Lumber

Re-manufactured lumber is the result of secondary or tertiary processing/cutting of previously milled lumber. Specifically, it is lumber cut for industrial or wood-packaging use. Lumber is cut by ripsaw or resaw to create dimensions that are not usually processed by a primary sawmill.

Re-sawing is the splitting of 25 by 300 millimetres (1 by 12 in) hardwood or softwood lumber into two or more thinner pieces of full-length boards. For example, splitting a 3-metre (10 ft) long 50 by 100 mm (2 by 4 in) into two 25 by 100 mm (1 by 4 in) of the same length is considered re-sawing.

Structural lumber may also be produced from recycled plastic and new plastic stock. Its introduction has been strongly opposed by the forestry industry.[4] Blending fiberglass in plastic lumber enhances its strength, durability, and fire resistance.[5] Plastic fiberglass structural lumber can have a "class 1 flame spread rating of 25 or less, when tested in accordance with ASTM standard E 84," which means it burns more slowly than almost all treated wood lumber.[6]

The length of a unit of dimensional lumber is limited by the height and girth of the tree it is milled from. In general the maximum length is 24 ft (7.32 m). Engineered wood products, manufactured by binding the strands, particles, fibers, or veneers of wood, together with adhesives, to form composite materials, offer more flexibility and greater structural strength than typical wood building materials.[7]

Pre-cut studs save a framer much time, because they are pre-cut by the manufacturer for use in 8-, 9-, and 10-foot ceiling applications, which means the manufacturer has removed a few inches or centimetres of the piece to allow for the sill plate and the double top plate with no additional sizing necessary.

In the Americas, two-bys (2×4s, 2×6s, 2×8s, 2×10s, and 2×12s), named for traditional board thickness in inches, along with the 4×4 (89 mm × 89 mm), are common lumber sizes used in modern construction. They are the basic building blocks for such common structures as balloon-frame or platform-frame housing. Dimensional lumber made from softwood is typically used for construction, while hardwood boards are more commonly used for making cabinets or furniture.

Lumber's nominal dimensions are larger than the actual standard dimensions of finished lumber. Historically, the nominal dimensions were the size of the green (not dried), rough (unfinished) boards that eventually became smaller finished lumber through drying and planing (to smooth the wood). Today, the standards specify the final finished dimensions and the mill cuts the logs to whatever size it needs to achieve those final dimensions. Typically, that rough cut is smaller than the nominal dimensions because modern technology makes it possible to use the logs more efficiently. For example, a "2×4" board historically started out as a green, rough board actually 2 by 4 inches (51 mm × 102 mm). After drying and planing, it would be smaller by a nonstandard amount. Today, a "2×4" board starts out as something smaller than 2 inches by 4 inches and not specified by standards, and after drying and planing is reliably 1 ¹⁄₂ by 3 ¹⁄₂ inches (38 mm × 89 mm).[8]

As previously noted, less wood is needed to produce a given finished size than when standards called for the green lumber to be the full nominal dimension. However, even the dimensions for finished lumber of a given nominal size have changed over time. In 1910, a typical finished 1-inch (25 mm) board was ¹³⁄₁₆ in (21 mm). In 1928, that was reduced by 4%, and yet again by 4% in 1956. In 1961, at a meeting in Scottsdale, Arizona, the Committee on Grade Simplification and Standardization agreed to what is now the current U.S. standard: in part, the dressed size of a 1-inch (nominal) board was fixed at ​³⁄₄ inch; while the dressed size of 2 inch (nominal) lumber was reduced from ​1 ⁵⁄₈ inch to the current ​1 ¹⁄₂ inch.[9]

Dimensional lumber is available in green, unfinished state, and for that kind of lumber, the nominal dimensions are the actual dimensions.

The longest plank in the world (2002) is in Poland and measures 36.83 metres (about 120 ft 10 in) long.

Individual pieces of lumber exhibit a wide range in quality and appearance with respect to knots, slope of grain, shakes and other natural characteristics. Therefore, they vary considerably in strength, utility, and value.

