Turn the building footprint, overhang and roof pitch into roof area, roofing squares and shingle bundles, and convert pitch, angle and percent slope.
Roof area = flat footprint area × pitch multiplier, where the multiplier is √(1 + (rise ÷ 12)²). A 40 ft by 30 ft building with a 12 in overhang has a footprint of 42 × 32 = 1,344 ft². At 6:12 the multiplier is √1.25 = 1.118, so the roof surface is 1,344 × 1.118 = 1,502.64 ft². This works for a gable or hip roof where every face has the same pitch, because each slope is longer than its flat footprint by the same factor. Type the pitch, the building length and width, and the overhang, and the page does the sum live in feet and inches or in metres.
Roof pitch x:12 means x inches of rise for every 12 inches of horizontal run. The angle is atan(x ÷ 12) and the percent slope is x ÷ 12 × 100. A 6:12 roof is 26.57 degrees and 50%. A 12:12 roof is 45 degrees and 100%, with a multiplier of 1.414. A 4:12 roof is 18.43 degrees and 33.3%, with a multiplier of 1.054. Choose which of the three you know, and the page shows the other two. A flat 0:12 roof has a multiplier of 1, so its area equals the footprint.
Roofers price in squares, and one square covers 100 ft² of roof. The page adds a waste allowance (10% to start with) for cutting field shingles (starter strips and ridge or hip caps are separate products and not included), then divides by 100. For the 1,502.64 ft² roof above, 10% waste gives 1,652.9 ft², or 16.53 squares. Bundles are the area with waste divided by the coverage printed on the bundle label, rounded up. At the default 33.3 ft² per bundle (3 bundles per square), 1,652.9 ft² needs 49.64, rounded up to 50 bundles. Some products list about 32.8 ft², which gives 51, so enter the label value. Below a 2:12 pitch the page shows the area only, because asphalt shingles are not for low-slope roofs (IRC R905.2.2); from 2:12 to 4:12 they need special underlayment (IRC R905.2.7). Metric input is converted with 1 m² = 10.764 ft².
The page assumes one pitch over the whole roof and a rectangular footprint. It does not add dormers, valleys, chimneys, skylights, flashing, underlayment, drip edge, ridge caps or local code requirements. Complex roofs are best split into rectangles: run each section and add the areas. Measure the footprint from the outside of the walls, and the overhang from the wall to the edge of the roof. Waste is your call: a simple gable might need 10%, while a roof with many hips and valleys often needs 15% or more.
Multiply the footprint (including overhang) by the pitch multiplier √(1 + (rise ÷ 12)²). A 1,344 ft² footprint at 6:12 gives 1,344 × 1.118 = 1,502.64 ft² of roof.
Divide the roof area plus waste by 100 ft². A 1,502.64 ft² roof with 10% waste is 1,652.9 ft², or 16.53 squares, so buy about 17 squares' worth.
Often 3, which is 33.3 ft² per bundle, but products differ, so enter the coverage per bundle from the label. 1,652.9 ft² at 33.3 ft² per bundle is 50 bundles; at 32.8 ft² it is 51.
1.118, which is √(1 + 0.5²). A 4:12 roof is 1.054, an 8:12 roof is 1.202 and a 12:12 roof is 1.414.
Degrees = atan(rise ÷ 12). 6:12 is 26.57°, 9:12 is 36.87° and 12:12 is 45°. For a percent slope, rise ÷ 12 × 100, so 6:12 is 50%.
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