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Woodworking Math

Roofing calculator

Enter the building's length and width, the overhang and the roof's pitch. You get the roof area in squares and the materials to cover it: bundles of shingles with waste, starter strip, hip and ridge cap, underlayment rolls laid to the code's laps, drip edge and sheathing sheets. The coverage figures are the shingle makers' own. Nothing is priced.

Roof shape
More: waste, bundles, products, drip edge
Blank: 12.5 %, the middle of the makers’ 10–15 %. More for valleys and dormers.
Blank: 3. Heavier designer shingles take 4 or 5: read your bundle’s wrapper.

Shingles

42 bundles

1,221 ft² of roof, 12.21 squares, plus 12.5 % for waste at 3 bundles a square

Median of 3 sources

Starter strip
2 bundles
Hip and ridge cap
2 bundles
Underlayment
2 rolls
Drip edge
15 pieces
Sheathing, 4 × 8 ft
39 sheets
Squares
12.21
Starter on the eaves and rakes, 142.1 ft at 120.33 lin ft a bundle; cap on the ridge, 42 ft at 33 lin ft a bundle.

Median of 3 sources

GAF FeltBuster, 48 in × 250 ft: one layer at 4/12 and up, lapped 2 in, ends lapped 4 in: 957 ft² a roll (IRC Table R905.1.1(2)).

39 sheets of 48 × 96 in (PS 1), no waste added. A 24/0 Rated Sheathing panel or higher (3/8, 7/16, 15/32, 1/2 in) may span supports 16 in apart (APA D510C); check the panel’s stamp and your code for edge support.

  • OK: At 4/12 or steeper: one layer of underlayment (IRC Table R905.1.1(2)).
  • OK: Drip edge on the eaves and rakes, 142.1 ft, pieces lapped at least 2 in (IRC R905.2.8.5).

One source: IRC 2024, the model building code

Quantities for asphalt shingles on a gable or hip roof with one pitch throughout. Valleys, dormers, chimneys, skylights and walls need flashing and add waste. The code’s limits are the IRC’s; your building department may add its own, such as an ice barrier or wind-rated shingles. Check your local building code.

Why: 12.21 squares of roof: 42 bundles of shingles with 12.5 % for waste.

Material list

  • 42 bundles of shingles (12.21 squares + 12.5 %)
  • 2 bundles of starter strip (142 lin ft)
  • 2 bundles of hip and ridge cap (42 lin ft)
  • 2 rolls of underlayment (GAF FeltBuster, 48 in × 250 ft)
  • 15 pieces of drip edge (142 lin ft)
  • 39 sheets of 4 × 8 ft sheathing, 24/0 or better

Quantities only, never prices. Add flashing, vents and nails for your roof; valleys, dormers and skylights add waste.

Why these numbers?

  1. The situation

    A roof of 1,092 ft² in plan, overhangs included, at one pitch throughout.

    From your input: the building and its roof

  2. What that means

    Every plane of a roof at one pitch is longer than its plan by the same factor, √(12² + rise²) ÷ 12 = 1.118, so the roof is 1,092 × 1.118 = 1,221 ft², 12.21 squares of 100 ft².

    Arithmetic: exact by definition

  3. What that means

    Standard 3-tab and architectural shingles come 3 bundles to a square.

    3 sources agree , sources [1, 2, 3]

  4. What that means

    Add 10 to 15 % for trim and waste; more on a roof with hips, valleys and dormers.

    3 sources agree , sources [1, 2, 3]

  5. So

    12.21 × 1.125 × 3 = 41.2, so 42 bundles.

    Arithmetic: exact by definition

  6. The number

    IRC R905.2.2 allows asphalt shingles from 2/12, with two layers of underlayment below 4/12 (Table R905.1.1(2): one layer lapped 2 in, or two lapped half a sheet plus 2 in, ends lapped 4 in). R905.2.8.5 requires drip edge at eaves and rakes, lapped 2 in.

    One source: International Code Council, the model building code , sources [4]

  7. The number

    Quantities for asphalt shingles on a gable or hip roof with one pitch throughout. Valleys, dormers, chimneys, skylights and walls need flashing and add waste. The code’s limits are the IRC’s; your building department may add its own, such as an ice barrier or wind-rated shingles. Check your local building code.