The move to set national standards for lumber in the United States began with the publication of the American Lumber Standard in 1924, which set specifications for lumber dimensions, grade, and moisture content; it also developed inspection and accreditation programs. These standards have changed over the years to meet the changing needs of manufacturers and distributors, with the goal of keeping lumber competitive with other construction products. Current standards are set by the American Lumber Standard Committee, appointed by the U.S. Secretary of Commerce.[10]

Design values for most species and grades of visually graded structural products are determined in accordance with ASTM standards, which consider the effect of strength reducing characteristics, load duration, safety, and other influencing factors. The applicable standards are based on results of tests conducted in cooperation with the USDA Forest Products Laboratory. Design Values for Wood Construction, which is a supplement to the ANSI/AF&PA National Design Specification® for Wood Construction, provides these lumber design values, which are recognized by the model building codes.[11]

Canada has grading rules that maintain a standard among mills manufacturing similar woods to assure customers of uniform quality. Grades standardize the quality of lumber at different levels and are based on moisture content, size, and manufacture at the time of grading, shipping, and unloading by the buyer. The National Lumber Grades Authority (NLGA)[12] is responsible for writing, interpreting and maintaining Canadian lumber grading rules and standards. The Canadian Lumber Standards Accreditation Board (CLSAB)[13] monitors the quality of Canada's lumber grading and identification system.

Attempts to maintain lumber quality over time have been challenged by historical changes in the timber resources of the United States – from the slow-growing virgin forests common over a century ago to the fast-growing plantations now common in today's commercial forests. Resulting declines in lumber quality have been of concern to both the lumber industry and consumers and have caused increased use of alternative construction products.[14][15]

Machine stress-rated and machine-evaluated lumber are readily available for end-uses where high strength is critical, such as trusses, rafters, laminating stock, I-beams and web joints. Machine grading measures a characteristic such as stiffness or density that correlates with the structural properties of interest, such as bending strength. The result is a more precise understanding of the strength of each piece of lumber than is possible with visually graded lumber, which allows designers to use full-design strength and avoid overbuilding.[16]

In Europe, strength grading of rectangular sawn timber (both softwood and hardwood) is done according to EN-14081 [17] and commonly sorted into classes defined by EN-338. For softwoods, the common classes are (in increasing strength) C16, C18, C24, and C30. There are also classes specifically for hardwoods and those in most common use (in increasing strength) are D24, D30, D40, D50, D60, and D70. For these classes, the number refers to the required 5th percentile bending strength in newtons per square millimetre. There are other strength classes, including T-classes based on tension intended for use in glulam.

• C14, used for scaffolding and formwork
• C16 and C24, general construction
• C30, prefab roof trusses and where design requires somewhat stronger joists than C24 can offer. TR26 is also a common trussed rafter strength class in long standing use in the UK.[18]
• C40, usually seen in glulam

Grading rules for African and South American sawn timber have been developed by ATIBT[19] according to the rules of the Sciages Avivés Tropicaux Africains (SATA) and is based on clear cuttings – established by the percentage of the clear surface.[20]

In North America, market practices for dimensional lumber made from hardwoods[lower-alpha 1] varies significantly from the regularized standardized 'dimension lumber' sizes used for sales and specification of softwoods – hardwood boards are often sold totally rough cut,[lower-alpha 2] or machine planed only on the two (broader) face sides. When hardwood boards are also supplied with planed faces, it is usually both by random widths of a specified thickness (normally matching milling of softwood dimensional lumbers) and somewhat random lengths. But besides those older (traditional and normal) situations, in recent years some product lines have been widened to also market boards in standard stock sizes; these usually retail in big-box stores and using only a relatively small set of specified lengths;[lower-alpha 3] in all cases hardwoods are sold to the consumer by the board-foot (144 cubic inches or 2,360 cubic centimetres), whereas that measure is not used for softwoods at the retailer (to the cognizance of the buyer).[lower-alpha 4]

Also in North America, hardwood lumber is commonly sold in a "quarter" system, when referring to thickness; 4/4 (four quarter) refers to a 1-inch-thick (25 mm) board, 8/4 (eight quarter) is a 2-inch-thick (51 mm) board, etc. This "quarter" system is rarely used for softwood lumber; although softwood decking is sometimes sold as 5/4, even though it is actually one-inch thick (from milling ¹⁄₈ in or 3.2 mm off each side in a motorized planing step of production). The "quarter" system of reference is a traditional North American lumber industry nomenclature used specifically to indicate the thickness of rough sawn hardwood lumber.

