← U.S. Residential Roofing Basics
Slope, roof area, and the roofing square
By the end: Read rise/run notation and distinguish roof surface area from a house footprint.
Residential roof steepness is often described as rise per 12 units of horizontal run. A 6:12 slope rises 6 inches for 12 horizontal inches. The run is not the sloping surface length. This distinction matters because roof coverings occupy the surface, not the flat footprint drawn on a floor plan.
For a simple rectangular plane, area equals sloping length times width. One roofing square means 100 square feet of roof area. That is an area unit, not one shingle bundle and not a square-shaped roof. Waste allowances, starter materials, ridge caps, and complex geometry are separate estimating considerations. A classroom calculation cannot determine an order quantity for an actual roof.
- A supplied drawing shows one rectangular plane 20 ft wide by 15 ft along its slope.
- Its surface area is 20 × 15 = 300 sq ft, or 3 roofing squares.
- Two identical planes total 600 sq ft, or 6 squares, before any waste or separate components.
- If 15 ft were a horizontal run instead, you would need more information before using it as the slope length.
Diagram description
A right triangle rises six units for twelve units of horizontal run. The sloping length is longer than the horizontal run. Roof surface area uses slope length, not run.
Quick check
A supplied roof-surface measurement is 1,800 sq ft. How many roofing squares is that?