Comprehensive Study Guide to Wood and Timber Technology

Biological Structure and Botanical Composition

Wood is defined as the tough, fibrous cellular substance that constitutes the majority of the stems and branches of trees located beneath the bark. The bark itself is the tough external covering of a woody stem, branch, or root, which is composed of a living inner layer known as the phloem and an outer bark consisting of corky, dead tissue. The phloem, also called the inner bark, is a specialized layer of tissue responsible for carrying food from the leaves to the growing parts of the tree. Situated between the phloem and the xylem is the cambium, a thin layer of reproductive tissue that produces new phloem on the outside and new xylem on the inside of stems, branches, and roots. The xylem is the woody tissue of a tree that provides structural support and conducts water and mineral nutrients upward from the roots.

At the very center of a newly formed stem is the pith, which is the soft, central core about which first growth takes place. As a tree matures, it develops an older, harder, and inactive core known as the heartwood, or duramen, which is typically darker, denser, and more durable than the surrounding wood. The younger, softer, and living portion of wood located between the cambium and the heartwood is called the sapwood, or alburnum. While sapwood is comparable in strength to heartwood, it is generally lighter in color, more permeable, and less durable. Supporting the entire structure is the trunk, the main stem of the tree apart from its branches and roots, and the crown, which encompasses the leaves and living branches.

Cellular and Microscopic Elements

The fundamental building blocks of wood are lignin and cellulose. Lignin is an organic substance that, together with cellulose, forms the woody cell walls of plants and serves as the cementing material between them. Cellulose is an inert carbohydrate that acts as the chief constituent of the cell walls of plants and dried woods, as well as jute, hemp, and cotton; it is used extensively in the manufacture of various synthetic building materials. The structural integrity and functional processes of wood are managed by specialized cells. Tracheids are elongated, supporting, and conductive cells in woody tissue characterized by tapering closed ends and lignified walls oriented parallel to the axis of a stem or branch. Vessels are tubular structures formed by the fusion and loss of end walls in a series of connected cells, used for conducting water and mineral nutrients.

Vertical bands of transverse cells that radiate between the pith and the bark for the storage and horizontal conduction of nutrients are called rays. Fibers are slender, thick-walled cells that serve together to strengthen plant tissue. In hardwood trees specifically, pores are relatively large vertical cells used for conducting sap. Sap is the vital fluid consisting of water, nitrogen, and mineral nutrients that circulates through a plant. In many conifers, resin ducts (also called resin canals) are tubular, intercellular spaces that secrete resins. Resin is a viscous, clear to translucent organic substance exuded by certain pines, often used in making varnishes, adhesives, and plastics, while pitch refers specifically to the resinous sap that exudes from various conifers.

Classification of Wood Types

Softwood refers to wood obtained from a conifer, though the term does not necessarily describe the actual softness of the material. A conifer is any of various predominantly evergreen, cone-bearing trees such as pine, fir, hemlock, and spruce. Evergreen plants have foliage that remains green and functional throughout the year or through more than one growing season. Hardwood is wood obtained from a broad-leaved flowering tree, such as cherry, maple, or oak; like softwood, this term is not descriptive of the actual hardness of the wood. Most hardwoods and a few softwoods are deciduous, meaning they shed their leaves annually or at the end of a growing season.

Wood growth is marked by the production of annual rings, also called growth rings, which are concentric layers produced during a single year's growth in temperate trees. An annual ring consists of two distinct parts: springwood and summerwood. Springwood, or early wood, is the softer, more porous portion that develops early in the growing season and is characterized by large, thin-walled cells. Summerwood, or late wood, is the harder, darker, and less porous portion that develops late in the season, characterized by compact, thick-walled cells.

Properties of Moisture and the Seasoning Process

Lumber is the timber product manufactured by sawing, resawing, passing lengthwise through a planing machine, cross-cutting to length, and grading, while timber refers to wood suitable for use as a building material. A log is a length of trunk or large limb of a felled tree ready for sawing. The moisture content of wood is the amount of water contained in a piece, expressed as a percentage of the weight of the wood when oven-dry. Oven-dry wood is lumber dried until no moisture can be extracted, typically by exposure in a kiln to a temperature of $214^{\circ}$ to $221^{\circ}F$ ($101^{\circ}$ to $105^{\circ}C$).

