How this is calculated
Roofing is quoted from the sloped surface, not the floor-plan rectangle. A 6/12 pitch is longer than a flat rectangle of the same footprint — the pitch factor is √(1 + (rise ÷ run)²), using a 12-inch run, which is how US roofers write pitch.
Materials are sold in squares of 100 square feet in the US (typically three bundles per square) and by pack coverage in the UK. A waste allowance covers hips, valleys, starter courses, and ridge caps — a simple gable needs less than a roof with dormers.
For an L-shaped building or roof sections at different pitches, calculate each plane group separately and add the adjusted areas. Drawing one large rectangle around the whole outline can substantially overstate or miss material.
The bundle conversion is a common asphalt-shingle planning assumption only. Ridge, starter, underlayment, membranes, battens, and tiles with product-specific laps need separate quantities confirmed from the manufacturer's data.
Formula & methodology
Footprint = Length × Width. Pitch factor = √(1 + (rise/12)²). Roof area = Footprint × pitch factor. With waste = Roof area × (1 + waste %). Squares = area (ft²) ÷ 100. Bundles ≈ squares × 3, rounded up.
Worked example
A 40 ft × 30 ft gable, 6/12 pitch, 10% waste
- 1.Footprint = 40 × 30 = 1,200 ft²
- 2.Pitch factor = √(1 + (6/12)²) = √1.25 ≈ 1.118
- 3.Roof area = 1,200 × 1.118 ≈ 1,342 ft²
- 4.With 10% waste = 1,476 ft² → 14.76 squares → 45 bundles at 3 per square