Gambrel Roof Calculator — Area, Squares & Shingle Bundles
A gambrel (barn-style) roof has two slopes per side — a steep lower section and a shallower upper section — to maximise loft space. Enter building dimensions and both pitch values to get roof area, squares and shingle bundles needed.
ft
ft
ft
/12
/12
%
Both sides of the gambrel combined
- 1
Upper section run
12 ft − 6 ft = 6Half the building width minus the lower horizontal run. - 2
Lower pitch factor
√(1 + (24 ÷ 12)²) = 2.2361 - 3
Upper pitch factor
√(1 + (6 ÷ 12)²) = 1.118 - 4
Lower slope length (per side)
6 ft × 2.2361 = 13.42 - 5
Upper slope length (per side)
6 ft × 1.118 = 6.71 - 6
Total roof area
2 × 40 ft × (13.42 + 6.71) ft = 1,610
How does this calculator work?
Gambrel roof area = 2 × length × (lower_run × √(1+(lp/12)²) + upper_run × √(1+(up/12)²)). Lower run + upper run = half the building width. Divide by 100 for roofing squares; multiply squares × 3 for bundles (add ~10% waste). Ridge height = lower_run×(lp/12) + upper_run×(up/12).
Formula
How this is calculated
A gambrel roof is symmetric: two slopes on each side meet at a horizontal purlin (knee wall) partway up the roof. The building's half-width is split between the lower horizontal run (user-specified) and the upper horizontal run (half-width minus lower run). Each section's true rafter length is derived from the standard slope formula: slope = run × √(1 + (rise/12)²), where rise/12 is the pitch ratio. Adding the lower and upper slope lengths gives the total rafter run per side; multiplying by two covers both sides, and multiplying by the building's length gives full roof area in square feet.
Roofing squares (1 square = 100 sq ft) and shingle bundles (3 bundles per square, the standard for asphalt shingles) follow directly from the area. A waste allowance of 10% is a common starting point; gambrel roofs can justify 12–15% due to the two-slope junction and hip/rake cuts on barn-style designs.
This model assumes a simple rectangular footprint — no dormers, skylights or overhangs. Ridge height is calculated as the sum of the two vertical rises: lower rise = lower run × (lower pitch/12) and upper rise = upper run × (upper pitch/12). Inputs use feet for lengths and rise-per-12 for pitch, following standard North American roofing notation.
Frequently asked questions
A gambrel (also called barn roof) has two slopes on each side. The lower slope is steep — typically 18–24/12 rise — and the upper slope is shallow — typically 4–8/12. This creates much more usable attic or loft space than a single-slope gable roof of the same wall height, which is why it became the classic barn and farmhouse design.
A gable roof applies one pitch factor to the full half-width. A gambrel applies two different pitch factors to two sub-widths and adds the results. Because the steep lower section has a large pitch factor, a gambrel typically has more total surface area than a gable with the same footprint — expect 10–30% more material depending on pitches.
Common designs pair a lower pitch of 18–24/12 (close to vertical) with an upper pitch of 4–8/12 (shallow). The lower horizontal run is often 40–50% of the half-width. Consult a structural engineer for your specific loading and snow requirements — these are starting estimates, not engineering specifications.
Also known as
TG we-Calculate Editorial Team. (2026). Gambrel Roof Calculator — Area, Squares & Shingle Bundles [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/gambrel-roof-calculator
TG we-Calculate Editorial Team. "Gambrel Roof Calculator — Area, Squares & Shingle Bundles." TG we-Calculate. 2026. https://we-calculate.com/calculator/gambrel-roof-calculator.
TG we-Calculate Editorial Team, "Gambrel Roof Calculator — Area, Squares & Shingle Bundles," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/gambrel-roof-calculator
@misc{wecalculate_gambrel_roof_calculator, title = {Gambrel Roof Calculator — Area, Squares & Shingle Bundles}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/gambrel-roof-calculator}}, year = {2026}, note = {TG we-Calculate} }
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