About Gambrel Roof Design
A gambrel roof (also known as a barn roof) has two distinct slopes on each side — a steep lower slope and a shallow upper slope. This classic design provides additional attic or loft space while maintaining efficient drainage and aesthetic appeal.
How the Calculator Works
The calculator estimates rafter lengths and roof area based on:
- Building width: total span between outer walls.
- Lower & upper pitches: angles that define each slope.
- Height ratio: proportion of total roof height occupied by the lower slope.
Formulas Used
Total Roof Height (H) = (½ × Width × tan(Lower Pitch) × Lower Ratio) + (½ × Width × tan(Upper Pitch) × (1 − Lower Ratio))
Rafter Lengths:
Lower Rafter = (½ × Width × Lower Ratio) ÷ cos(Lower Pitch)
Upper Rafter = (½ × Width × (1 − Lower Ratio)) ÷ cos(Upper Pitch)
Roof Area = 2 × (Total Rafter × Building Width)
Example Calculation
For a building width of 24 ft, lower pitch = 60°, upper pitch = 30°, lower height ratio = 0.6:
- Total Roof Height = 24/2 × tan(60°) × 0.6 + 24/2 × tan(30°) × 0.4 = 12.47 ft
- Lower Rafter Length = (12 × 0.6) ÷ cos(60°) = 8.64 ft
- Upper Rafter Length = (12 × 0.4) ÷ cos(30°) = 5.55 ft
- Total Roof Area = 2 × (14.19 × 24) = 682 sq.ft
Advantages of Gambrel Roofs
- Maximizes attic or storage space.
- Provides efficient water runoff and snow shedding.
- Gives buildings a traditional barn or colonial aesthetic.
- Economical in material use compared to mansard roofs.
Common Applications
- Barns and agricultural sheds.
- Residential homes and loft conversions.
- Workshops and garages needing extra overhead space.
Design Tips
- Keep lower pitch between 55°–70° and upper pitch 20°–40° for balanced aesthetics.
- Ensure ridge and wall connections are reinforced against wind uplift.
- Use metal or shingle roofing materials for better water protection.
- Consider overhangs for added rain protection and visual appeal.