How Much Waste Should I Add?
Sensible buffers by material, and why 10% is not a universal answer.
Waste depends on cuts, not on price
Materials cut to fit generate offcuts. The more cuts and the more complex the layout, the higher the buffer needs to be.
Typical buffers
Paint 5–10%. Flooring 10% straight lay, 15% diagonal, 20% herringbone. Tile 10–15%. Concrete 5–10% for uneven excavation. Decking 10%. Roofing 10% simple, 15% with hips and valleys.
Buy a little extra deliberately: a sealed spare box from the same batch is the cheapest insurance against a future repair.