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Wood as a Material

How wood's structure and moisture govern its behavior, and how to buy and choose wood stock.

  1. Wood Structure and Species
  2. Grain and Moisture Movement
  3. Buying Lumber and Board Feet
  4. Engineered Wood Products
Materials and Manufacturing Processes/Wood

Wood Structure and Species

Read tree structure, distinguish botanical groups, and choose species for their properties rather than their names.

Wood belongs to the Wood & Composites family

Wood belongs to the Wood & Composites family

Wood is a cellular material whose properties depend on direction. Long cells run mainly along the trunk; their arrangement helps explain why a board carries loads and splits differently along and across its grain.

In the material-family model, wood belongs to Wood & Composites. Engineered wood products combine veneers, strands, particles, or fibers with binders. These products are why composites share this family. See Engineered Wood Products for the product choices; the material families and manufacturing process categories are foundation topics.

Supporting detail: Addendum 1 — What makes the cell walls work together?.

Read a trunk from bark to heartwood

Read a trunk from bark to heartwood

Trunk cross-section: outer bark, inner bark, thin cambium, sapwood, heartwood, central pith, growth rings and radial rays.

Bark
Outer bark protects the tree. Inner bark carries sugars made by the leaves.
Cambium
A thin growing layer produces new wood toward the inside and new inner bark toward the outside.
Sapwood
The outer wood conducts water and contains living storage cells. Many of its conducting cells are already dead at maturity.
Heartwood
Older inner wood no longer conducts water. Deposits can change its color and decay resistance; heartwood is not automatically stronger or decay-resistant in every species.
Pith and rays
The pith marks the central stem tissue. Rays run radially across the wood and contribute to storage and transport across the trunk.

A growth ring records a growth cycle. In many temperate trees, earlywood develops earlier in the season and latewood later. Their cells differ in size and wall thickness, creating visible bands. Ring contrast varies by species; ring width alone is not a strength grade.

Hardwood and softwood describe the tree, not its hardness; Practice

Hardwood and softwood describe the tree, not its hardness

Hardwoods come from angiosperm trees; softwoods come from gymnosperm trees, commonly conifers. Leaf shedding is not the defining test: some hardwood trees are evergreen, and some conifers shed their needles.

Balsa is a very soft hardwood. Southern yellow pine is a softwood but can be much harder than balsa. Ask about the species and the property you need, rather than treating the category name as a test result.

Supporting detail: Addendum 2 — How the cell structure differs.

Practice: balsa Practice: yellow pine

Match common species to the job

Match common species to the job

These are typical starting points, not grades or guaranteed design values. Inspect the actual board for grain, knots, moisture, and defects.

Common North American woods and familiar uses
Species or groupTypical characterCommon uses
PineVaries widely: eastern white pine is relatively light and soft; southern yellow pines are generally denser.Trim, construction lumber, furniture parts.
Yellow-poplar (tulip-poplar)Relatively soft hardwood; easy to machine and often painted. Not the same tree as aspen.Painted trim, cabinet parts, concealed furniture parts.
Red oakHard, prominent open pores and grain.Flooring, furniture, cabinetry.
White oakHard, prominent rays; heartwood is generally more decay-resistant than red oak heartwood.Furniture, flooring, cooperage such as barrels.
Hard mapleHard, fine texture, good wear resistance.Flooring, work surfaces, furniture.
Black walnutDark heartwood and valued figure; sapwood is lighter.Furniture, cabinetry, decorative veneer.
Black cherryFine texture; heartwood darkens with exposure.Furniture, trim, veneer.
Yellow birchHard, fine texture; often used as veneer.Cabinetry, furniture, plywood faces.

Continue with Grain and Moisture Movement before selecting a board by appearance alone.

Further reading on wood properties and products

Further reading on wood properties and products

Supporting detail: Addendum 3 — Reference resources.

Materials and Manufacturing Processes/Wood

Grain and Moisture Movement

Connect grain orientation and moisture changes to board movement, defects, and design choices.

