Why a roof is bigger than the house footprint — footprint area times the pitch slope multiplier equals roof squares
Your roof is always bigger than your house footprint. Multiply the flat footprint area by the pitch slope multiplier (1.118 for a 6:12) to get true roof area, then divide by 100 to get roofing squares. A 1,500 sq ft footprint at 6:12 ≈ 1,677 sq ft ≈ 16.8 squares.
What this diagram shows
Diagram showing why a sloped roof has more surface area than the flat house footprint beneath it. A right triangle shows that 12 inches of horizontal run becomes a longer rafter on the slope, so a flat footprint must be multiplied by a slope (pitch) multiplier to get the true roof area. A worked example multiplies a 1,500-square-foot footprint by the 6:12 multiplier of 1.118 to get 1,677 square feet of roof, which divided by 100 square feet per roofing square is about 16.8 squares. A table lists the multiplier for common pitches: 1.054 at 4:12, 1.118 at 6:12, 1.202 at 8:12, and 1.414 at 12:12. The steeper the pitch, the more roof surface and the more shingles a given footprint needs.
Roof Pitch Calculator
Convert roof pitch between rise:run, degrees, percent grade and slope multiplier. Get sloped roof area, rafter length and code-allowed coverings. Free.
