Roofing Calculator
Roofing Calculator
Estimate roof surface area and materials. Enter footprint dimensions and pitch to get sloped area, add a waste allowance, and convert to shingle squares and bundles — or metal sheet/panel counts by coverage area.
Roofing Calculator
Estimate roof area and materials (asphalt shingles or metal sheets) with waste allowance
Results
Enter dimensions above to calculate
How Roof Pitch Affects Surface Area
The pitch (rise over run) increases the actual sloped surface area beyond the floor footprint. The diagram below shows how to read pitch and the slope factor formula.
Common Pitch Values & Slope Factors
Use this table to quickly check the slope factor for your roof. The higher the pitch, the more material you need beyond the footprint area.
| Pitch (rise/run) | Angle | Slope Factor | Category |
|---|---|---|---|
| 2/12 | 9.5° | 1.014 | Low-slope / flat equivalent |
| 3/12 | 14.0° | 1.031 | Low-slope |
| 4/12 | 18.4° | 1.054 | Common residential |
| 5/12 | 22.6° | 1.083 | Common residential |
| 6/12 | 26.6° | 1.118 | Common residential |
| 7/12 | 30.3° | 1.158 | Steeper residential |
| 8/12 | 33.7° | 1.202 | Steep |
| 9/12 | 36.9° | 1.250 | Steep |
| 12/12 | 45.0° | 1.414 | Very steep |
Step-by-Step Formulas
1. Footprint Area
Start with the roof footprint (horizontal projection). This is the building length × width including overhangs. Use the same units consistently.
2. Convert Pitch to Slope Factor
Pitch increases the actual surface area compared to the horizontal footprint. A 6/12 pitch has a slope factor of 1.118 — meaning you need 11.8% more material than a flat surface of the same footprint.
3. Roof Surface Area
For gable and shed roofs, total sloped area = footprint × slope factor. Hip roofs are similar because all four faces together multiply by the same factor.
4. Waste Allowance
Waste covers starter courses, ridge caps, valley cuts, hip wrapping, and edge trim. 10% suits simple gable roofs; 15–20% is safer for hips, multiple dormers, or complex layouts.
5. Convert to Materials
Asphalt shingles are sold by the roofing square (100 ft²) and typically 3 bundles per square. Metal panels/sheets are estimated by coverage per sheet from the manufacturer.
Worked Example
Simple gable roof: 40 ft × 28 ft footprint, 6/12 pitch, 10% waste, asphalt shingles.
💡 Pro tip
Always order a minimum of 10% waste. On roofs with hips, valleys, dormers, or skylights, budget 15–20%. Running short mid-job means a second delivery charge and potential dye-lot mismatch on shingles.
Frequently Asked Questions
- What waste percentage should I use for roofing?
- For simple gable roofs, 10% waste is common. For hips, valleys, lots of penetrations, or complex layouts, 15% or more may be needed.
- What is a roofing square?
- A roofing square is 100 square feet of roof surface area. Shingles are often priced and estimated using squares and bundles (3 bundles typically cover 1 square).
- Can I use custom roof area instead of length/width and pitch?
- Yes. If you already have the sloped roof area from plans or a measurement, use Custom Roof Area to estimate materials directly.
- Does the calculator work for hip roofs?
- Yes — the slope factor approach works for hip roofs too, because all four faces share the same pitch. Total area = footprint × slope factor.