Follow the grain through the face, edge, and end

Follow the grain through the face, edge, and end

Grain describes the direction of wood cells. Figure is the visible pattern created by growth structure and the way the wood is cut. A board's broad surface is its face, its narrow long surface is its edge, and its cut end exposes end grain. Face and edge grain usually run generally along the board; they are not separate botanical structures.

Wood is generally stronger in tension along the grain than across it. A short projection with grain running across its narrow base can split off even if the rest of the part is thick. Knots divert nearby grain, and sloping or interlocked grain makes a board less predictable.

During machining, a cutting edge can lift fibers ahead of the cut and tear them out. Grain direction helps predict that result, but does not replace the machine's operating procedure. See Machine Hazards and Housekeeping and Equipment Operating Procedure for safety; wood separating and finishing are later topics.

Sawing orientation changes figure and movement

Sawing orientation changes figure and movement

Three selected boards in log cross-sections: plain-sawn tangent to rings, quarter-sawn radial, and rift-sawn oblique. Face sketches compare cathedral figure, straight grain with possible ray fleck, and straight grain.

Read the growth rings on the end of a board. Plain-sawn boards have rings more nearly parallel to the broad face and often show cathedral-shaped face figure. Quarter-sawn boards have rings more nearly perpendicular to the face; their width follows the radial direction more closely. Rift-sawn boards have an intermediate, oblique ring angle and commonly show straight face grain.

Quarter-sawn oak can expose conspicuous ray fleck. Rift-sawn oak is often selected for straight grain with less prominent fleck. Trade descriptions and angle boundaries vary, so inspect the end grain as well as the sales label. The diagram shows selected orientations, not a mill's complete cutting plan.

Moisture content uses oven-dry wood mass as its reference

Moisture content uses oven-dry wood mass as its reference

MC (%) = (wet mass − oven-dry mass) / oven-dry mass × 100. A sample weighing 150 g before drying and 100 g after oven drying has 50% MC. MC can exceed 100% because the reference is dry wood mass, not the original wet mass.

Water occupies cell cavities and is also held in cell walls. At the fiber saturation point, the walls are saturated while the cavities contain no free liquid water. Below this region, loss of water from the cell walls causes appreciable shrinkage. It is not one exact threshold for every species and condition.

Equilibrium moisture content (EMC) is the moisture content wood approaches in a particular surrounding humidity and temperature. A board can be dry today and absorb moisture later.

Green
Freshly cut or unseasoned; moisture varies greatly.
Air-dried
Dried by surrounding air; the achievable condition depends on the climate and storage.
Kiln-dried
Dried in a controlled chamber. This is a drying history, not a promise that the board will never move or that every kiln-dried product has the same MC.

Choose and acclimate material for its intended service environment. Use measurements to judge readiness; a fixed number of days cannot guarantee equilibrium for every thickness and starting condition.

Most movement is across the grain; Interactive

Most movement is across the grain

End-grain view labels tangential direction around a growth ring and radial direction toward the center; longitudinal direction is along the trunk, out of the view. Typical movement is greatest tangentially, then radially, and much smaller longitudinally.

For normal straight-grained wood, shrinkage is usually greatest tangentially (along growth rings), smaller radially (across rings), and much smaller longitudinally (along the cells). Swelling reverses the moisture-loss trend. Actual amounts depend on species, grain orientation, and the moisture interval.

Do not apply a green-to-oven-dry shrinkage percentage directly to a small seasonal humidity change. A movement estimate needs a species-specific coefficient, board orientation, starting MC, and expected service MC. Reaction wood and juvenile wood can also depart from ordinary longitudinal behavior.

Estimate movement using starting dimension × coefficient × (final MC − starting MC), with both MC values within 6–14%. The attachment experiment below separates total width change from travel at a particular point.

Wood movement and attachment clearance — both moisture contents must be 6–14%.

Change = starting dimension × coefficient × (final MC − starting MC), with MC entered as a percentage (8, not 0.08). Negative means shrinkage; positive means swelling. Use the tangential coefficient CT for the width of a flat-sawn board or when the ring orientation is unknown, and the radial coefficient CR for the width of a quarter-sawn board or the thickness of a flat-sawn one.

