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Roofing calculator: roof area, pitch, squares and shingle bundles

From the building footprint and the roof pitch, get the pitch angle, rafter length, roof area in squares, shingle bundles and underlayment rolls.

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Results

Roof area

1,409

Roofing squares

14.1 squares

Shingle bundles to buy

47 bundles

Pitch angle
26.6 °
Conventional pitch Walkable and suited to standard shingle installation.
Rafter length including overhang
16.77 ft
Ridge height above the walls
7.00 ft
Ridge length
42.0 ft
Synthetic underlayment (10-square rolls)
2 rolls

Roof area and bundles by pitch

Pitch angleRoofing squaresShingle bundles to buy
3/12 current14.013.043
4/12 current18.413.344
6/12 current26.614.147
8/12 current33.715.150
10/12 current39.816.455
12/12 current45.017.859

From footprint to roof area

You usually know a house’s footprint, but not the actual surface of its roof. That surface drives how many shingles, how much underlayment and how much labor a roofing job needs. This calculator takes the length, width, pitch and overhang and returns the pitch angle, rafter length, roof area, roofing squares and shingle bundles.

The formulas

  1. Pitch angle = arctan(rise ÷ 12). A 12/12 roof is 45°.
  2. Horizontal run per side = half the building width (gable) or the full width (shed), plus the overhang.
  3. Rafter length = run ÷ cos(angle), equivalent to run × √(1 + (pitch ÷ 12)²).
  4. Roof area = rafter length × (building length + 2 × overhang) × number of sides.
  5. Squares = roof area ÷ 100.
  6. Bundles = squares × (1 + waste) × bundles per square.

Worked example: a 40 × 28 ft house with a 6/12 gable roof

12 in overhangs, standard shingles at 3 bundles per square, 10% waste.

  1. Angle: arctan(6 ÷ 12) ≈ 26.6°.
  2. Run per side: 28 ÷ 2 + 1 = 15 ft.
  3. Rafter length: 15 ÷ cos(26.6°) ≈ 16.77 ft.
  4. Roof area: 16.77 × (40 + 2) × 2 ≈ 1,409 sq ft.
  5. Squares: 14.1.
  6. Bundles: 14.1 × 1.10 × 3 → 47 bundles.
  7. Ridge: 42 ft long, 7 ft above the top of the walls; underlayment: 2 rolls of 10 squares.

How pitch changes the job (same house)

Pitch Angle Squares Bundles
3/12 14.0° 13.0 43
4/12 18.4° 13.3 44
6/12 26.6° 14.1 47
8/12 33.7° 15.1 50
10/12 39.8° 16.4 55
12/12 45.0° 17.8 59

A 12/12 roof needs over a third more material than a 3/12 roof on the same footprint, and steep roofs also cost more in labor and safety equipment.

Pitch rules for asphalt shingles

The International Residential Code allows asphalt shingles on slopes of 2/12 and steeper. Between 2/12 and 4/12, two layers of underlayment are required. In areas with a history of ice damming, an ice-barrier membrane must extend from the eave to at least 24 in inside the exterior wall line. Below 2/12, use a low-slope system such as TPO, EPDM or modified bitumen. Metal roofing has its own minimum slopes depending on the panel profile.

Other materials to order

  • Starter strip along every eave and rake.
  • Hip and ridge cap shingles, sold by the linear foot of ridge and hips.
  • Drip edge around the perimeter.
  • Flashing at walls, chimneys and skylights, and ventilation for the attic.
  • Nails: about 320 roofing nails per square with four nails per shingle, more in high-wind zones.

Hips, valleys and dormers increase both area and waste, so have a roofer measure on site for an exact quote.

Frequently asked questions

How do I calculate roof area from the footprint?

Each roof plane is the rafter length times the roof length. The rafter length is the horizontal run divided by the cosine of the pitch angle, or run × √(1 + (pitch ÷ 12)²). A 40 × 28 ft house with a 6/12 gable roof and 1 ft overhangs has 15 ft of run per side, a 16.77 ft rafter and about 1,409 sq ft of roof, or 14.1 squares.

What is a roofing square?

A square is 100 sq ft of roof surface. Roofers price and order materials by the square. Most architectural and 3-tab asphalt shingles come three bundles to the square, though some heavy designer lines need four or five.

How do I convert roof pitch to degrees?

Angle = arctan(rise ÷ run). A 4/12 pitch is 18.4°, 6/12 is 26.6°, 8/12 is 33.7° and 12/12 is 45°. You can measure pitch with a level held 12 in from the roof surface and a tape measuring the vertical rise.

How much waste should I allow for shingles?

About 10% on a simple gable roof and 15% or more on roofs with hips, valleys, dormers or skylights. Starter strips and hip and ridge caps are usually bought separately, by the linear foot of eaves and ridges.

What is the minimum pitch for asphalt shingles?

The International Residential Code allows asphalt shingles on slopes of 2/12 or more, with double underlayment required between 2/12 and 4/12. Below 2/12, use a low-slope roofing system instead.

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Results are estimates provided for information purposes. They do not replace professional advice.

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