Gambrel Roof Calculator

Step 1: Determine Pitch Combinations

Choose appropriate pitch combinations for lower and upper sections:

• Lower Section: Typically 12/12 (45°) for optimal water shedding
• Upper Section: Usually 4/12 (18°) for economical framing
• Alternative: 14/12 lower with 3/12 upper for steeper profile
• Maintain at least 3/12 pitch on upper section for proper drainage

Step 2: Calculate Knee Wall Height

The knee wall height determines usable attic space:

• Standard height: 8 feet for full headroom
• Minimum: 6 feet for limited storage use
• Maximum: 10 feet for extra space (increases cost)
• Height affects lower rafter run and overall geometry

Step 3: Material Calculations

Gambrel roofs require specific material considerations:

• Lower Rafters: Usually 2x8 or 2x10 for steep angle
• Upper Rafters: 2x6 typically sufficient for shallow pitch
• Knee Wall: 2x4 framing with proper bracing
• Valley Flashing: Required at pitch change intersection

Space Efficiency

The dual-slope design maximizes usable attic space compared to conventional gable roofs. The steep lower pitch allows for near-vertical walls in the attic, while the shallow upper pitch keeps the overall height manageable.

Weather Performance

The steep lower section sheds water and snow effectively, while the upper shallow section reduces wind load. Valley flashing at the pitch change requires careful installation but provides reliable weather protection.

Construction Considerations

• Knee walls must be properly supported and braced
• Valley created at pitch change needs special attention
• Consider collar ties for wind resistance
• May require engineered design for large spans
• Professional installation recommended for complex geometry

Common Applications

• Barns and agricultural buildings
• Dutch Colonial style homes
• Storage buildings and workshops
• Homes requiring extra attic space
• Historic restoration projects