Worked example: a flat-sawn white fir board 9.15 in wide at 8% MC, CT = 0.00245, reaching 11% MC: 9.15 × 0.00245 × (11 − 8) = 0.0672525 in wider, about 9.2173 in final width. The exact conversion starts at 232.41 mm and ends at about 234.118 mm. The Handbook's separately rounded 232 mm example ends at about 233.705 mm.

Predict: with one ideal fixed reference on a stationary support, does center fixing make the right edge move half as far, the same distance, or not at all, compared with fixing the left edge?

Same total width change, different edge travel: 8% to 11% MC
Fixed referenceLeft edge travelRight edge travel
Center0.0336 in outward0.0336 in outward
Left edge0 (fixed)0.0673 in outward

For uniform free strain, signed point displacement = (signed distance from the fixed reference ÷ installed width) × total width change. Center fixing halves the edge travel, not the total width change. A point closer to the reference travels less.

Compare available travel in each direction from the installed position. With 0.040 in outward travel, center-fixed edges remain inside the travel window, while the opposite edge of the left-edge-fixed board exceeds it. Drying reverses the movement: 8% to 6% MC gives 0.0224 in inward travel at each center-fixed edge, exceeding a separate 0.020 in inward allowance. Enough travel in one direction does not supply travel in the other.

Each estimate starts from the installed dimension and MC. Edge points are geometric tracking points, not a fastening layout. The support is stationary, with one ideal fixed reference and otherwise free uniform width change; friction, warp, strength, restraint stress and cracking are not calculated. Exceeding a travel window does not predict what the restrained board will do. A result within the window does not approve a joint.

Commercial lumber is rarely perfectly flat- or quarter-sawn, so this method can overestimate width shrinkage and underestimate thickness shrinkage. Board properties and ring angles vary. These are approximate estimates; no correction or safety factor is added. Outside 6–14% MC a different method applies.

Moisture-movement source: USDA Wood Handbook (2021), chapter 13, Equation 13-3 and Table 13-5. The point-travel equation is derived geometry under the stated assumptions.

Name the distortion before deciding what to do; Practice

Name the distortion before deciding what to do

Defects described by shape
DefectWhat you see
CupCurvature across the width: the face resembles a shallow trough.
BowCurvature along the length, out of the broad face's plane.
CrookCurvature along the length within the face's plane; the edge wanders sideways.
TwistThe corners no longer lie in a common plane.
Check / splitA separation along the grain. A check may be confined to a surface or end; a split extends through the piece.

Unequal shrinkage, uneven moisture, and irregular grain can contribute to warp. Drying stresses can open checks and splits. A stain is a color change, not automatically proof of lost strength; decay is a different concern.

Practice: cup Practice: bow Practice: crook Practice: twist Practice: check

Allow solid wood to change width; Checklist; Practice

Allow solid wood to change width

A solid-wood tabletop needs room to expand and contract across its grain. Suitable slotted attachments let it move relative to a supporting frame. A panel held in grooves needs clearance rather than rigid restraint across its full width. Details depend on the joint and expected movement.

Choose the fixed reference deliberately, then check travel from each attachment's installed position in both moisture directions. Slot length alone does not give clear travel: the fastener occupies space, and its starting position divides the available movement. The experiment above uses clear travel directly rather than inferring a hardware size.

Do not assume that a finish locks moisture out. It can slow moisture exchange, but does not eliminate seasonal movement. Plywood often solves a panel-stability problem better than simply making a solid board thicker.

  • Identify the grain direction and width that can move.
  • Check moisture against the intended service environment.
  • Inspect for warp, splits, and grain interruptions before laying out parts.
  • Check that attachments allow the expected movement.

Wood conditioning and fabricating cover drying and joint-making in more detail; Engineered Wood Products compares alternatives to solid lumber.

Practice: center-fixed travel Practice: directional clearance

Further reading on wood properties and products

Further reading on wood properties and products

Supporting detail: Addendum 4 — Reference resources.

Materials and Manufacturing Processes/Wood

Buying Lumber and Board Feet

Separate sales size from measured size and calculate board footage using an explicit dimension convention.

