope and a shallower lower slope. This creates a barn-like appearance and provides more headroom in upper floors compared to standard pitched roofs.
Roof Pitches: Upper pitches are typically steep (16-20/12), while lower pitches are shallower (5-10/12). The combination creates the characteristic gambrel shape. Different pitches affect roof area, height, and material requirements.
Material Calculation: Roof area is calculated from rafter lengths and building length. Typically, 3 bundles of shingles cover 1 square (100 sq ft) of roof area. Always add 10-15% extra for waste and overlaps.
Tips for Best Results
- Measure Accurately: Use accurate building dimensions for precise roof calculations
- Verify Pitches: Confirm roof pitches with architect or designer before construction
- Add Waste Factor: Add 10-15% extra materials for waste, cuts, and overlaps
- Check Local Codes: Verify roof pitch requirements with local building codes
- Consult Professionals: For complex projects, consult architects or structural engineers
Frequently Asked Questions
What is a gambrel roof?
A gambrel roof has two slopes on each side—a steeper upper slope and a shallower lower slope. This creates a barn-like appearance and provides more headroom in upper floors compared to standard pitched roofs.
What are typical gambrel roof pitches?
Upper pitches are typically steep (16-20/12), while lower pitches are shallower (5-10/12). Common combinations include 18/12 upper with 7/12 lower, or 16/12 upper with 6/12 lower. The calculator works with any pitch combination.
How do I calculate gambrel roof area?
Calculate rafter lengths for upper and lower sections, then multiply total rafter length by building length for each side, and add both sides. The calculator performs these calculations automatically based on building dimensions and pitches.
How many shingles do I need for a gambrel roof?
Typically, 3 bundles of shingles cover 1 square (100 sq ft) of roof area. For example, a 2,000 sq ft gambrel roof needs about 60 bundles (2,000 / 100 × 3). Always add 10-15% extra for waste. The calculator shows exact bundles needed.
What's the difference between upper and lower rafter lengths?
Upper rafters support the steeper upper section of the gambrel roof, while lower rafters support the shallower lower section. Total rafter length per side is upper rafter + lower rafter + overhang. The calculator shows both lengths separately and combined.