01
Make the allowance visible
If a measured field requires 4,000 pieces, a hypothetical 10 percent allowance adds 400 pieces. That example illustrates arithmetic, not a recommended allowance for every roof. Cutting patterns, roof geometry, delivery condition, and the intended stock of spares can change the appropriate order. Ask the estimator to explain what the extra pieces cover.
02
Avoid counting the same provision twice
An estimate may include separate lines for breakage, cutting, starters, and spare stock. Check whether the percentage already includes any of these. A clearly labeled schedule helps the supplier fulfill the order and makes later shortages easier to investigate. Keep ordered quantity, installed quantity, and retained spares as separate records at completion.