Nominal size is a name; actual size is what fits; Practice

Nominal size is a name; actual size is what fits

A nominal lumber size identifies a product category. Dry, surfaced construction lumber sold as a 2 × 4 commonly measures 1-1/2 × 3-1/2 in in cross-section. Rough, green, specialty, and reclaimed stock can differ. Measure the piece when fit matters.

Drying and surfacing help explain the difference, but nominal names are not a universal subtraction rule. Do not assume that removing the same amount from every nominal size will give its actual dimensions.

Use Measurement Language for units and Dimensional Analysis for conversions. Keep length units explicit: an 8 ft board is 96 in long, not 8 in.

Practice: actual width

Quarter thickness and surfacing describe different things

Quarter thickness and surfacing describe different things

Hardwood thickness is often named in quarter-inch increments before final surfacing: 4/4 means nominal one-inch stock and 8/4 means nominal two-inch stock. Neither guarantees that much finished thickness. Drying, flattening, and planing reduce the usable material.

Rough
Sawn surfaces remain; allow material for flattening and sizing.
S2S
Surfaced on two sides, normally the two broad faces. Do not assume the edges are straight or ready for a joint.
S4S
Surfaced on all four long sides: both faces and both edges. Ends are not included in the four.

Ask for the supplied thickness, width, length, moisture condition, grade, and surfacing. A thickness label alone is not a complete purchase specification.

A board foot is volume, not length

A board foot is volume, not length

One board foot (BF) is the volume of a piece 1 in × 12 in × 12 in: 144 cubic inches. With thickness and width in inches and length in feet:

BF = thickness (in) × width (in) × length (ft) / 12.

If all three dimensions are in inches, divide by 144 instead. Multiply by the number of identical boards only after finding the volume of one.

Supporting detail: Addendum 5 — Why the mixed-unit formula divides by 12.

State the measuring convention. A physical-volume calculation uses actual dimensions. A sales tally may use nominal thickness or other trade rules and may round individual board measurements. Ask the seller; do not replace a quoted tally basis with finished dimensions without saying so. The examples here explicitly specify their basis and use unrounded arithmetic until the final answer.

Keep purchase tally and finished volume separate; Interactive; Practice

Keep purchase tally and finished volume separate

Suppose a seller specifies a tally using nominal 1 in thickness, measured 7 in width, and 9 ft length. The tally is 1 × 7 × 9 / 12 = 5.25 BF.

If that board finishes at 3/4 in thick with the same width and length, its physical volume is 0.75 × 7 × 9 / 12 = 3.9375 BF, or 3.94 BF to two decimals. These answers describe different quantities. The finished volume does not change the stated sales agreement.

A board tally is not a finished-parts yield. Allow for saw cuts, defects, flattening, trimming, and the layout of the parts. Appearance grades and structural grades answer different questions; an attractive board is not automatically suitable for a load-bearing member.

Build a list of boards below to compare the sales tally with actual volume, calculate cost, and estimate a material allowance.

Say which allowance you mean. This calculator treats the allowance as the share of the material you buy that will not end up in parts: BF to buy = needed BF ÷ (1 − allowance), so a 20% allowance means buying 1.25 times what you need. Many shop guides instead say “add 20%” to what you need (× 1.20), which is a different definition and buys less. Both are used; check which one a source or an instructor means before you compare numbers.

Practice: sales tally Practice: physical volume Practice: several boards Practice: all-inch dimensions Practice: material allowance

Further reading on wood properties and products

Further reading on wood properties and products

Supporting detail: Addendum 6 — Reference resources.

Materials and Manufacturing Processes/Wood

Engineered Wood Products

Compare veneers, strands, particles, fibers, and laminations to choose a wood product for its intended use.

Engineered wood changes the arrangement of the wood

Engineered wood changes the arrangement of the wood

Engineered products combine wood pieces with binders to make sheets or members that a single log could not supply as consistently. They belong to Wood & Composites: the wood supplies much of the structure, while the binder holds the selected arrangement together.

Engineering redistributes natural variability; it does not make all products equally strong, waterproof, or free of manufacturing defects. Select the product grade, thickness, orientation, exposure suitability, and fastening details for the job.

