Last reviewed 16 Sept 2026 · 13 min read
Classification of trees
| Class | Growth | Examples |
|---|---|---|
| Exogenous trees | Grow outward by adding annual rings under the bark — yield useful timber | Softwoods and hardwoods |
| Endogenous trees | Grow inward; fibrous mass, no annual rings | Bamboo, cane, palm — limited use as timber (bamboo widely used for scaffolding, structures) |
Softwoods and hardwoods
| Softwoods | Hardwoods |
|---|---|
| From conifers — needle-like leaves, mostly evergreen | From broad-leaved (deciduous) trees |
| Distinct annual rings | Annual rings less distinct |
| Generally light in colour and weight, resinous | Generally darker and heavier, non-resinous |
| Weaker, easier to work | Stronger, harder, more durable (with exceptions) |
| Examples: deodar, chir (pine), fir, spruce, kail | Examples: teak, sal, shisham (rosewood), oak, mahogany, babul, mango |
Structure of a tree (cross-section)
| Part | Description |
|---|---|
| Pith (medulla) | Innermost central core — dies and decays in old trees |
| Heartwood | Inner, dead portion — darker, harder, stronger and more durable; gives useful timber |
| Sapwood | Outer, living portion near the bark — lighter colour, carries sap, less durable, more prone to decay and insects |
| Cambium layer | Thin layer between sapwood and inner bark — where new wood cells form |
| Inner bark (bast) and outer bark | Protective coverings |
| Annual rings | Concentric rings of spring (light) and autumn (dark) wood — one ring per year indicates age |
| Medullary rays | Thin radial lines from pith towards bark — hold annual rings together, carry sap |
Felling of trees
- Trees are felled when mature (maturity varies from a few decades to over a century depending on species); immature timber has more sapwood, and over-mature timber may have decay.
- Felling in a season when sap is at its minimum is preferred (to reduce decay and speed seasoning) — commonly mid-winter in plains and mid-summer in hilly regions.
Qualities of good timber
Sound (clear ringing sound when struck); uniform colour and straight, close grain; free from defects (knots, shakes, decay, insect attack); adequate strength, hardness, toughness and elasticity; durability against fungi and insects; well seasoned (appropriate moisture content, no warping); heavy for its species (denser timber is stronger); fibrous and not brittle; sweet smell (fresh cut); good workability and ability to hold nails and screws; fire resistance for large sections.
Seasoning of timber
Seasoning is the controlled drying of timber to reduce its moisture content to a level in equilibrium with the atmosphere where it will be used.
Objectives
- Reduces shrinkage, warping and cracking in service.
- Increases strength, hardness, stiffness and durability (more resistance to decay).
- Reduces weight (economical transport).
- Makes timber easier to work, paint, glue and treat with preservatives.
- Improves resistance to fungi and insects.
Freshly felled (green) timber may contain moisture exceeding its dry weight; well-seasoned timber commonly has about 10–15% moisture content (lower for furniture in dry interiors, somewhat higher for outdoor/structural use).
FormulaMoisture content of timber
( = oven-dry weight.) The fibre saturation point (about 25–30%) is the moisture content at which cell cavities are empty but cell walls saturated — shrinkage and strength changes occur only below it.
Methods
| Method | Description | Remarks |
|---|---|---|
| Natural (air) seasoning | Stacks of timber with spacers (sticks) under a shed, allowing air circulation | Cheap, slow (months to years), no control, possible defects |
| Water seasoning | Logs immersed in running water (removes sap), then air dried | Reduces sap; timber may lose some strength and elasticity |
| Kiln seasoning | Drying in kilns with controlled temperature, humidity and air circulation — compartment kilns and progressive kilns | Quick, well-controlled, uniform — most common commercially; costly |
| Boiling | Boiling/steaming timber — kills fungi and insects, removes sap | Quick; reduces strength and elasticity; costly for large quantities |
| Chemical (salt) seasoning | Timber treated with hygroscopic salts (e.g. urea) that slow surface drying relative to the interior | Reduces surface checking |
| Electrical seasoning | High-frequency current heats the timber | Rapid; expensive |
| Solar kilns | Use solar heat | Low-cost controlled drying |
Conversion (sawing) of timber
Conversion is sawing logs into marketable sizes (planks, battens, scantlings).
| Method | Features |
|---|---|
| Ordinary (flat/through-and-through) sawing | Parallel cuts through the log — simple, economical, least waste; boards prone to warping |
| Quarter sawing | Log quartered, then sawn roughly radially — attractive figure, less warping and shrinkage, more waste |
| Tangential sawing | Cuts tangential to annual rings — shows grain figure; shrinks more in width |
| Radial sawing | Cuts along medullary rays — best quality boards, least warping; most wasteful and costly |
Defects in timber
Defects due to natural forces
| Defect | Description |
|---|---|
| Knots | Bases of branches embedded in the trunk — live (sound, tight) knots, dead/loose knots (fall out); reduce strength (especially in tension zones) |
| Shakes | Cracks separating fibres — heart shakes (from the pith, due to shrinkage of heartwood), cup/ring shakes (along annual rings, due to frost or uneven growth), star shakes (from bark inward, due to extreme heat or frost), radial shakes |
| Twisted fibres (wandering heart) | Fibres twisted spirally due to wind action — difficult to work |
| Upsets (ruptures) | Crushed fibres due to felling or strong wind |
| Rind galls | Curved swellings where branches were improperly cut |
| Burls, callus, foxiness, dote, wind cracks | Other growth irregularities and early decay signs |
Defects due to fungi
- Dry rot — fungi attack timber in damp, poorly ventilated, warm conditions (e.g. floor boards on damp walls); timber becomes dry, brittle and powdery; can spread through masonry.
