A 20-by-20-foot garage has a 400-square-foot footprint, but its roof usually covers more than 400 square feet because of the roof’s slope and any overhangs. For a basic estimate, a simple low-slope roof may need about five roofing squares before waste; a typical pitched roof may need roughly five to six squares. The exact amount depends on roof design, measurements, and the shingles’ stated coverage.
Quick estimate: A simple 20×20 garage roof commonly calls for about 17 to 20 bundles of standard asphalt shingles, assuming three bundles cover one square and the estimate includes typical waste. Check the package label: bundle coverage varies by product, and ridge caps and starter shingles may be sold separately.
What “Roofing Square” Means
Roofing materials are often estimated in squares. One roofing square equals 100 square feet of roof surface. Shingles are commonly sold in bundles, and many standard asphalt shingles require three bundles per square. That is a useful estimating rule, not a guarantee: the manufacturer’s packaging and installation instructions determine the actual coverage.
For a 400-square-foot roof, the basic calculation is 400 ÷ 100 = 4 squares. But that figure treats the roof as a flat surface with no overhangs. A pitched roof has more surface area than the horizontal footprint, and cutting, fitting, and damaged shingles also affect the order quantity.
How Many Shingles a 20×20 Garage May Need
The table uses a 20-by-20-foot building footprint, excludes overhangs, and estimates roof area using common pitch factors. It then adds 10% for waste and assumes three bundles per square. Actual results can vary with roof dimensions, product coverage, and layout.
| Roof Pitch | Approximate Roof Area | With 10% Waste | Approximate Bundles |
|---|---|---|---|
| Low-slope reference | 400 sq. ft. | 440 sq. ft. | 14 bundles |
| 4/12 | 422 sq. ft. | 464 sq. ft. | 14 bundles |
| 6/12 | 447 sq. ft. | 492 sq. ft. | 15 bundles |
| 8/12 | 481 sq. ft. | 529 sq. ft. | 16 bundles |
| 10/12 | 521 sq. ft. | 573 sq. ft. | 18 bundles |
| 12/12 | 566 sq. ft. | 622 sq. ft. | 19 bundles |
These bundle estimates are rounded up to the next whole bundle, based on three bundles per 100 square feet. A low-slope reference is not a recommendation to install shingles on any roof below the product’s minimum permitted pitch. Asphalt-shingle manufacturers specify minimum slope requirements and underlayment details; the installation instructions and local code should be followed.
For a common 6/12 roof, the estimate is about 15 bundles with 10% waste. A roof with a 12/12 pitch is closer to 19 bundles under the same assumptions. If the garage has wide eaves, hips, valleys, dormers, or other details, the order quantity can be higher.
How to Calculate the Shingle Quantity
Measure the roof, not just the garage floor
The most reliable starting point is the roof’s actual dimensions. Measure each roof plane from eave to ridge and along the roof’s length, accounting for overhangs. For a simple gable roof, there are two rectangular planes. Multiply the length by the sloped distance from eave to ridge for each plane, then add the two areas.
For example, a 20-foot-wide gable garage with a 6/12 pitch has a half-span of 10 feet. A 6/12 pitch rises 6 inches for every 12 inches of horizontal run, or 0.5 foot per foot. The rise is 10 × 0.5 = 5 feet. The sloped distance from eave to ridge is approximately √(10² + 5²), or 11.18 feet. If the roof is 20 feet long and has no overhangs, the two planes total about 20 × 11.18 × 2 = 447 square feet.
Add overhangs and roof features
Overhangs extend the roof beyond the building’s walls, increasing the area. A 20-by-20-foot garage with 1-foot overhangs on all sides has a larger roof plan than a 20-by-20-foot footprint. The exact increase depends on the roof shape and slope. Measure to the roof edges rather than assuming the building dimensions equal the roof dimensions.
Chimneys, vents, skylights, valleys, and hips also affect the estimate. A large opening can reduce the shingle area slightly, but cutting around it can create extra waste. Valleys, hips, and complicated roof intersections often require more careful cutting and may increase material needs. A straightforward roof with two clean planes is generally easier to estimate than a roof with several intersecting sections.
Apply the roof-pitch factor
If only the horizontal roof footprint is known, a pitch factor can provide a preliminary estimate. Multiply the footprint area by the appropriate factor, then add overhang area if applicable. Common approximate factors are 1.054 for 4/12, 1.118 for 6/12, 1.202 for 8/12, 1.302 for 10/12, and 1.414 for 12/12. These values convert horizontal area to sloped surface area; they do not include waste.
Pitch is commonly written as rise over a 12-inch horizontal run. A 6/12 roof rises 6 inches for each foot of horizontal distance. The pitch can be checked with a level and tape measure, but measuring a roof from the ground or ladder carries fall risk. A qualified roofer can measure the roof safely when access is difficult.