In rough-sawn lumber it immediately clarifies that the lumber is not yet milled, avoiding confusion with milled dimension lumber which is measured as actual thickness after machining. Examples – ​³⁄₄-inch, 19 mm, or 1x. In recent years architects, designers, and builders have begun to use the "quarter" system in specifications as a vogue of insider knowledge, though the materials being specified are finished lumber, thus conflating the separate systems and causing confusion.

Hardwoods cut for furniture are cut in the fall and winter, after the sap has stopped running in the trees. If hardwoods are cut in the spring or summer the sap ruins the natural color of the timber and decreases the value of the timber for furniture.

Engineered lumber is lumber created by a manufacturer and designed for a certain structural purpose. The main categories of engineered lumber are:[21]

• Laminated veneer lumber (LVL) – LVL comes in ​1 ³⁄₄ inch thicknesses with depths such as ​9 ¹⁄₂, ​11 ⁷⁄₈, 14, 16, 18, and 24 inches, and are often doubled or tripled up. They function as beams to provide support over large spans, such as removed support walls and garage door openings, places where dimensional lumber is insufficient, and also in areas where a heavy load is bearing from a floor, wall or roof above on a somewhat short span where dimensional lumber is impractical. This type of lumber is compromised if it is altered by holes or notches anywhere within the span or at the ends, but nails can be driven into it wherever necessary to anchor the beam or to add hangers for I-joists or dimensional lumber joists that terminate at an LVL beam.
• Wooden I-joists – sometimes called "TJI", "Trus Joists" or "BCI", all of which are brands of wooden I-joists, they are used for floor joists on upper floors and also in first floor conventional foundation construction on piers as opposed to slab floor construction. They are engineered for long spans and are doubled up in places where a wall will be aligned over them, and sometimes tripled where heavy roof-loaded support walls are placed above them. They consist of a top and bottom chord or flange made from dimensional lumber with a webbing in-between made from oriented strand board (OSB) (or, latterly, steel mesh forms which allow passage of services without cutting). The webbing can be removed up to certain sizes or shapes according to the manufacturer's or engineer's specifications, but for small holes, wooden I-joists come with "knockouts", which are perforated, pre-cut areas where holes can be made easily, typically without engineering approval. When large holes are needed, they can typically be made in the webbing only and only in the center third of the span; the top and bottom chords lose their integrity if cut. Sizes and shapes of the hole, and typically the placing of a hole itself, must be approved by an engineer prior to the cutting of the hole and in many areas, a sheet showing the calculations made by the engineer must be provided to the building inspection authorities before the hole will be approved. Some I-joists are made with W-style webbing like a truss to eliminate cutting and to allow ductwork to pass through.
  

  Freshly cut logs showing sap running from beneath bark
• Finger-jointed lumber – solid dimensional lumber lengths typically are limited to lengths of 22 to 24 feet, but can be made longer by the technique of "finger-jointing" by using small solid pieces, usually 18 to 24 inches long, and joining them together using finger joints and glue to produce lengths that can be up to 36 feet long in 2×6 size. Finger-jointing also is predominant in precut wall studs. It is also an affordable alternative for non-structural hardwood that will be painted (staining would leave the finger-joints visible). Care is taken during construction to avoid nailing directly into a glued joint as stud breakage can occur.
• Glulam beams – created from 2×4 or 2×6 stock by gluing the faces together to create beams such as 4×12 or 6×16. As such, a beam acts as one larger piece of lumber – thus eliminating the need to harvest larger, older trees for the same size beam.
• Manufactured trusses – trusses are used in home construction as a pre-fabricated replacement for roof rafters and ceiling joists (stick-framing). It is seen as an easier installation and a better solution for supporting roofs than the use of dimensional lumber's struts and purlins as bracing. In the southern U.S. and elsewhere, stick-framing with dimensional lumber roof support is still predominant. The main drawbacks of trusses are reduced attic space, time required for engineering and ordering, and a cost higher than the dimensional lumber needed if the same project were conventionally framed. The advantages are significantly reduced labor costs (installation is faster than conventional framing), consistency, and overall schedule savings.

