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.
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
Frequently asked questions
How do I measure roof length and width without climbing on the roof?
Use the building's exterior footprint from the ground or a plan: length along the ridge, width from eave to eave. This calculator applies the pitch factor so you do not need the slope length.
What pitch should I enter for a nearly flat roof?
Enter 0 for a membrane or felt flat roof. Low-slope roofs (1/12–2/12) still need a small pitch factor; when in doubt, a roofer's takeoff from drawings is safer than guessing pitch from the street.
Does this include underlayment, flashing, or ridge cap?
No — it estimates field shingles or tiles only. Underlayment is typically the same area plus overlap; flashing and ridge are separate line items on a materials list.
Why add waste if I measured carefully?
Starter strips, ridge, hips, valleys, and cut-offs around chimneys are not in the simple rectangle. Ten percent is a planning default for a simple gable; complicated roofs often need 15% or more.
Last updated 2026-08-01. Formulas are planning methods — confirm with a supplier before you order.