Roof Area Calculator
Work out the surface area of a roof from the building's length and width, the overhang and the pitch, and how much roofing to order with a waste allowance.
Roof surface area
1,503 sq ft (139.6 m²)
- Material to order
- 1,653 sq ft
- Roofing squares (100 sq ft)
- 17
- Pitch factor
- 1.118
A 40 × 30 ft building with a 6-in-12 roof has about 1,503 sq ft of roof; order about 1,653 sq ft.
How it's worked out
- Footprint with overhang
42 × 32 ft1,344 sq ft - Pitch factor
√(1 + (6 ÷ 12)²)1.118 - Roof area
1,344 × 1.1181,503 sq ft
The working
Pitch factor = √(rise² + 12²) ÷ 12
Roof area = (length + 2 × overhang) × (width + 2 × overhang) × pitch factor
Example
A 40 × 30 ft house with a 1 ft overhang has a 42 × 32 ft footprint, 1,344 sq ft. At a 6-in-12 pitch that is 1,503 sq ft of roof. With 10% waste you need about 1,653 sq ft, or 17 squares.
Works the same in metres; the pitch factor has no units.
Questions people ask
How do I calculate roof area?
Multiply the footprint, including the overhang, by the pitch factor. A 6-in-12 roof has a factor of 1.118, so it has about 12% more area than the flat footprint.
What does a 6/12 pitch mean?
The roof rises 6 inches for every 12 inches across. Steeper roofs have a bigger pitch factor and more surface to cover.
What is a roofing square?
100 square feet of roof, the unit shingles and sheets are often sold in.
How much waste should I allow?
About 10% for a simple gable roof and 15% or more for hips, valleys and dormers.
About this calculator
Formula and rules last checked 25 October 2026Written and tested by the CalculatorDex teamReport a problem