Gambrel Roof Calculator

Two-Pitch & Half-Circle methods • Rafter thickness • Plumb cuts • Live diagram

📐 Two-Pitch Method

⭕ Half-Circle Method

Units

Roof Pitches (Two-Pitch)

Lower Pitch (θ)

Upper Pitch (φ)

Optional: Lower Run (x₁)

Optional: Upper Run (x₂)

Optional: Total Height

Building Dimensions

Building Width (W)

Building Length (L)

Eaves Overhang (e)

Gable Overhang (g)

Rafter Thickness (t)

Lower Pitch

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Upper Pitch

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Total Height

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Rafter R₁ (actual)

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Rafter R₂ (actual)

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Inside Width

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Inside Height

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Roof Area

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Attic Volume

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Interactive diagram updates with your inputs

Gambrel Roof Formulas

R₁ = x₁ / cosθ
Lower rafter (centerline)

R₂ = x₂ / cosφ
Upper rafter (centerline)

y₁ = x₁·tanθ
Lower rise height

y₂ = x₂·tanφ
Upper rise height

Area = 2·[(L+2g)·(R₁+e) + (L+2g)·R₂]

Volume = 2L·[½x₁y₁ + ½x₂y₂ + x₂y₁]

Plumb cut offset = t / tan((90+θ)/2)

Half-Circle: φ = θ - 45°

Disclaimer: This gambrel roof calculator provides estimates based on standard geometric formulas. Actual rafter lengths may vary due to material selection, cutting methods, and local building codes. Always verify all measurements with a licensed structural engineer or qualified contractor before cutting any materials. For informational use only.