Gambrel roof area = building length × 2 × (lower slope length + upper slope length). A 36 × 24 ft barn with 1 ft overhangs, an 18/12 lower pitch and a 6/12 upper pitch has 1,469 sq ft of roof, 14.69 squares, and its ridge stands 12 ft above the wall plates.
How the gambrel roof is calculated
A gambrel has two slopes on each side: a steep lower one and a shallow upper one, meeting at a break line. The calculator takes the half-span (wall to ridge), splits it at the break point you choose, and treats each part as its own right triangle. The lower slope also carries the eave overhang.
Slope length = level run × √(1 + (pitch ÷ 12)²) · Area = (building length + 2 × rake) × 2 × (lower slope + upper slope) · Ridge height = lower run × lower slope + upper run × upper slope
Worked example with the defaults: width 24 ft, so the half-span is 12 ft, broken at 50%. The lower slope covers 6 ft of wall run plus 1 ft of overhang, 7 ft × 1.8028 (18/12) = 12.62 ft. The upper slope covers the other 6 ft × 1.1180 (6/12) = 6.71 ft. The roof is 38 ft long with rake overhangs, so the area is 38 × 2 × (12.62 + 6.71) = 1,468.9 sq ft. The lower planes are 959 sq ft and the upper planes 510 sq ft. The break stands 6 × 18/12 = 9 ft above the plates, and the ridge 9 + 6 × 6/12 = 12 ft.
Where to break the slope
There is no single code number for the break. The calculator's default is halfway up the half-span. Moving the break outward makes the steep part wider and the upper roof lower; moving it inward gives a tall, narrow upper roof. Keep the lower slope steeper than the upper one: the calculator warns when it is not, because that shape is closer to a hip or a mansard than a gambrel. The knee span in the drawing, 12 ft at the default, is the width between the two break lines, and it is what the upper rafters have to cross.
Gambrel roof rafters
A gambrel needs two rafter lengths per side, joined by a gusset or a knee plate at the break. The Figures give both: 12.62 ft below and 6.71 ft above at the defaults, before overhang tails and cuts. Do not frame the break from a calculator alone: the joint carries load from both slopes, so have an engineer or a truss manufacturer check it. The roof truss calculator covers common trusses, and the rafter calculator lays out one straight rafter.
Mansard roof mode
Switch the shape to Mansard for a roof that is steep on all four sides with a shallow hip on top. You give the steep pitch (24/12 by default), the top pitch (4/12), and how far in from the wall the steep band reaches (4 ft). For a 36 × 24 ft building with 1 ft overhangs that is 1,207 sq ft on the steep band and 472 sq ft on top, 1,680 sq ft or 16.80 squares, with the ridge 10.67 ft above the plates.
Shingles for a gambrel
The default 15% waste covers the extra cuts at the break and the eaves. The 14.69 squares become 16.89 with waste, or 51 bundles of architectural shingles. Steep planes above about 9/12 are hard to work on and often add labor, which the roof replacement cost calculator accounts for. For the underlayment, drip edge and ice barrier lists use the roofing calculator, and for the deck the roof sheathing calculator.
What to work out next
Compare with a simpler shape in the hip roof calculator, which also has a gable mode. If the pitch numbers are new to you, the roof pitch calculator turns degrees or rise and run into x/12. To convert the area into whole squares for an order, use the roof square calculator. All roofing tools are in the roofing and siding calculators.