Squares of shingles (100 sq ft each).
A roofing 'square' is 100 sq ft. Roofing materials are ordered by the square.
Add ~10% waste for cuts, hips, and valleys. Steeper pitches raise waste.
The pitch factor exists because a sloped roof surface is always larger than its flat footprint on the ground — the steeper the slope, the more the actual surface area exceeds what a simple footprint measurement would suggest.
2,000 sq ft roof area ≈ 22 squares (includes waste).
Multiply footprint by a pitch factor: 4/12 pitch ≈ 1.054×; 8/12 ≈ 1.202×; 12/12 ≈ 1.414×.
Standard 3-tab: 3 bundles per square. Architectural: 3–4.
It means the roof rises 6 inches vertically for every 12 inches of horizontal run — the higher the first number, the steeper the roof.