• Square and rectangular forms: Plank, slat, batten, board, lath, strapping (typically ​³⁄₄ in × ​1 ¹⁄₂ in), cant (A partially sawn log such as sawn on two sides or squared to a large size and later resawn into lumber. A flitch is a type of cant with wane on one or both sides). Various pieces are also known by their uses such as post, beam, (girt), stud, rafter, joist, sill plate, wall plate.
• Rod forms: pole, (dowel), stick (staff, baton)

In the United States, pilings are mainly cut from southern yellow pines and Douglas firs. Treated pilings are available in Chromated copper arsenate retentions of 0.60, 0.80 and 2.50 pounds per cubic foot (9.6, 12.8 and 40.0 kg/m³) if treatment is required.

Under the prescription of the Method of Construction (營造法式) issued by the Song dynasty government in the early 12th century, timbers were standardized to eight cross-sectional dimensions.[22] Regardless of the actual dimensions of the timber, the ratio between width and height was maintained at 1:1.5. Units are in Song Dynasty inches (31.2 mm).

Timber smaller than the 8th class were called "unclassed" (等外). The width of a timber is referred to as one "timber" (材), and the dimensions of other structural components were quoted in multiples of "timber"; thus, as the width of the actual timber varied, the dimensions of other components were easily calculated, without resorting to specific figures for each scale. The dimensions of timbers in similar applications show a gradual diminution from the Sui Dyansty (580–618) to the modern era; a 1st class timber during the Sui was reconstructed as 15×10 (Sui Dynasty inches, or 29.4 mm).[23]

During the process of converting timber to commercial form the following defects may occur:

• Chip mark: this defect is indicated by the marks or signs placed by chips on the finished surface of timber
• Diagonal grain: improper sawing of timber
• Torn grain: when a small dent is made on the finished surface due to falling of some tool
• Wane: presence of original rounded surface in the finished product

Fungi attack timber when these conditions are all present:

• The timber moisture content is above 25% on a dry-weight basis
• The environment is sufficiently warm
• Oxygen (O₂) is present

Wood with less than 25% moisture (dry weight basis) can remain free of decay for centuries. Similarly, wood submerged in water may not be attacked by fungi if the amount of oxygen is inadequate.

Fungi timber defects:

• Blue stain
• Brown rot
• Dry rot
• Heart rot
• Sap stain
• Wet rot
• White rot

Following are the insects and molluscs which are usually responsible for the decay of timber:

• Woodboring beetles
• Marine borers (Barnea similis)
• Teredos (Teredo navalis)
• Termites
• Carpenter ants
• Carpenter bees

There are two main natural forces responsible for causing defects in timber: abnormal growth and rupture of tissues. Rupture of tissue includes cracks or splits in the wood called "shakes". "Ring shake", "wind shake", or "ring failure" is when the wood grain separates around the growth rings either while standing or during felling. Shakes may reduce the strength of a timber and the appearance thus reduce lumber grade and may capture moisture, promoting decay. Eastern hemlock is known for having ring shake.[24] A "check" is a crack on the surface of the wood caused by the outside of a timber shrinking as it seasons. Checks may extend to the pith and follow the grain. Like shakes, checks can hold water promoting rot. A "split" goes all the way through a timber. Checks and splits occur more frequently at the ends of lumber because of the more rapid drying in these locations.[24]

The seasoning of lumber is typically either kiln- or air-dried. Defects due to seasoning are the main cause of splits, bowing and honeycombing.[25]

Wood is a hygroscopic material, which means it naturally absorbs and releases water to balance its internal moisture content with the surrounding environment. The moisture content of wood is measured by the weight of water as a percentage of the oven-dry weight of the wood fiber. The key to controlling decay is controlling moisture. Once decay fungi are established, the minimum moisture content for decay to propagate is 22 to 24 percent, so building experts recommend 19 percent as the maximum safe moisture content for untreated wood in service. Water by itself does not harm the wood, but rather, wood with consistently high moisture content enables fungal organisms to grow.

