Lumber is a generic term that applies to various lengths of wood used as construction materials. Pieces of lumber are cut lengthwise from the trunks of trees and are characterized by having generally rectangular or square cross sections, as opposed to poles or pilings, which have round cross sections.
The use of wood as a construction material predates written history. The earliest evidence of wood construction comes from a site near Nice, France, where a series of post holes seems to indicate that a hut 20 ft (6m) wide by 50 ft (15 m) long was built there 400,000 years ago using wood posts for support. The oldest wood construction found intact is located in northwest Germany, and was built about 7,300 years ago. By 500 B.C. iron axes, saws, and chisels were commonly used to cut and shape wood. The first reference to cutting wood in a sawmill, rather than using hand tools, comes from northern Europe and dates from about 375. The sawmill was powered by the flow of water.
In North America, European colonists found vast forests of trees, and wood became the principal building material. The circular saw, which had been developed in England, was introduced in the United States in 1814 and was widely used in sawmills. A large-scale bandsaw was developed and patented by Jacob R. Hoffman in 1869 and replaced the circular saw for many sawmill operations.
Lumber produced in early sawmills had varying dimensions depending on the customer's specific order or the mill's standard practice. Today, lumber pieces used in construction have standard dimensions and are divided into three categories, depending on the thickness of the piece. Lumber with nominal thicknesses of less than 2 in (5 cm) are classified as boards. Those with nominal thicknesses of 2 in (5 cm) but less than 5 in (13 cm) are classified as dimension. Those with nominal thicknesses of 5 in (12.5 cm) and greater are classified as timbers. The nominal widths of these pieces vary from 2-16 in (5-40 cm) in 1 in (2.5 cm) increments. Most rough-cut lumber pieces are dried and then finished, or surfaced, by running them through a planer to smooth all four sides. As a result, the actual dimensions are smaller than the nominal dimensions. For example, a standard two-by-four piece of dried, surfaced dimension lumber actually measures 1.5 in (3.8 cm) by 3.5 in (8.9 cm).
Pieces of lumber that are not only surfaced, but also machined to produce a specific cross sectional shape are classified as worked lumber or pattern lumber. Decorative molding, tongue-and-groove flooring, and shiplap siding are examples of pattern lumber.
Today, processing wood products is a billion-dollar, worldwide industry. It not only produces construction lumber, but also plywood, fiberboard, paper, cardboard, turpentine, rosin, textiles, and a wide variety of industrial chemicals.
The trees from which lumber is produced are classified as hardwoods or softwoods. Although the woods of many hardwoods are hard, and the woods of many softwoods are soft, that is not the defining characteristic. Most hardwood trees have leaves,
Hardwoods are generally more expensive than softwoods and are used for flooring, cabinetry, paneling, doors, and trimwork. They are also extensively used to manufacture furniture. Hardwoods are available in lengths from 4-16 ft (1.2-4.8 m). Softwoods are used for wall studs, joists, planks, rafters, beams, stringers, posts, decking, sheathing, subflooring, and concrete forms. They are available in lengths from 4-24 ft (1.2-7.3 m).
Both hardwood and softwood lumber pieces are graded according to the number and size of defects in the wood. Defects include knots, holes, pitch pockets, splits, and missing pieces on the edges or corners, called wanes. These defects primarily affect the appearance, but may also affect the strength of the piece. The higher grades are called select grades. Hardwoods may also be graded as firsts or seconds, which are even higher than select. These grades have very few defects and are used for trim, molding, and finish woodwork where appearance is important. The higher the grade, the fewer the number of defects. The lower grades are called common grades and are used for general construction where the wood will be covered or where defects will not be objectionable. Common grades are designated in descending order of quality by a number such as #1 common, #2 common, and so on. Pieces of softwood common grade lumber may also be designated by an equivalent name, such as select merchantable, construction, and so on. Lumber intended for uses other than construction, such as boxes or ladders, are given other grading designations.
In the United States, most trees destined to be cut into lumber are grown in managed forests either owned by the lumber company or leased from the government. After the trees have reached an appropriate size, they are cut down and transported to a lumber mill where they are cut into various sizes of lumber.
Here is a typical sequence of operations for processing trees into lumber.
There are very few pieces of perfect lumber. Even though great care is taken to avoid or minimize defects when sawing the wood to the required sizes, there are almost always some defects present. The number and location of these defects determines the grade of the lumber, and the purchaser must choose the grade that is appropriate for each specific application.
As the number of older trees available for logging diminishes, so does the lumber industry's ability to selectively cut pieces of lumber to the sizes needed for construction. Many of the trees being logged today are second-generation or third-generation trees that are younger and smaller in diameter than the original old-growth trees. These younger trees also contain a higher percentage of juvenile wood, which is less dimensionally stable than older wood.
To counter this trend, the lumber industry is literally taking trees apart and putting them back together again to manufacture the sizes, strengths, and stability required for construction. Actually, they have been doing this for decades in the form of plywood and glue-laminated beams, and some of the new products use similar technology.
One of the new manufactured lumber products is called parallel strand lumber. It begins much like plywood with a thin veneer of wood being peeled off a log. The veneer passes under a fiber-optic scanner that spots defects and cuts them out, sort of like an automated cookie cutter. The veneer is then dried and cut into 0.5 in (1.3 cm) wide strips. The strips are fed into one end of a machine, which coats them with a phenolic resin glue and stacks them side-to-side and end-to-end to form a solid 12 in by 17 in (30 cm by 43 cm) beam of wood. The beam is zapped with 400,000 watts of microwave energy, which hardens the glue almost instantly. As the beam emerges from the other end of the machine, it is cut into 60 ft (18.3 m) lengths. It is then further cut into various sizes of lumber, and sanded smooth. The resulting pieces are significantly stronger and more dimensionally stable than natural wood, while being attractive enough to be used for exposed beams and other visible applications.
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— Chris Cavette