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How to measure a roof and order shingles

You can estimate a shingle order without climbing onto the roof. Measure the footprint of the house, find the pitch from the attic or a ladder, and let the pitch factor turn flat area into the sloped roof area.

Updated By Matholio Editorial Team3 min read

The short answer

Roofing is ordered in squares of 100 sq ft, and most shingles come 3 bundles to a square. Measure the footprint including overhangs, multiply by the pitch factor, add 10 to 15% for waste and divide by 100. A 40 × 30 ft footprint at a 6/12 pitch is about 1,342 sq ft of roof: 14.76 squares, or 45 bundles.

What a roofing square is

GAF explains that one roofing square is 100 sq ft of roof, a 10 × 10 ft patch. Shingles are packed in bundles, and most 3-tab and architectural shingles take 3 bundles to cover a square. Some heavier products need more, so check the wrapper of the shingle you plan to use.

Step 1: Measure the footprint

Measure the length and width of the house at ground level, out to the edge of the overhangs, not the walls. For a house with wings or an L-shape, split the outline into rectangles, measure each and add their areas.

Footprint = length × width, measured to the edge of the overhangs

Step 2: Find the pitch

Pitch is the number of inches a roof rises for every 12 inches it runs horizontally. A roof that rises 6 inches in 12 has a 6/12 pitch. This Old House describes the simple method: hold a level horizontal against a rafter, mark 12 inches along it, then measure straight up from the mark to the underside of the rafter. That vertical distance is the rise.

The attic is the safest place to take the measurement. You can also enter the rise and run into the roof pitch calculator to get the pitch, angle and pitch factor.

Roof pitch, angle and pitch factor
Pitch Angle Pitch factor
4/12 18.4° 1.054
6/12 26.6° 1.118
8/12 33.7° 1.202
10/12 39.8° 1.302
12/12 45.0° 1.414

Step 3: From footprint to bundles

A sloped roof has more surface than the flat footprint under it. The pitch factor, √(1 + (rise ÷ 12)²), converts one to the other.

Roof area = footprint × pitch factor

Squares = roof area ÷ 100 × (1 + waste %)

Bundles = squares × bundles per square, rounded up

  1. Footprint: 40 × 30 = 1,200 sq ft.
  2. Pitch factor for 6/12: 1.118.
  3. Roof area: 1,200 × 1.118 = 1,341.6 sq ft.
  4. Squares with 10% waste: 1,341.6 ÷ 100 × 1.10 = 14.76 squares.
  5. Bundles: 14.76 × 3 = 44.3, rounded up to 45 bundles.
Shingle orders for sample roofs (3 bundles per square)
Footprint Pitch Roof area Waste Squares Bundles
40 × 30 ft 6/12 1,341.6 sq ft 10% 14.76 45
40 × 30 ft 6/12 1,341.6 sq ft 15% 15.43 47
50 × 34 ft 8/12 2,043.1 sq ft 10% 22.47 68

How much waste to add

GAF recommends adding 10 to 15% for trim and waste. A simple gable roof sits near 10%. Use the higher figure when the roof has hips, valleys, dormers or many small planes, because every cut leaves offcuts you cannot reuse.

What the shingle count leaves out

  • Starter strip shingles along the eaves and rakes.
  • Hip and ridge cap shingles.
  • Underlayment, ice and water barrier, drip edge and flashing.
  • Roof vents and nails.

Ask your supplier or roofer to price these from the roof drawings. They depend on edge lengths, not on the roof area.

Common mistakes

  • Measuring the walls instead of the footprint with overhangs.
  • Forgetting the pitch factor on a steep roof.
  • Ordering 3 bundles per square for a shingle that needs more.
  • Using 10% waste on a roof with many hips and valleys.
  • Climbing onto a steep or wet roof to measure it when the attic or ground measurements would do.

Calculators for this job

These calculators do the arithmetic from this guide. Each card shows the result for a typical project; open one to enter your own measurements.

Roofing Calculator 40 × 30 ft roof at 6/12 14.76 squares Open calculator Roof Pitch Calculator 6 in rise over 12 in 6/12 Open calculator

Frequently asked questions

What is a roofing square?

A roofing square is 100 sq ft of roof area. A roof with 24 squares has about 2,400 sq ft of surface.

How many bundles of shingles are in a square?

Most 3-tab and architectural shingles come 3 bundles to a square, according to GAF. Some heavier products need more, so read the wrapper.

How do I measure roof pitch without climbing on the roof?

Measure from the attic: hold a level horizontal against a rafter, mark 12 inches along it and measure straight up from the mark to the rafter. The distance in inches is the rise of a pitch written as rise/12.

What is a 6/12 pitch in degrees?

A 6/12 pitch rises 6 inches over 12 inches of run, which is about 26.6 degrees. Its pitch factor is 1.118.

Sources

  1. GAF, How Many Bundles of Shingles Are in a Square?
  2. GAF, How to Measure a Roofing Square
  3. This Old House, How to Determine Roof Pitch
Paint How much paint do I need for a room? Measure the walls, subtract openings and turn square feet into gallons and quarts, with a worked 12 × 12 ft example. Flooring How much flooring do I need? How to measure a floor, how much waste to add for plank and tile layouts, and how to round up to boxes. Concrete How much concrete do I need for a slab? Turn slab dimensions into cubic yards for a truck or bags for hand mixing, with worked examples for 4 inch slabs.