Barn Gambrel Roof Rafter & Surface Area Calculator

A Gambrel roof is a classic two-pitch barn roof featuring a steep lower slope and a shallower upper slope on each side to maximize upper loft attic space.

To use the Barn Gambrel Roof Rafter & Surface Area Calculator, enter your Building Span Width, Building Length, Lower Rafter Slope Angle and other values. The calculator instantly computes Lower Rafter Length, Upper Rafter Length and more. Results update in real time as you change any input — no submit button needed.

How to Use This Calculator

Enter your Building Span Width (e.g. 24 in ft), Building Length (e.g. 36 in ft), Lower Rafter Slope Angle (e.g. 60 in degrees (steep)), and Upper Rafter Slope Angle (e.g. 30 in degrees (shallow)). Measure twice before entering dimensions — small measurement errors scale up significantly in material estimates.

The Formula

Lower Rafter Length = (Half Span × 0.55) ÷ cos(Lower Angle)

Upper Rafter Length = (Half Span × 0.55) ÷ cos(Upper Angle)

Total Roof Area (sq ft) = 2 × (Lower Rafter Length + Upper Rafter Length) × Building Length

Worked Example

24 ft wide × 36 ft long barn with 60° lower slope and 30° upper slope:

Lower Rafter = 13.20 ft | Upper Rafter = 7.62 ft.

Total Slope Width = 41.64 ft | Roof Surface Area = 1,499.0 sq ft (50 shingle bundles).

Understanding Your Result

Roofing materials are measured in "squares" — 1 square = 100 sq ft of roof surface. Add 10–15% waste factor for standard gable roofs; up to 20% for complex hip roofs with multiple valleys. Minimum slope for asphalt shingles is 2:12 (2 inches rise per 12 inches run); below this requires low-slope roofing products. Measure the actual roof surface, not the footprint — pitch increases surface area by 5% (2:12 pitch) to 25% (8:12 pitch) above the horizontal footprint.