CourseWiki/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

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

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

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 C_T for the width of a flat-sawn board or when the ring orientation is unknown, and the radial coefficient C_R 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, C_T = 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 reference	Left edge travel	Right edge travel
Center	0.0336 in outward	0.0336 in outward
Left edge	0 (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
Defect	What you see
Cup	Curvature across the width: the face resembles a shallow trough.
Bow	Curvature along the length, out of the broad face's plane.
Crook	Curvature along the length within the face's plane; the edge wanders sideways.
Twist	The corners no longer lie in a common plane.
Check / split	A 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

Reference resources

Appears in

DET 130 DET Wood as a Material