Roofing Calculator Widget
Let homeowners estimate a re-roof before they call a contractor. They enter the building's footprint, the eave overhang and the roof pitch, and see the sloped roof area, the number of squares, the shingle bundles and the underlayment rolls to order.
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<iframe src="https://a2z.tools/embed/w/roofing-calculator" title="Roofing Calculator by A2Z Tools" width="100%" height="860" style="border:0;width:100%" loading="lazy" allow="clipboard-write"></iframe>
A plain iframe. Works everywhere, including site builders that strip scripts. Adjust height if your content needs more room.
<div data-a2z-widget="roofing-calculator" data-height="860"></div> <script async src="https://a2z.tools/embed.js"></script>
Adds a small script (what it does) that sizes the widget to fit its content, loads it lazily and keeps it isolated from your page's CSS.
Works with
How it works
Roofers measure in squares of 100 square feet. The widget first adds the overhang on all four sides to the footprint, then multiplies the plan area by the pitch factor, the square root of 1 + (rise / 12)², which converts a flat plan into area along the slope. For any roof whose planes share one pitch - a plain gable or a hip roof - this gives the true surface without measuring each plane. A 40 x 30 ft house with a 1 ft overhang has a 1,344 sq ft plan; at 6/12 the factor is 1.118, giving 1,503 sq ft or 15.03 squares. The waste allowance (10% for simple roofs, more where hips and valleys need cutting) is added, and the result times the bundles per square (three for most asphalt shingles) is rounded up to whole bundles. Underlayment rolls use the chosen roll's coverage. Below 4/12 a caution appears, because shingle makers specify a special low-slope underlayment method there, and below 2/12 shingles are outside their pitch range.
Calculation method
- Plan area = (length + 2 x overhang) x (width + 2 x overhang)
- Pitch factor = sqrt(1 + (rise / 12)^2); from degrees, rise = 12 x tan(angle)
- Roof area = plan area x pitch factor; squares = roof area / 100 sq ft (9.2903 m2)
- Bundles = ceil(squares x (1 + waste / 100) x bundles per square)
- Underlayment rolls = ceil(roof area x (1 + waste / 100) / roll coverage); synthetic 1000 sq ft, #15 felt 400 sq ft, #30 felt 200 sq ft (typical nominal roll sizes - check the label)
Worked examples
Two-storey house, 6/12 gable
Inputs: 40 x 30 ft; 1 ft overhang; 6/12; 10% waste; 3 bundles per square; synthetic underlayment
Result: 1,503 sq ft (15.03 squares); 50 bundles; 2 rolls
42 x 32 = 1,344 sq ft x 1.118 = 1,502.6; x 1.10 = 16.53 squares x 3 = 49.6, rounded up to 50.
Steep 12/12 shed
Inputs: 10 x 10 ft; no overhang; 12/12; 0% waste
Result: 141 sq ft; 5 bundles
100 x 1.4142 = 141.4 sq ft = 1.414 squares x 3 = 4.24, rounded up to 5.
Limitations
- Assumes one pitch throughout; roofs mixing pitches should be split and run plane by plane.
- Ridge cap, hip cap, starter strip, drip edge, ice-and-water shield and flashing are not counted.
- Gable-end overhangs (rakes) are treated as equal to the eave overhang.
- Underlayment roll coverages are typical nominal figures; laps and the product's own coverage change the count.
Where publishers use it
- Roofing contractors' lead pages that ask for a rough size first
- Shingle distributors and home centres listing bundle prices
- Storm-damage and insurance-claim guides explaining squares
- Shed and garage roofing tutorials
- Real-estate inspection blogs estimating a roof replacement
Questions
What is a roofing square?
A square is 100 square feet of roof surface, about 9.29 m². Shingles, underlayment and labour are priced per square, so a 1,503 sq ft roof is just over 15 squares.
How many bundles of shingles are in a square?
Most three-tab and architectural asphalt shingles come three bundles to the square, but some heavier premium lines are packed four or five to the square - read the wrapper. The widget lets you change the figure.
Does a hip roof need a different calculation?
Not for area. If every plane has the same pitch, the sloped area is the plan area times the pitch factor whether the roof is a gable or a hip. Hips and valleys do need more cutting waste and extra ridge/hip cap.
How much waste do I add for shingles?
Around 10% for a straight gable and 15% or more for roofs with hips, valleys, dormers or skylights, because each cut line wastes part of a shingle.
Can I enter the pitch in degrees?
Yes. Switch 'Pitch given as' to degrees; 25 degrees is the same as 5.6/12 and gives a pitch factor of 1.1034.
What is the lowest pitch for asphalt shingles?
Shingle specifications cover roofs from 2:12 upward, and between 2:12 and 4:12 they require the maker's low-slope underlayment method printed on the wrapper or the local code's. Flatter roofs need a membrane system instead.
Sources
- Roofing glossary: bundle and square - Owens Corning . A square covers 100 square feet; there are typically 3, 4 or 5 bundles per square.
- Section 07 31 13 Asphalt Shingles - guide specification - Owens Corning . Bundles per square: 3 bundles of 20 or 22 shingles (Duration), 4 for some premium lines; on pitches from 2:12 to less than 4:12 follow the low-slope application instructions on the wrapper or local code.
Cite or recommend this tool
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A2Z Tools Roofing Calculator https://a2z.tools/embed/roofing-calculator
<a href="https://a2z.tools/embed/roofing-calculator">A2Z Tools Roofing Calculator</a>
[A2Z Tools Roofing Calculator](https://a2z.tools/embed/roofing-calculator)
Roofing Calculator by A2Z Tools - https://a2z.tools/embed/roofing-calculator
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