What waste factor should you use for roof pitch and shingles?
Short answer. Pitch and waste are separate adjustments: the pitch factor converts footprint to true roof area, and the waste factor adds extra material for cuts. Common rules of thumb are about 10 percent waste on a simple gable roof, about 15 percent on a hip roof, and more on cut up roofs with many valleys and dormers.
Keep the two numbers apart
First get true roof area: footprint × pitch factor (for example 1.118 at 6/12). Then add waste on top of that. Folding waste into the pitch factor, or skipping one of them, is how homemade estimates end up a few squares short.
What actually creates waste
- Hips: every course ends in an angled cut along each hip, and the offcut is rarely usable.
- Valleys: shingles are cut along the valley line on one or both sides.
- Dormers, chimneys and skylights: short courses and cut pieces around each one.
- Small planes: porch roofs and bump outs use partial bundles.
- Steepness, indirectly: steep roofs tend to be the cut up ones, and handling on a steep slope damages more pieces.
Items not covered by the waste factor
Starter strip, ridge cap, drip edge, leak barrier and underlayment are measured by linear feet or rolls. Many measurement reports list several waste percentages and leave the choice to the estimator.
Waste on insurance estimates
Insurance estimates generally include a waste percentage on the shingle line. If the waste allowed looks low for a hip roof with several valleys, your roofer can point that out with measurements; read about Xactimate roof estimates. For the measuring side, see how roofers measure a roof.