The fiber-saturation point is the stage in drying or wetting where cell walls are fully saturated but cell cavities are void of water, ranging from $25\%$ to $32\%$ moisture content for common species; drying beyond this point results in shrinkage and generally higher strength, stiffness, and density. Equilibrium moisture content occurs when wood neither gains nor loses moisture in air at a given temperature and relative humidity. Seasoned lumber has been dried to improve its serviceability, either as kiln-dried (controlled heat, air, and humidity) or air-dried (exposed to the atmosphere). Surfaced green lumber has a moisture content exceeding $19\%$ at manufacture, while surfaced dry lumber has $19\%$ or less. Working refers to the alternate swelling and shrinkage of seasoned wood due to humidity changes. To acclimatize wood means to store it in an interior space until it adapts to the moisture and temperature of the new environment.

Dimensions and Measurement of Lumber

Shrinkage is the dimensional contraction of wood when moisture falls below the fiber-saturation point. This occurs minimally along the grain but significantly across it. Tangential shrinkage occurs tangent to growth rings and is about double the radial shrinkage. Radial shrinkage happens perpendicular to the grain, across the rings. Longitudinal shrinkage happens parallel to the grain and is approximately $2\%$ of radial shrinkage.

Lumber quantity is measured in board feet (board measure). A board foot is a unit equal to the volume of a piece with nominal dimensions of $12\,\text{in.}$ ($304.8\,\text{mm}$) square and $1\,\text{in.}$ ($25.4\,\text{mm}$) thick. The nominal dimension (or nominal size) is the size before drying and surfacing, used for convenience and written without inch marks. The dressed size (or dressed dimension) is the size after seasoning and surfacing, which is typically $3/8$ to $3/4\,\text{in.}$ ($9.5$ to $19.1\,\text{mm}$) less than the nominal dimension and is always written with inch marks (\").

Wood Grain Characteristics and Sawing Methods

Grain describes the direction, size, arrangement, and appearance of fibers in dressed wood. Edge grain (vertical grain) results from quartersawing, where annual rings form an angle of $45^{\circ}$ or more with the broad faces. To quartersaw means to saw quartered logs approximately at right angles to the annual rings. Flat grain results from plain-sawing (also called bastard-sawing), where annual rings form an angle of less than $45^{\circ}$ with the broad faces. Mixed grain refers to a combination of edge and flat grain. End grain results from a cut across the grain (a crosscut). Diagonal grain occurs when rings are at an angle to the piece length due to sawing at an angle to the log axis. Cross grain occurs when fibers run transversely or diagonally due to sawing or growth characteristics.

Close grain features narrow, inconspicuous annual rings with little difference between springwood and summerwood, while coarse grain has wide, conspicuous rings with high contrast. Coarse texture (open grain) has large pores, while fine texture has small, closely spaced pores. Raised grain is a surface where denser summerwood rises above softer springwood. Wood can be sawn in the direction of the grain (ripping) or across it (crosscutting).

Material Defects and Biological Degradation

Warp is any deviation from a true surface, usually caused by uneven drying. Specific types include cup (curvature across the width), bow (curvature along the length), crook (curvature along the edge), and twist (edges turning in opposite directions). Structural separations include shakes (separations along the grain between rings from tree stress), pitch pockets (openings in softwoods containing pitch), checks (lengthwise separations across rings from rapid seasoning), and splits (checks extending through the board, also called through checks).

Manufacturing defects include wane (bark or missing wood at an edge), skip (area missed by a planer), and machine burn (charring from blades or belts). Knots are the bases of branches enclosed by subsequent growth. Live knots (intergrown knots) have rings integrated with the surrounding wood and are allowable within limits. Sound knots are solid, hard, and undecayed. Tight knots are held firmly by position. Dead knots (encased or loose knots) have rings not intergrown and are considered defects as they can loosen. Decay is decomposition by fungi, causing softening and weight loss. Dry rot is a specific decay where fungi consume cellulose, leaving a brittle skeleton. Pecky wood has isolated spots of incipient decay, common in cypress or cedar.

