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Sheet S-130
PPDPDD CE

Woodconcept

Wood structure: grain, hardwood vs softwood, and shrinkage

One-line orientation

Wood is strongest along the grain and shrinks by different amounts in different directions. This directional behavior is called anisotropy.

Key points

  • Grain direction and strength: Cells grow linearly, so wood is stronger along (parallel to) the grain and weaker across it. Good in tension and compression; comparatively weak in shear.
  • Growth bands:
    • Springwood — fast growth, less dense, lower strength.
    • Summerwood — slow growth, denser, higher strength.
    • Annual rings — alternating springwood/summerwood bands; count indicates age.
  • Cross-section layers (bark → core): dead bark (protection) · cambium (living growth layer) · sapwood (moves water/nutrients) · heartwood (structural, rot-resistant) · pith (core, least stable).
  • Hardwood vs softwood:
    • Hardwood — deciduous trees; durable, intricate grain; flooring and furniture.
    • Softwood — evergreen/coniferous trees; cost-effective; the common choice for structural applications.
  • Shrinkage by direction: Tangential > radial > longitudinal (longitudinal nearly negligible). The tangential/radial mismatch causes cupping, warping, and checks (cracks).

Wood shrinkage: tangential > radial > longitudinal

Unequal movement across the rings distorts the section; length barely changes.

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Wood shrinkage directions and their magnitude ranking An end-grain log section identifies tangential movement along the growth rings and radial movement from pith to bark. On the same section, the outermost growth ring is drawn heavier as a boundary: the narrow band outside it, against the bark line, is labelled sapwood, and a longer leader reaching past that boundary into the wood inside it is labelled heartwood. A separate miniature side-view trunk carries a horizontal arrow along its long axis, clarifying that longitudinal means along the trunk or member length. Beside it, proportional bars rank tangential shrinkage greatest, radial at about half, and longitudinal as nearly negligible. Below the bars, a straight board cross-section changes into a visibly cupped and checked section. A ruled species-class zone under the log names softwood as coniferous structural lumber and hardwood as deciduous, for flooring and furniture. An amber note warns that longitudinal, radial, tangential is the reversed order. DIRECTIONSRELATIVE SHRINKAGESapwoodHeartwoodTangentialalong growth ringsLongitudinalside view · member lengthRadialpith to barkDISTORTIONCheckunequal T vs R opens cracksand cups boardsSPECIES CLASSSoftwoodconiferous · structural lumberHardwooddeciduous · flooring and furnitureTangentialgreatestRadialabout halfLongitudinal≈0 · negligibleUNEQUAL T VS R MOVEMENTbefore · straightafter · cup + checktangential movement dominates

Trap: longitudinal > radial > tangential is reversed.

Heartwood is the structural, rot-resistant inner wood; sapwood is the outer band that still moves water.

Confusions / comparison

TraitHardwoodSoftwood
Tree typeDeciduousEvergreen / coniferous
Typical useFlooring, furnitureStructural lumber, framing
CostHigherCost-effective
GrainIntricate patternsGenerally simpler
Shrinkage directionRelative amountEffect
Tangential (along rings)GreatestDrives cupping / checking
Radial (pith to bark)ModerateLess distortion than tangential
Longitudinal (along length)NegligibleLength stays essentially stable

→ Wood moisture content and seasoning (this topic): why shrinkage happens only below the fiber saturation point · Lumber sizing and defects (this topic): how shrinkage shows up as cup, bow, and check · Engineered wood products (this topic): how cross-lamination cancels directional movement.

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