Methodology
How we calculate, and where the numbers come from.
Every constant below is sourced from manufacturer spec sheets or trade-standard references. Defaults are starting points — you can override product-specific figures in the calculator.
Two figures we deliberately do not invent: cost (you supply your own price
per square, since prices swing by region and year) and slate piece counts (these depend on
slate size and exposure, so we ask you to enter your product's coverage).
| Constant | Default | Source |
|---|---|---|
| Roofing square | 100 sq ft | Universal trade unit |
| 3-tab shingles | 3 bundles per square | GAF / industry convention |
| Architectural shingles | 4 bundles per square | GAF / WeatherShield selector |
| Shingle bundle coverage | 33.3 sq ft (~29 shingles) | Manufacturer convention |
| Starter strip | 120.33 lin ft per bundle (38 pcs) | GAF Pro-Start spec sheet |
| Hip & ridge cap | 25 lin ft per bundle @ 6 2/3" exposure | GAF Seal-A-Ridge spec sheet |
| 15 lb felt | ~400 sq ft per roll | Inch Calculator |
| 30 lb felt | ~200 sq ft per roll | Inch Calculator |
| Metal panel coverage | 16" standing seam / 24" 5V / 36" R-panel (coverage width) | Manufacturer specs |
| Concrete S-tile | ~89 pieces per square (12 3/8" x 17") | Eagle Capistrano spec |
| Cedar shakes | ~5 bundles per square @ 10" exposure (24") | Cedar Shake & Shingle Bureau / suppliers |
| Slate | User-input (varies by size & exposure) | No safe fixed default |
| Penetration deductions | User-supplied opening area, subtracted before waste | Chimneys & skylights; small vents left as waste cover |
| Step flashing | ~1 piece per linear foot of wall/chimney side | Trade estimate; 5"–7" pieces with overlap |
| Chimney & skylight flashing | One kit per unit (step + counter, or head/sill/side) | Trade convention; inferred from penetrations entered |
| Valley flashing | 10 ft per stick of open-valley metal | Manufacturer / supplier convention |
| Pipe boots | One boot per pipe or vent penetration | Trade convention |
| Waste factor | 10% simple to 25% very complex | Trade convention |
| Cost | User-input price per square only | No fabricated pricing |
The core formula
For each roof plane: true area = footprint × pitch multiplier, where the multiplier is √(run² + rise²) ÷ run. Planes are summed, waste is added, and the total is divided by 100 to get roofing squares. Materials follow from there.
See the guides for worked examples, or the full calculator to build your own roof.