TradesHow this works
Start with the roof footprint, the flat area it covers on the ground, and the pitch. The tool multiplies the footprint by the pitch factor to get the real sloped area, converts that to squares at 100 square feet each, and turns squares into shingle bundles at about three per square. If you have already measured the sloped area, switch the input and it skips the pitch step.
The real decision is how much to buy. Order exactly and one bad cut means a return trip and a possible dye-lot mismatch. Order too much and the leftovers are money on the ground. The tool recommends a waste allowance for the roof's complexity and shows a conservative option for a cut-up or steep roof.
Common questions
What is a roofing square?
A roofing square is 100 square feet of roof surface. Shingles, underlayment, and labor are all quoted in squares. Asphalt shingles run about three bundles to a square.
Why multiply the footprint by the pitch?
The footprint is the flat area the roof covers on the ground. The actual roof surface is larger because it slopes. The pitch factor, the square root of one plus the rise-over-run squared, converts the footprint to the real sloped area. A 6/12 roof is about 1.12 times the footprint; a 12/12 roof is about 1.41 times.
How much waste should I add?
It depends on how cut-up the roof is. A plain gable wastes little, around 10 percent. A roof with hips, valleys, and dormers wastes more from all the cut courses, 15 to 20 percent. The tool recommends by complexity and shows a conservative option for very cut-up roofs.
Does this include starter and ridge?
The waste allowance is meant to cover starter courses and ridge caps along with the cut waste for a typical roof. For a big or complex roof, price starter and ridge as their own line items to be safe.
Sources: Roofing geometry: 1 square = 100 sq ft, pitch factor = sqrt(1 + (rise/12)^2) · Asphalt shingles ~3 bundles per square; industry waste allowances by complexity. Engine version 1.0.0. Risk tier 1.