Waste follows the layout
Straight layouts parallel to the long wall waste the least — 10% is a common baseline. Diagonal layouts cut every edge board at an angle and often need 15%. Herringbone and patterned work can run higher still. The layout is chosen before the estimate, so the waste allowance should be too.
Boxes round up, always
Order math happens in boxes using the coverage printed on the label: area × (1 + waste) ÷ coverage per box, rounded up. A 400 sq ft job at 10% waste with 20 sq ft boxes is exactly 22 boxes — and if the math had said 21.2, the answer would still be 22. One box short on the last row costs a crew day and risks a dye-lot mismatch.
Keep the attic stock honest
Some contractors buy an extra box as attic stock for future repairs and say so on the estimate. Customers understand a labeled extra box; they do not understand a mysteriously inflated waste number. Make the choice visible either way.