Wood's structural properties derive from fibers aligned along the tree's length, contributing significantly to its mechanical strength. Wood exhibits up to twenty times greater tensile strength along these fibers compared to across them, and generally shows better performance under compression than tension. The length of fibers varies, with hardwoods having fibers around one twenty-fifth inch long and softwoods ranging from one-eighth to one-third inch.

The strength characteristics of wood are evaluated through laboratory tests assessing its behavior under bending, shear, tension, and compression forces. This testing is crucial for understanding how wood will handle various stresses in practical applications.

Lumber, a primary form of processed wood, is categorized based on size into boards, dimension lumber, and timbers. Boards are defined by a thickness of less than two inches, including sizes like one by two, or one by six. Dimension lumber ranges from two to four inches in thickness and is typically sold in lengths from eight to twenty-four feet, including sizes such as two by four, or three by ten. Timbers, the largest category, start at five inches thick with examples like five by ten or six by six, used in more substantial structural applications. Nominal sizes in lumber refer to dimensions before finishing, while actual sizes are the true measurements after processing. For example, a 2 by 4 inches is actually 1.5 by 3.5 inches.

These categories of lumber are tailored for specific uses in construction, from general buildings to specific, load-bearing structures, ensuring versatility and reliability in a wide range of building projects.