Before stacking used lumber, what material must be removed?

The correct answer is nails. Before stacking used lumber, it is essential to remove nails to ensure safety and maintain the integrity of the wood. Nails can cause injury to individuals handling the lumber, leading to cuts or puncture wounds. Additionally, leaving nails in the wood can damage cutting equipment, such as saw blades, during further processing or use of the lumber. Removing nails also improves the usability of the wood for future projects, as it allows for easier handling and more efficient storage.

In this context, it's important to recognize that while splinters, rust, and debris might also be concerns when dealing with used lumber, they do not present the same immediate risk or functional problem as nails do. Splinters are typically a part of the wood’s surface and can be managed with care, rust primarily affects metal components rather than the lumber itself, and debris can often be cleared away without consequence. However, nails directly impact the safety and future usability of the stacked lumber, making their removal a priority.