    From your input: where you build

How we source these numbers

How the roof area is worked out

Measure the building outside of the walls and add the overhang on every side: that is the roof's plan. Every plane of a roof at one pitch is longer than its plan by the same factor, √(12² + rise²) ÷ 12, so a gable or a hip roof at 6/12 has 1.118 times its plan area. A square is 100 square feet of roof. The eaves, rakes, ridge and hips are measured the same way: rakes climb by the factor, and a hip runs the diagonal, √(288 + rise²) ÷ 12 per foot of common run.

Where each figure comes from

Bundles a square: GAF, Owens Corning and IKO each print three bundles to a square for their standard 3-tab and architectural shingles; heavier designer shingles take four or five, so read your bundle's wrapper and type it in.Waste: all three say to add 10 to 15 percent for trim and waste, more on a roof cut up by valleys and dormers, and the calculator starts at the middle, 12.5 percent.

Starter strip runs along the eaves and rakes; hip and ridge cap along the ridge and hips. Each maker prints its own bundle's coverage:

Pick a product to use its own figure, or leave “any maker” to use the median of the three, shown with their span.Underlayment is laid to the IRC's Table R905.1.1(2): at 4/12 and steeper one layer lapped 2 inches; from 2/12 up to 4/12 two layers, a half-width strip at the eaves and then full sheets lapped half their width plus 2 inches; ends lapped 4 inches. Drip edge goes on the eaves and rakes, its pieces lapped at least 2 inches (R905.2.8.5).Sheathing is counted in 4 × 8 ft sheets with no waste, and the plywood thickness chart lists which panels each span rating covers.

What the calculator will not do

It counts asphalt shingles on a gable or hip roof with one pitch throughout. Asphalt shingles are not allowed below 2/12 (IRC R905.2.2), so a flatter roof is refused. Valleys, dormers, chimneys, skylights and walls need flashing and more waste, metal and tile roofs need their makers' own figures, and it never prices anything: prices change by the week and by the town. Where ice dams are a history, your code requires an ice barrier from the eave edge to 24 inches inside the wall (R905.1.2); tick the box to size it. Check your local building code.

Framing the roof as well? The deck calculator and stair calculator use the same plain-geometry method, and the plywood sizes page shows a sheet's true size.

Sources

  1. GAF, How many bundles of shingles are in a square: three for standard 3-tab and architectural shingles; add 10–15% for trim and waste, read 2026-11-08
  2. Owens Corning, DIY roofing: three bundles of laminate shingles a square; add 10%–15% to all material totals for waste, read 2026-11-08
  3. IKO, How many square feet in a bundle of shingles: Cambridge, Dynasty, Royal Estate 3 bundles a square; Leading Edge Plus starter 123 lin ft a bundle; Hip & Ridge 12 36.5 lin ft a bundle; waste about 10–15%, read 2026-11-08
  4. GAF, Pro-Start Starter Strip Shingles: a bundle covers about 120.33 linear feet, read 2026-11-08
  5. Owens Corning, Starter Strip Plus: 105 linear feet of actual coverage a bundle, read 2026-11-08
  6. GAF, Seal-A-Ridge ridge cap shingles data sheet: 4 bundles cover 100 linear feet, read 2026-11-08
  7. Owens Corning, ProEdge Hip & Ridge Shingles: 33 lineal feet a bundle at a 6 in exposure, read 2026-11-08
  8. GAF, FeltBuster synthetic roofing felt data sheet: roll 48 in × 250 ft, 10 squares excluding laps, read 2026-11-08
  9. Malarkey, Secure Start Plus underlayment: 250 ft × 48 in, 10 squares a roll, read 2026-11-08
  10. Owens Corning, ProArmor synthetic roof underlayment: roll 286 ft × 42 in, 10 squares, read 2026-11-08
  11. International Code Council, 2024 International Residential Code, Chapter 9: R905.1.1 and Table R905.1.1(2) Underlayment, R905.1.2 Ice barriers, R905.2.2 Slope, R905.2.8.5 Drip edge (free read-only text), read 2026-11-08

How we source these numbers