The primary objective when addressing moisture loads is to keep water from entering the building envelope in the first place and to balance the moisture content within the building itself. Moisture control by means of accepted design and construction details is a simple and practical method of protecting a wood-frame building against decay. For applications with a high risk of staying wet, designers specify durable materials such as naturally decay-resistant species or wood that has been treated with preservatives. Cladding, shingles, sill plates and exposed timbers or glulam beams are examples of potential applications for treated wood.

For buildings in termite zones, basic protection practices addressed in current building codes include (but are not limited to) the following:

• Grading the building site away from the foundation to provide proper drainage
• Covering exposed ground in any crawl spaces with 6-mil polyethylene film and maintaining at least 12 to 18 inches (300 to 460 mm) of clearance between the ground and the bottom of framing members above (12 inches to beams or girders, 18 inches to joists or plank flooring members)
• Supporting post columns by concrete piers so that there is at least 6 inches (150 mm) of clear space between the wood and exposed earth
• Installing wood framing and sheathing in exterior walls at least eight inches above exposed earth; locating siding at least six inches from the finished grade
• Where appropriate, ventilating crawl spaces according to local building codes
• Removing building material scraps from the job site before backfilling.
• If allowed by local regulation, treating the soil around the foundation with an approved termiticide to provide protection against subterranean termites

Special fasteners are used with treated lumber because of the corrosive chemicals used in its preservation process.

To avoid decay and termite infestation, untreated wood is separated from the ground and other sources of moisture. These separations are required by many building codes and are considered necessary to maintain wood elements in permanent structures at a safe moisture content for decay protection. When it is not possible to separate wood from the sources of moisture, designers often rely on preservative-treated wood.[26]

Wood can be treated with a preservative that improves service life under severe conditions without altering its basic characteristics. It can also be pressure-impregnated with fire-retardant chemicals that improve its performance in a fire.[27] One of the early treatments to "fireproof lumber", which retard fires, was developed in 1936 by the Protexol Corporation, in which lumber is heavily treated with salt.[28] Wood does not deteriorate simply because it gets wet. When wood breaks down, it is because an organism is eating it. Preservatives work by making the food source inedible to these organisms. Properly preservative-treated wood can have 5 to 10 times the service life of untreated wood. Preserved wood is used most often for railroad ties, utility poles, marine piles, decks, fences and other outdoor applications. Various treatment methods and types of chemicals are available, depending on the attributes required in the particular application and the level of protection needed.[29]

There are two basic methods of treating: with and without pressure. Non-pressure methods are the application of preservatives by brushing, spraying, or dipping the piece to be treated. Deeper, more thorough penetration is achieved by driving the preservative into the wood cells with pressure. Various combinations of pressure and vacuum are used to force adequate levels of chemical into the wood. Pressure-treating preservatives consist of chemicals carried in a solvent. Chromated copper arsenate, once the most commonly used wood preservative in North America began being phased out of most residential applications in 2004. Replacing it are amine copper quat and copper azole.

All wood preservatives used in the United States and Canada are registered and regularly re-examined for safety by the U.S. Environmental Protection Agency and Health Canada's Pest Management and Regulatory Agency, respectively.[29]

The conversion from coal to biomass power is a growing trend in the United States.[33]

The United Kingdom, Uzbekistan, Kazakhstan, Australia, Fiji, Madagascar, Mongolia, Russia, Denmark, Switzerland, and Swaziland governments all support an increased role for energy derived from biomass, which are organic materials available on a renewable basis and include residues and/or byproducts of the logging, sawmilling and papermaking processes. In particular, they view it as a way to lower greenhouse gas emissions by reducing the consumption of oil and gas while supporting the growth of forestry, agriculture and rural economies. Studies by the U.S. government have found the country's combined forest and agriculture land resources have the power to sustainably supply more than one-third of its current petroleum consumption.[34]

Biomass is already an important source of energy for the North American forest products industry. It is common for companies to have cogeneration facilities, also known as combined heat and power, which convert some of the biomass that results from wood and paper manufacturing to electrical and thermal energy in the form of steam. The electricity is used to, among other things, dry lumber and supply heat to the dryers used in paper-making.