Add an appropriate waste allowance
After calculating the roof surface, add material for cuts and installation waste. Ten percent is a common starting estimate for a simple roof. A roof with multiple valleys, hips, dormers, unusual geometry, or a complex shingle pattern may require 15% or more. Waste is not a fixed rule; the roof layout and installer’s method affect the amount.
Use this formula:
Roof area × (1 + waste percentage) ÷ 100 = squares to order
For example, a 447-square-foot 6/12 roof with 10% waste is 447 × 1.10 = 491.7 square feet, or about 4.92 squares. Round up to five squares. If the selected shingles cover 100 square feet per three bundles, five squares require 15 bundles.
Do Not Forget Starter Shingles and Ridge Caps
Field shingles cover the main roof planes, but a complete installation can require other shingle products or accessories. Starter shingles are installed along eaves and, depending on the installation system, at rakes. Ridge-cap shingles cover the roof peak; hip shingles may be needed on a hip roof. These products may be sold separately or may have different coverage from standard field shingles.
Underlayment, drip edge, flashing, and ventilation components are also part of a roofing project, but they are not counted as field-shingle bundles. The exact materials depend on the roof design, manufacturer’s instructions, and local requirements. Before ordering, check whether the shingle package or roofing estimate includes starter and ridge-cap products.
What Can Change the Estimate?
- Overhangs: Eaves and rakes add roof surface beyond the garage’s wall dimensions.
- Pitch: A steeper roof has more surface area than a low-slope roof with the same footprint.
- Roof shape: Gable, hip, and intersecting roofs have different layouts and cutting needs.
- Waste allowance: Complex designs generally need more extra material than simple rectangular planes.
- Shingle coverage: Bundle coverage varies by shingle style and manufacturer. Verify the package label.
- Installation details: Starter strips and ridge caps may be separate products and need separate quantities.
- Existing roof layers: A tear-off or overlay can affect project scope, disposal, and preparation, although it does not change the measured area of the new roof.
How to Estimate Starter and Ridge-Cap Materials
Starter material is generally estimated by the linear footage of the eaves and sometimes the rakes, according to the manufacturer’s system. Ridge-cap material is estimated by the length of the roof ridge. The number of pieces or bundles depends on the product’s stated coverage. For a simple 20-foot-long gable roof, the ridge is approximately 20 feet long before accounting for roof extensions. A hip roof has additional hip lines that may need cap shingles.
Product instructions matter because some shingles can be cut and used for caps, while others call for dedicated ridge-cap shingles. Using field shingles as caps is appropriate only when the manufacturer permits that method. The product’s installation guide provides the relevant coverage and fastening details.
Common Estimating Mistakes
Ordering for 400 square feet without adjusting for pitch
The garage floor area is not automatically the roof area. On a 6/12 gable roof with no overhangs, the sloped roof area is about 447 square feet, not 400. Adding a 10% waste allowance brings the estimate to about 492 square feet.
Assuming every bundle covers the same area
Three bundles per square is a familiar rule for many asphalt shingles, but packaging and coverage can differ. Architectural shingles, specialty products, and ridge-cap materials should be checked individually. The manufacturer’s label is more reliable than a generic bundle estimate.
Leaving out roof edges and accessories
Overhangs add area, while starter courses and ridge caps require their own material calculations. A roof estimate that includes only field shingles may not represent the complete shingle order.
Ignoring matching and future repairs
Buying slightly more than the calculated amount can be useful if shingles are damaged during installation or repairs are needed later. However, excessive leftovers are costly, and asphalt shingles can differ between production batches. Ask the supplier about ordering enough from the same batch and about the return policy for unopened bundles.
Practical Ordering Checklist
- Confirm whether 20×20 describes the building’s exterior dimensions or the roof edges.
- Identify the roof type and measure each roof plane, including overhangs.
- Calculate total sloped roof area, or use a pitch factor for a preliminary estimate.
- Add a waste allowance suited to the roof’s complexity.
- Convert square footage to roofing squares and round up to whole squares or bundles as the supplier sells them.
- Verify the selected shingle’s coverage on its packaging or manufacturer’s product literature.
- Estimate starter strips and ridge or hip caps separately, and confirm underlayment and flashing needs.
- Check local building requirements and product instructions before installation.
Reliable Sources for Product and Code Details
For shingle coverage and installation requirements, consult the selected manufacturer’s documentation. GAF’s roofing resources and Owens Corning’s roofing resources provide product information and homeowner guidance. The National Roofing Contractors Association offers consumer information about roofing work. Local building departments can clarify applicable permit and code requirements.
For a typical 20×20 garage with a simple 6/12 gable roof and no overhangs, a practical preliminary order is about five squares, or 15 bundles when each bundle covers roughly one-third of a square, plus separately calculated starter and ridge-cap materials. Recalculate if the garage has overhangs, a steeper pitch, or a more complex roof.