Gambrel Roof Calculator

A gambrel roof has two slopes on each side of the ridge. The steeper lower section creates useful headroom, while the shallower upper section closes the roof at the ridge. This familiar barn profile also appears on garages, sheds, and houses. Estimating its surface area requires measuring both sloped sections rather than treating the roof as a single simple pitch. The calculator models a symmetrical cross-section. Building width is split at the ridge. The entered lower run occupies part of each half-width, and the remaining horizontal distance becomes the upper run. Each sloped length is found with the Pythagorean theorem from its horizontal run and vertical rise. The sum of the lower and upper slope lengths is multiplied by building length and by two roof sides to obtain net roof area. Waste allowance increases the net area for cuts, starter courses, damaged pieces, pattern alignment, and future repairs. Roofing suppliers commonly describe quantity in squares, where one roofing square covers 100 square feet. The calculator converts the waste-adjusted area to squares without rounding it to package quantities. If a material price per square foot is entered, cost is based on the same waste-adjusted area. Accurate geometry matters. Measure building dimensions at the roof-bearing lines and use the designed rise and run for each segment. This simplified model does not add eave or gable overhangs, dormers, valleys, intersecting wings, cupolas, or other penetrations. To include a consistent eave overhang, adjust dimensions according to the roof plan or calculate those surfaces separately. Complex roofs should be broken into measurable planes and checked against construction drawings. The estimate supports early budgeting and material comparison, but it is not a structural design. Rafters, trusses, connections, sheathing, uplift resistance, snow load, wind load, ventilation, drainage, and local code requirements need professional consideration. Roofing packages also have specific coverage, lap, starter, ridge, and fastener requirements. Use the calculated area to request quotes and compare options, then confirm final quantities with the manufacturer, supplier, plans, and a qualified roofing or structural professional before ordering or building.