Waste factor
Also: waste percentage · waste allowance
A waste factor is a percentage added to a net material quantity to cover cuts, breakage, and overage, so enough is ordered to finish the job. It varies by trade — drywall and flooring run around 10 percent, roofing shingles higher on cut-up roofs — and is applied before rounding to package sizes.
Waste is trade-specific and geometry-specific. A simple gable roof wastes fewer shingles than a hip roof with valleys; tile with a diagonal layout wastes more than a straight lay. Applying a flat factor everywhere either over-orders cheap materials or under-orders expensive ones.
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