Final answer:
Heavy, bulky, or unstable materials such as lumber, steel beams, and concrete blocks require racking or blocking for safe storage.
Step-by-step explanation:
Materials that need to be racked or blocked to prevent spreading or tilting are typically those that are heavy, bulky, or otherwise unstable when stored. This often includes building materials like lumber, steel beams, concrete blocks, and pipes, which can be prone to shifting or toppling if not secured. Proper storage solutions, including racks, blocks, stands, or braces, are crucial for maintaining safety and organization in environments such as warehouses, construction sites, and manufacturing facilities.