Roof Area Calculator
Calculate the total area of your roof based on dimensions, pitch, and roof type
Building Footprint
Along the ridge (eave to eave for hip)
Perpendicular to the ridge
Roof Pitch
e.g., 6 means 6:12 pitch
Mansard Lower Slope
Steep lower portion (typically 18:12 to 24:12)
Overhang (Eave)
Typical: 12" - 24" (30 - 60 cm)
Results
Step-by-Step Calculation
Understanding Roof Area Calculations
Roof area is always larger than the building footprint because of the slope. The steeper the pitch, the greater the difference between floor area and roof area. The pitch multiplier (also called the roof area factor) is used to convert a flat footprint area into the actual sloped roof area.
The Pitch Multiplier Formula:
Multiplier = √(rise² + run²) / run
For a 6:12 pitch: √(6² + 12²) / 12 = √(180) / 12 = 13.416 / 12 = 1.118
Roof Area = Footprint Area × Pitch Multiplier
Common Pitch Multipliers:
- Flat (0:12): 1.000 — No increase
- Low slope (3:12): 1.031 — 3.1% more area
- Standard (6:12): 1.118 — 11.8% more area
- Moderate (8:12): 1.202 — 20.2% more area
- Steep (12:12): 1.414 — 41.4% more area
Roof Types Explained
Gable Roof
The most common roof type with two sloping sides meeting at a central ridge.
- • Two rectangular sloped surfaces
- • Triangular gable ends (vertical walls)
- • Good water and snow shedding
- • Simple to build and maintain
Hip Roof
All four sides slope downward to the walls with no vertical gable ends.
- • Two trapezoidal and two triangular surfaces
- • More stable in high winds
- • Requires more roofing material
- • More complex construction
Flat Roof
A nearly level roof with minimal pitch (typically 1/4" to 1/2" per foot for drainage).
- • Roof area ≈ footprint area
- • Common on commercial buildings
- • Allows rooftop use (HVAC, decks)
- • Requires special waterproof membranes
Shed / Lean-To Roof
A single sloping surface, like half of a gable roof.
- • One rectangular sloped surface
- • Simplest roof to build
- • Common for additions and outbuildings
- • Good for solar panel installations
Mansard Roof
A four-sided roof with two slopes on each side — a steep lower slope and a flatter upper slope.
- • Maximizes usable attic space
- • Distinctive French architectural style
- • Lower slope is nearly vertical (18:12 to 24:12)
- • Upper slope is shallow (similar to low-pitch gable)
Roof Pitch Reference Table
| Pitch (X:12) | Angle (°) | Multiplier | % Increase | Category |
|---|---|---|---|---|
| 1:12 | 4.8° | 1.003 | 0.3% | Low slope |
| 2:12 | 9.5° | 1.014 | 1.4% | Low slope |
| 3:12 | 14.0° | 1.031 | 3.1% | Low slope |
| 4:12 | 18.4° | 1.054 | 5.4% | Conventional |
| 5:12 | 22.6° | 1.083 | 8.3% | Conventional |
| 6:12 | 26.6° | 1.118 | 11.8% | Most common |
| 8:12 | 33.7° | 1.202 | 20.2% | Moderate |
| 10:12 | 39.8° | 1.302 | 30.2% | Steep |
| 12:12 | 45.0° | 1.414 | 41.4% | Steep |
| 18:12 | 56.3° | 1.803 | 80.3% | Very steep (mansard) |
Understanding Roofing Squares
In the roofing industry, materials are often sold by the "square." One roofing square equals 100 square feet (9.29 m²) of roof area. This standard unit simplifies ordering and cost estimation.
Shingles
3 bundles per square (for standard 3-tab shingles). Architectural shingles may require 4-5 bundles per square.
Underlayment
Typically sold in rolls covering 4 squares (400 sq ft). Allow 10-15% overlap.
Metal Roofing
Panels are sold by the linear foot or square. Standard panels are 36" wide with varying lengths.
Waste factor: Always add 10-15% for waste due to cutting, overlaps, and mistakes. Complex roofs with many valleys, hips, and dormers require a higher waste factor (15-20%).
Related Calculators
Note: This calculator provides estimates based on standard geometric formulas and uniform roof pitch. Real roofs may have dormers, valleys, chimneys, skylights, and other features that affect total area. For complex roofs, consider measuring each section separately. For the most accurate measurements, a professional roof inspection or aerial measurement service is recommended.
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