Structural Lumber Classifications and Grading Systems

Lumber is classified by its intended use and size. Yard lumber is intended for general building and includes boards (less than $2\,\text{in.}$ thick and $2\,\text{in.}$ or more wide), dimension lumber ($2$ to $4\,\text{in.}$ thick and $2\,\text{in.}$ or more wide), and timbers ($5\,\text{in.}$ or more in the least dimension). Structural lumber (framing lumber) is graded based on strength and includes joists and planks ($2$ to $4\,\text{in.}$ thick, more than $4\,\text{in.}$ wide, graded for bending on the narrow or wide face), light framing ($2$ to $4\,\text{in.}$ thick and wide, for low strength needs), and decking ($2$ to $4\,\text{in.}$ thick, $4\,\text{in.}$ or more wide, graded for bending on the wide face).

Heavier structural members include beams and stringers (rectangular, at least $5\,\text{in.}$ thick, width more than $2\,\text{in.}$ greater than thickness, graded for bending on the narrow face) and posts and timbers (square or nearly square, $5\times5\,\text{in.}$ or larger, width not more than $2\,\text{in.}$ greater than thickness, graded for axial loads). Appearance lumber is selected for finish quality rather than strength. Factory lumber (shop lumber) is for further manufacture into cuttings. Matched lumber features tongue-and-groove edges, and patterned lumber is shaped to a molded form. Face width refers to the width of matched or shiplapped lumber excluding the tongue.

Certification and Environmental Standards

Certified wood comes from responsibly managed forests as verified by independent organizations. Key programs include the Forest Stewardship Council (FSC), the Sustainable Forestry Initiative (SFI), the American Tree Farm System, the Canadian Standards Association, and the Programme for the Endorsement of Forest Certification (PEFC). The Forest Stewardship Council (FSC) is a nongovernmental, nonprofit organization established in 1993 to promote environmentally appropriate, socially beneficial, and economically viable forest management, providing accreditation services for forest products in over 50 countries.

Structural Design Values and Adjustment Factors

Lumber is graded through visual grading (inspection of quality-reducing characteristics) or machine rating (machine stress-rating), where a machine flexes wood to measure bending resistance and computes the modulus of elasticity. A grademark is a stamp indicating the stress grade, mill, moisture content, species, and grading authority. Stress grade is a category for which base values and modulus of elasticity are established. Design values are allowable unit stresses modified by factors like size and use conditions.

Base values include allowable stresses for bending, compression (perpendicular and parallel to grain), tension (parallel to grain), and horizontal shear. These are modified by several factors: the size factor (coefficient based on cross-section), the repetitive member factor (increases values for three or more members spaced not more than $24\,\text{in.}$ or $610\,\text{mm}$ on center), the duration of load factor (increases values for short-term loads), the horizontal shear factor (adjusts for shakes, checks, or splits), the flat use factor (increases bending value for planks $4\,\text{in.}$ or wider), and the wet use factor (decreases values for wood used where moisture content will exceed $19\%$). Slope of grain refers to the angle of grain relative to the piece length.

Preservation and Chemical Treatments

Treated wood is coated or impregnated with chemicals for resistance to decay, insects, or fire. Pressure-treated wood utilizes pressure for deep penetration. The full-cell process first uses a vacuum to remove air so cell walls absorb preservative, followed by pressure to fill cell cavities, leaving a maximum amount of chemical in the wood. The empty-cell process uses pressure that entraps air; when released, air expands to expel excess chemical, yielding a drier product with deep penetration. Non-pressure treatments include coating, dipping, or the vacuum process (where atmospheric pressure forces preservative into a vacuum-exhausted cell).

Preservatives include water-borne preservatives like ammoniacal copper arsenite (ACA) and chromated copper arsenite (CCA), which affix to cell walls and are odorless and paintable; the copper acts as a fungicide and arsenate as an insecticide. Oil-borne preservatives include organic chemicals like pentachlorophenol (highly toxic to humans and plants, with a persistent odor) or copper naphthenate. Creosote is an oily liquid from coal tar distillation used for marine or severe exposures; it has a penetrating odor and makes wood unpaintable. Fire-retardant wood is treated with mineral salts that react below wood's ignition point to break down combustible vapors into water and carbon dioxide.

Wood Components and Structural Formats

Specialized wood components include ferrules (metal rings to prevent splitting at post ends), bolsters (horizontal timbers on posts to enlarge bearing areas), and brackets (used for eccentric loads below column ends). Columns include solid columns (single piece of solid-sawn or glulam timber), tapered columns (cross section diminishes along length), built-up columns (fastened or glued cover plates, never as strong as solid), box columns (hollow square or rectangular sections), and spaced columns (parallel members separated by blocking and joined by timber connectors).

