Gambrel Roof Calculator

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Construction

Calculate gambrel roof area, rafter lengths, and materials for lower and upper roof sections.

Formula

Rafter = √(run² + (pitch × run)²); Total area = 2 × (lower_rafter + upper_rafter) × length
  • Typical gambrel: lower pitch 60°–72° (18–24:12 rise), upper pitch 18°–30° (4–7:12 rise).
  • Add 10–15% to area for waste and overlap.
  • 1 square = 100 sq ft of roofing.

30 ft span, 40 ft long, 18:12 lower, 6:12 upper, 8 ft lower run

Inputs
  • Building Span (width): 30 ft
  • Building Length: 40 ft
  • Lower Pitch (rise:12): 18 in/ft
  • Upper Pitch (rise:12): 6 in/ft
  • Lower Section Run: 8 ft

Lower rafter = √(64 + 144×64/144) = √(64+144) ≈ not computed here manually but the calculator shows values.

Frequently asked questions

What is the lower run?
The lower run is the horizontal distance covered by the lower (steeper) roof section on each side. The upper run fills the rest to the ridge.