Key Takeaways
- Roofing materials are sold by the square (100 square feet of surface area)
- Roof pitch increases surface area beyond the plan footprint
- Hip and gable roofs with the same pitch have the same surface area
- Flat roofs typically use 1 bundle per square; pitched roofs use 3 bundles
- A 10 percent waste allowance covers cuts at ridges, valleys and edges
Roofing square is a unit of area equal to 100 square feet of roof surface, the standard unit for ordering roofing materials in the US.
Pitch is the slope of a roof, expressed as rise over run (for example, 4/12 means 4 inches of rise for every 12 inches of horizontal run).
Estimating roofing materials starts with understanding that the number you see on a floor plan is not the number you need to order. A roof is sloped, which means the actual surface area is larger than the area underneath it. The steeper the pitch, the bigger the difference.
Here is how to work through the calculation.
Plan area is the floor footprint
Multiply the length by the width of the building. A house with a 40 foot by 30 foot footprint has a plan area of 1,200 square feet. That number is the basis for everything else, but it is not the number of shingles you need.
Pitch changes everything
Roof pitch is the ratio of vertical rise to horizontal run, expressed as something like 4/12 or 6/12. The pitch multiplier converts the plan area into actual roof surface area.
The formula is: pitchMultiplier = sqrt(1 + (rise / run)^2)
For a 4/12 pitch: pitchMultiplier = sqrt(1 + (4/12)^2) = sqrt(1 + 0.1111) = 1.0541
That means the actual roof surface is about 5.4 percent larger than the plan area. For a 1,200 square foot plan, the roof surface is 1,200 x 1,0541 = 1,265 square feet.
A 12/12 pitch nearly doubles the surface area. A flat roof has a multiplier of 1.
Roof type affects the count
A gable roof has two sloping sides meeting at a ridge. A hip roof has four sloping sides, all with the same pitch. Because every face shares the same slope angle, the total surface area equals the plan area times the pitch multiplier, the same as a gable roof. Hip roofs may generate more waste during cutting, which is handled through the waste percentage.
Flat roofs have no pitch adjustment but may need different materials and different waste allowances.
Roofing squares and bundles
Roofing materials are sold by the square, and one square covers 100 square feet of roof surface. Standard three-tab shingles require 3 bundles per square. Flat roof materials typically use 1 bundle per square.
The calculator first converts the roof area into squares (including waste), then multiplies by the bundles per square to get the total bundle count.
The waste allowance covers cuts
Cutting shingles at ridges, valleys, edges and penetrations creates waste. Ten percent is a reasonable allowance for a standard gable or hip roof. Complex roof shapes with many valleys or dormers may need more.
A caution worth reading
The numbers above are shopping estimates, not engineering specifications. For anything structural or anything your local building department has an opinion about, use what they require. This site produces material quantities, not engineering drawings.
Once you have your dimensions, the roofing calculator turns them into a complete material list with squares, shingles and total area.
Editorial note: This article is reviewed for accuracy and updated regularly. All calculations use standard industry formulas. For the latest density figures, consult your supplier directly.
Frequently asked questions
How many shingles do I need for a 2000 square foot roof?
A 2,000 square foot plan area at a 4/12 pitch has an actual roof surface of about 2,120 square feet, or roughly 21 squares. Standard three-tab shingles require 3 bundles per square, so you would need about 63 bundles. Add 10 percent for waste and you are ordering about 70 bundles.
What is a roofing square?
One roofing square equals 100 square feet of roof surface area. It is the standard unit that roofing materials are sold and priced by. If your roof measures 2,500 square feet of actual surface, that is 25 squares.
How does roof pitch affect the material count?
Roof pitch increases the actual surface area beyond the plan footprint. A 4/12 pitch adds about 5 percent to the surface area. A 12/12 pitch nearly doubles it. The steeper the pitch, the more material you need.
How many bundles of shingles per square?
Standard three-tab shingles require 3 bundles per square (100 square feet). Architectural or laminated shingles may also use 3 bundles per square but cover more area per bundle. Flat roof materials typically use 1 bundle per square.