Beam types include built-up beams (vertically laminated by fastening smaller members), box beams (hollow sections with plywood or OSB webs glued to sawn or LVL flanges), and I-beams (or I-joists, featuring sawn or LVL flanges and a single plywood or OSB web). A flitch beam (flitch girder or sandwich beam) is a vertically laminated beam consisting of timbers on edge bolted to steel flitchplates. Glued-laminated timber (glulam) is made by laminating stress-grade lumber with grain usually parallel, often end-joined with scarf or finger joints. Glulam appearance grades include premium, architectural, and industrial.

Engineered Wood Panels and Veneer Production

A veneer is a thin sheet of wood rotary cut, sliced, or sawn from a log or flitch (a log section to be cut into veneers). Cutting methods include rotary cutting (rotating against a knife for a variegated ripple figure), flat slicing (plain slicing for a wavy figure), quarter slicing (perpendicular to rings for stripes), and half-round slicing (off-center mounting for mixed characteristics). Rift cutting slices oak perpendicular to rays to minimize their appearance. Sheets are matched to emphasize color and figure (the natural pattern from rings, knots, burls, etc.). Matching types include book matching (mirror images), herringbone matching (book matching with opposing slopes), slip matching (repeated figure side-by-side), diamond matching (four sheets form a diamond), and random matching (casual, unmatched appearance).

Plywood is made by bonding veneers under heat and pressure, usually with grains at right angles and symmetrical about a center ply. Components include face plies, crossbands (at right angles to face plies), and the core (center of veneers, lumber, or composition board). Banding is solid wood around panel sides. Decorative plywood is used for cabinetry, while specialty panels like texture 1-11 feature specific grooves (e.g., $1/4\,\text{in.}$ deep, $3/8\,\text{in.}$ wide, spaced $4$ or $8\,\text{in.}$ on center). Performance-enhanced panels include high-density overlay (HDO, resin-fiber on both sides for concrete forms) and medium-density overlay (MDO, phenolic/melamine resin base for painting).

Grading and Durability of Wood Panels

Structural wood panels are identified by a gradestamp, such as from the American Plywood Association (APA), which lists panel grade, thickness, span rating, exposure durability, and mill number. Span rating indicates maximum support spacing in inches. Group numbers (1 to 5) classify species based on bending strength and stiffness (Group 1 is stiffest). Exposure durability classifications include Exterior (waterproof glueline for continuous exposure), Exposure 1 (exterior glueline for protected construction subject to wetting), and Exposure 2 (intermediate glueline for fully protected construction).

Panel grades include engineered grades (for sheathing or subflooring). Softwood veneer grades range from N-grade (smooth, heartwood/sapwood, few repairs), A-grade (smooth, paintable, limited repairs), B-grade (solid surface, circular plugs allowed), C-grade (tight knots, limited holes, sanding defects), C-plugged grade (improved C-grade with smaller knots), to D-grade (large knots, holes, pitch pockets). Hardwood veneer grades include premium grade (highest, few burls/knots), good grade (similar to premium, no face matching required), sound grade (smooth, free of open defects but discolored), utility grade (allows knots, holes, splits), and backing grade (allows larger defects not affecting strength).

Manufactured Wood Products and Proprietary Systems

Non-veneered panels include oriented strandboard (OSB), made by bonding long wood strands with the surface aligned parallel to the long axis; waferboard, made from thin wood flakes with random grain orientation; and particleboard (chipboard), made from small wood particles used as core material. Fiberboard is made of compressed plant fibers; hardboard is a dense version. Tempered hardboard is impregnated with drying oil and baked, such as Masonite or Peg-Board (which has perforations for hooks).

Structural composite lumber includes parallel strand lumber (PSL), marketed as Parallam, made by bonding long, narrow wood strands for use as beams and columns. Laminated veneer lumber (LVL), marketed as Microlam, is made by bonding veneer layers with grain running in the same longitudinal direction, providing high strength for beam or plank loading and often serving as flanges for wood I-joists. Composite panels consist of face veneers bonded to a reconstituted wood core.