The roof surface is 1,503 ft² (139.6 m²), which with 10% waste is 16.53 squares: 50 bundles of shingles, 4 rolls of #15 felt and 2 bundles of ridge cap for 42 ft of ridge.
Roof cross-section
計算方法 S
Footprint with overhangs
(40+2×1)(30+2×1)=1,344ft2
Slope factor
1+0.52=1.11803
Pitch 6 in 12 = 26.57°. A hip roof with the same pitch on all four sides has the same area factor.
Roof area
1,344×1.11803=1,502.638ft2
Squares and bundles
1001,502.638×1.1=16.529squares×3⇒50bundles
Rounded up to whole bundles.
Underlayment
⌈415.026⌉=4rolls
Roll coverage already allows for laps; rounded up to whole rolls.
Ridge
ridge=42ft
Ridge cap: 42 ft ÷ 25 ft per bundle, rounded up to 2.
Roofing and shingle calculatorについて
Roof area is the footprint including overhangs multiplied by the slope factor √(1 + (rise ÷ 12)²), which turns plan area into sloped surface. A hip roof with the same pitch on all four sides has the same factor as a gable. Area plus waste is divided into squares of 100 ft² and multiplied by bundles per square; the ridge and hip length divided by the coverage of one ridge-cap bundle gives the ridge bundles.
The default 40 × 30 ft gable with 1 ft overhangs at a 6/12 pitch has a slope factor of 1.118 and 1,502.64 ft² of roof. With 10% waste that is 16.53 squares, or 50 bundles at three per square, plus 4 rolls of #15 felt and 2 bundles of ridge cap.
The pitch is checked against the asphalt-shingle slope rules in 2021 IRC R905.2.2. Valleys, dormers, starter strip and drip edge are not counted.
照合元：Python math: 11 × 9 m ÷ cos 30° = 114.3154 m² = 1230.48 ft² × 1.1 ÷ 100 × 3 = 40.6 → 41
よくある質問
How many bundles of shingles do I need?
Divide the roof area by 100 to get squares, add waste, and multiply by the bundles per square printed on the wrapper (three is the default here). A 40 × 30 ft gable with 1 ft overhangs at 6/12 has 1,502.64 ft² of roof, or 16.53 squares with 10% waste: 49.6 bundles, bought as 50. The waste presets are 10% for gables and 15% for hips, where every course is cut along the hips.
How do you convert roof pitch to degrees and a slope factor?
Pitch is the rise in inches per 12 in of run. A 6/12 roof rises at arctan(6 ÷ 12) = 26.57°, and its slope factor √(1 + 0.5²) = 1.118 means each square foot of plan carries 1.118 ft² of roof. A 4/12 roof is 18.43° with a factor of 1.054; a 12/12 roof is 45° with a factor of 1.414.
What is the minimum roof pitch for asphalt shingles?
2:12, about 9.5°. Under 2021 IRC R905.2.2, asphalt shingles may be used only on slopes of 2 in 12 or steeper, and slopes from 2:12 up to 4:12 need double underlayment. Flatter roofs need a low-slope covering such as a membrane. In the 2:12 to 4:12 range this calculator doubles the underlayment rolls.
What is a roofing square?
100 ft² of roof surface, about 9.29 m². Shingles and underlayment are ordered by the square, so a roof of 1,502.64 ft² is 15.03 squares before waste and 16.53 after 10% waste. With the #15 felt option at 4 squares per roll, that roof takes 4 rolls of underlayment.
How do you calculate the area of a hip roof?
Use the same slope factor as a gable: when all four sides share one pitch, sloped area = plan area × √(1 + (rise ÷ 12)²). A 40 × 30 ft hip roof at 6/12 with no overhang is 1,200 × 1.118 = 1,341.64 ft². Its ridge is 40 − 30 = 10 ft and each of the four hips is 22.5 ft, so ridge cap covers 100 ft.
International Code Council — 2021 IRC R905.2.2: asphalt shingles on slopes of 2:12 or steeper, double underlayment from 2:12 up to 4:12; GAF Seal-A-Ridge — 25 linear ft per bundle; Owens Corning ProEdge — 33 linear ft per bundle.