How the calculator works it out
Roofers measure in squares, and a square is 100 square feet of roof surface. The calculator takes the plan area of the roof (the house plus the overhang on every side), stretches it by the pitch multiplier to get the real sloped area, then adds waste and divides into squares and bundles.
Plan area = (length + 2 × overhang) × (width + 2 × overhang)
Pitch multiplier = √(1 + (rise ÷ 12)²)
Roof area = plan area × pitch multiplier
Squares = roof area × (1 + waste %) ÷ 100 · Bundles = squares × 3, rounded up
Worked example: a 40 × 30 ft house
With a 12 inch overhang the roof covers 42 × 32 = 1,344 square feet of plan. At 6/12 the multiplier is √(1 + 0.5²) = 1.118, so the roof itself is 1,344 × 1.118 = 1,502.6 square feet. Add 10% for a gable roof: 1,652.9 square feet, or 16.53 squares. At three bundles per square that is 49.6, so you buy 50 bundles. Underlayment is 2 rolls of synthetic or 5 rolls of #15 felt.
Why hip and gable roofs have the same area
Every face of the roof is tilted at the same pitch, and a tilted surface is always its flat footprint times the same multiplier. A gable roof splits the plan into two rectangles; a hip roof splits the same plan into two trapezoids and two triangles. Either way the pieces add up to the same plan area, so the sloped areas match too. What changes is the waste: a hip roof has diagonal cuts along every hip and more edges, which is why the calculator starts hip roofs at 15% waste. Drip edge changes too. A gable roof has two eaves and four sloped rake edges, while a hip roof has eaves all the way round and no rakes.
This assumes every face shares one pitch. For a roof with dormers, valleys, different pitches or a lower porch roof, work each section out separately and add them.
Pitch multiplier table
| Pitch | Angle | Multiplier | Roof area per 1,000 sq ft of plan |
|---|---|---|---|
| 1/12 | 4.8° | 1.003 | 1,003 sq ft |
| 2/12 | 9.5° | 1.014 | 1,014 sq ft |
| 3/12 | 14.0° | 1.031 | 1,031 sq ft |
| 4/12 | 18.4° | 1.054 | 1,054 sq ft |
| 5/12 | 22.6° | 1.083 | 1,083 sq ft |
| 6/12 | 26.6° | 1.118 | 1,118 sq ft |
| 7/12 | 30.3° | 1.158 | 1,158 sq ft |
| 8/12 | 33.7° | 1.202 | 1,202 sq ft |
| 9/12 | 36.9° | 1.250 | 1,250 sq ft |
| 10/12 | 39.8° | 1.302 | 1,302 sq ft |
| 11/12 | 42.5° | 1.357 | 1,357 sq ft |
| 12/12 | 45.0° | 1.414 | 1,414 sq ft |
| 13/12 | 47.3° | 1.474 | 1,474 sq ft |
| 14/12 | 49.4° | 1.537 | 1,537 sq ft |
| 15/12 | 51.3° | 1.601 | 1,601 sq ft |
| 16/12 | 53.1° | 1.667 | 1,667 sq ft |
| 17/12 | 54.8° | 1.734 | 1,734 sq ft |
| 18/12 | 56.3° | 1.803 | 1,803 sq ft |
What the numbers are based on
- Bundles. GAF's guide to bundles per square and Owens Corning's roofing estimating guide both give three bundles per square for most 3-tab and architectural shingles. Some lines differ: Owens Corning's glossary says 3, 4 or 5 bundles per square, and GAF Woodland comes four to a square.
- Waste. Both manufacturers suggest adding 10 to 15%. Owens Corning applies it to underlayment as well, and so does this calculator.
- Underlayment. Owens Corning gives 10 squares per roll of synthetic and 4 squares per roll of #15 felt. Many #15 rolls are printed as 432 square feet; the extra goes into the overlaps.
- Low slopes. The 2021 International Residential Code, section R905.2.2, allows asphalt shingles only at 2/12 or steeper and requires double underlayment from 2/12 up to 4/12. The calculator doubles the underlayment for 2/12 and 3/12. Your local code may be amended, so confirm with your building department.
- Drip edge. Section R905.2.8.5 of the same code requires drip edge at eaves and rake edges of shingle roofs, with pieces lapped at least 2 inches. The piece count assumes 10 ft lengths, the usual size at supply stores, lapped 2 inches.
Before you order
- Measure the overhang on the gable ends too. Rake overhangs are often shorter than eave overhangs; if so, the result runs slightly high, which is the safe direction.
- Ridge caps, starter strip, ice and water barrier and vents are not included.
- For a tear-off, ask how many layers are on the roof now. Disposal is priced by weight.
Questions people ask
How many bundles of shingles are in a square?
Three bundles per square for most standard 3-tab and architectural asphalt shingles, according to both GAF and Owens Corning. A square is 100 square feet of roof. Some heavier or specialty lines are packed four or five bundles per square, such as GAF Woodland at four, so read the bundle label before you order.
How do I work out roof area from the ground?
Measure the house length and width, add the overhang on both sides, and multiply to get the plan area. Then multiply by the pitch factor for your slope. A 42 × 32 ft plan at 6/12 is 1,344 × 1.118 = 1,503 square feet of roof. Owens Corning describes the same plan-area times correction-factor method.
Why does a hip roof get more waste than a gable roof?
The surface area is the same at the same pitch, but every hip is a diagonal line that shingles have to be cut along, and hip roofs have four sloped edges instead of two. That means more offcuts. GAF and Owens Corning both suggest 10 to 15% waste; this calculator starts at 10% for gable and 15% for hip.
What is the lowest pitch I can use asphalt shingles on?
Under the International Residential Code (section R905.2.2), asphalt shingles need a slope of at least 2/12. From 2/12 up to 4/12 the code requires a double layer of underlayment. Below 2/12 you need a low-slope roofing system instead. Local codes can differ, so check with your building department.
How many rolls of underlayment do I need?
Owens Corning gives 10 squares (1,000 sq ft) for a roll of synthetic underlayment and 4 squares (400 sq ft) for a roll of #15 felt. Divide your roof area including waste by those figures and round up. Roll sizes vary between brands, so check the product you are buying.
Does this include ridge caps and starter strip?
No. Hip and ridge cap shingles and starter strip are sold as separate products with their own coverage per bundle. Measure the total length of ridges and hips for the caps, and the length of the eaves (plus rakes, if the manufacturer calls for it) for the starter.