Plywood cross-laminates veneers to restrain movement

Plywood cross-laminates veneers to restrain movement

Exploded five-ply plywood: veneer grain alternates lengthwise and crosswise; outer layers run the same way and the layup is symmetric about its middle ply.

A veneer is a thin sheet of wood. In plywood, adjacent veneer layers commonly have grain at right angles. Each layer restrains the cross-grain movement of neighboring layers, making the panel more stable in its plane than a comparable wide solid-wood board.

Conventional plywood commonly has an odd number of plies in a balanced layup, with matching face-grain directions. Odd ply count is not a universal rule for every panel construction. The grain direction of the faces still matters to panel stiffness and strength; cross-lamination does not mean equal performance in every direction.

Plywood serves cabinet boxes, furniture panels, sheathing, and subfloors when the appropriate product is selected. Face appearance, core voids, bond durability, and panel rating vary. A decorative face veneer does not establish a structural rating.

Choose the sheet by its structure and service conditions; Practice

Choose the sheet by its structure and service conditions

Sheet goods: useful differences
ProductStructure and strengthsLimits and typical uses
OSB: oriented strand boardWood strands bonded in layers with deliberate orientation. Structural grades provide useful panel strength and stiffness.Common in wall, roof, and floor sheathing. Follow the marked strength direction and rating. Edges can swell when wetted; it is not a decorative substitute for every plywood face.
MDF: medium-density fiberboardFine wood fibers and resin produce a smooth surface without solid wood's visible grain pattern. Well suited to painted profiles.Common in interior cabinet doors and trim. Heavy; ordinary grades swell with moisture. Edge fastening needs suitable hardware and preparation. Moisture-resistant grades are not automatically waterproof.
ParticleboardSmall wood particles bonded into an economical, comparatively uniform panel; useful as a substrate under a decorative surface.Common in interior furniture and laminate countertops. Ordinary grades are moisture-sensitive, edges are vulnerable, and fastener holding and shelf deflection need attention.

Painting a moisture-sensitive board does not turn it into a product rated for prolonged wet exposure. For wood dust and product-handling hazards, use Personal Protective Equipment and Chemicals and Safety Data Sheets rather than inferring precautions from a product's name.

Practice: painted panel Practice: rated sheathing Practice: laminate substrate

Glulam and LVL put wood along the member

Glulam and LVL put wood along the member

Glulam bonds lumber laminations with their grain generally parallel to the member's length. It is used for structural beams and columns, including large members and curved forms.

LVL (laminated veneer lumber) bonds thin veneers with grain predominantly along the member. It is part of the structural composite lumber group and is commonly used for beams and headers over openings. It is not simply thick plywood: its layup favors lengthwise performance.

Use the manufacturer's design information for a particular structural product. Product names alone do not give allowable loads, spans, or permission to cut holes. Lamination and bending methods belong to the later wood forming and fabricating topics.

Further reading on wood properties and products

Further reading on wood properties and products

Supporting detail: Addendum 7 — Reference resources.

Supporting detail

Addendum

Additional evidence, standards context, and background referenced from the main text.

  1. Materials and Manufacturing Processes/Wood/Wood Structure and Species

    Addendum 1. What makes the cell walls work together?

    Cellulose reinforces the cell wall, while lignin and hemicelluloses help form its surrounding structure. This is a useful natural-composite analogy, but a tree is more complex than fibers suspended in glue. Cell arrangement, moisture, knots, and grain direction all affect a real board.

    Back to reference 1

  2. Materials and Manufacturing Processes/Wood/Wood Structure and Species

    Addendum 2. How the cell structure differs

    Most commercial hardwoods have vessels, visible as pores in end grain, along with supporting fibers. Softwoods rely mainly on long cells called tracheids for support and water transport. These differences influence appearance and processing, but do not make every member of either group behave alike.

    Back to reference 2

  5. Materials and Manufacturing Processes/Wood/Buying Lumber and Board Feet

    Addendum 5. Why the mixed-unit formula divides by 12

    Convert the length in feet to inches by multiplying by 12. Dividing the resulting cubic inches by 144 gives 12/144 = 1/12. The formula counts 144-cubic-inch units; it is not a linear-foot conversion.

    Back to reference 5