Waste Factor

The waste factor (or waste allowance) is the additional material an estimator adds on top of the measured roof area. No roof can be covered with exactly its surface area of material: pieces are cut at hips, valleys, rakes, dormers and penetrations, some offcuts are too small to reuse and a few units arrive damaged or break during installation.

How it is applied

Waste is usually expressed as a percentage of the measured roof area:

Material needed = roof area x (1 + waste factor)

For example, 20 squares (about 186 m²) of roof with a 10 percent waste factor calls for 22 squares of material. Starter strips, ridge and hip caps, and underlayment laps are often calculated separately rather than included in the general waste figure.

What affects waste

• Roof shape. A simple gable roof produces little waste; a hip roof, many valleys or numerous dormers produce more.
• Material. Large or brittle units (slate, tile) or patterned layouts generate more offcuts than flexible shingles. Metal panels cut to length can produce very little waste on simple roofs but more on complex hips.
• Exposure and pattern. Some products, such as wood shakes, vary in coverage with exposure.
• Installer practice. Planning cuts and reusing offcuts reduces waste.

Published guidance

Canadian guidance for wood shakes and shingles recommends adding 5 to 10 percent to total material requirements, plus extra for valleys. Shingle, tile and metal manufacturers, as well as estimating software, publish their own allowances for different roof shapes.

Over- and under-ordering

Too low a waste factor can stop work while more material is ordered, possibly from a different production batch with slight colour differences. Too high a factor wastes money and adds disposal costs. Many contractors keep a few spare units for future repairs, which can be useful when a product is later discontinued.