- Wet rot — decay under alternate wetting and drying (e.g. posts at ground level); timber becomes soft and spongy.
- Brown rot, white rot, heart rot, sap stain, blue stain.
Defects due to insects
Termites (white ants) — hollow out timber internally; beetles (powder-post beetles) — reduce sapwood to powder; marine borers (shipworms) — attack timber in sea water.
Defects due to seasoning
| Defect | Description |
|---|---|
| Bow | Curvature along the length in the direction of thickness |
| Cup | Curvature across the width |
| Twist (wind) | Spiral distortion along the length |
| Spring (crook) | Curvature along the length in the plane of the wide face |
| Checks and splits | Surface cracks (checks) and cracks through thickness at ends (splits) due to rapid drying |
| Honeycombing | Internal cracks due to case hardening |
| Case hardening | Outer layers dry and set in a stretched condition while the core is wet — internal stresses |
| Warp | General distortion |
Defects due to conversion
Chip mark, diagonal grain (improper sawing), torn grain (tool action), wane (original rounded surface/bark on the edge of a sawn piece).
Preservation of timber
Objectives: increase durability and life; protect from fungi, insects and marine borers; reduce fire hazard (with fire retardants).
Preservatives
| Type | Examples | Remarks |
|---|---|---|
| Oil-type | Creosote (coal tar distillate), coal tar | Very effective, especially for outdoor timber (poles, sleepers, piles); dark colour, odour, not paintable |
| Water-borne (salt) preservatives | Copper–chromium–arsenic (CCA / ASCU), copper–chromium–boron (CCB), zinc chloride, sodium fluoride, boric acid–borax | Clean, odourless, paintable; fixed salts resist leaching (CCA); arsenic-containing preservatives require careful handling |
| Solvent-type (organic solvent) | Pentachlorophenol (restricted), copper naphthenate, synthetic insecticides in solvents | Good penetration, quick drying |
Methods of application
- Brushing and spraying — surface treatment; limited penetration; maintenance treatments.
- Dipping and steeping — immersion for minutes to days.
- Charring — surface burnt to form a protective charcoal layer (e.g. ends of posts in the ground).
- Hot and cold open tank process — timber heated in preservative, then cooled so that preservative is drawn in.
- Pressure impregnation — in closed cylinders:
- Full-cell (Bethell) process — vacuum then pressure — maximum retention (marine piles).
- Empty-cell processes (Rueping and Lowry) — deep penetration with less retention (economical).
- Boucherie process — sap displacement in green poles.
Fire resistance
Timber is combustible, but large sections perform relatively well in fire: the surface chars, and the char layer insulates the inner wood, which retains strength. Fire resistance is improved by fire-retardant chemicals (e.g. ammonium phosphate, ammonium sulphate, borax, boric acid), fire-retardant paints, larger sections, and protection with plaster or boards.
Properties of timber
- Anisotropic — properties differ parallel and perpendicular to the grain: compressive and tensile strengths parallel to grain are much higher than perpendicular to grain.
- Strength decreases with increasing moisture (below fibre saturation point) and with defects (knots, sloping grain).
- Density — denser timber is generally stronger and harder.
- Good strength-to-weight ratio, low thermal conductivity (good insulator), good sound absorption, easy to work.
- Shrinkage: tangential shrinkage > radial shrinkage > longitudinal shrinkage (negligible).
Common Indian timbers
| Timber | Type | Features / uses |
|---|---|---|
| Teak | Hardwood | Highly durable, termite resistant, stable, easy to work, takes good polish — doors, windows, furniture, ship building, panelling |
| Sal | Hardwood | Hard, strong, heavy, durable — railway sleepers, beams, trusses, piles |
| Shisham (Indian rosewood) | Hardwood | Strong, durable, attractive — furniture, carving, cabinet work |
| Deodar | Softwood | Durable, aromatic, easy to work — doors, windows, beams in hill buildings, sleepers |
| Chir (pine) | Softwood | Light, less durable — packing boxes, cheap furniture, temporary work |
| Babul | Hardwood | Hard, tough — tool handles, cart wheels, agricultural implements |
| Mango | Hardwood | Cheap, not durable — packing cases, cheap furniture |
| Jackfruit, haldu, mahogany, oak, walnut | Hardwood | Furniture, decorative work |
| Bamboo | Endogenous | High tensile strength — scaffolding, low-cost structures, reinforcement in some rural works, mats and boards |
Wood-based products
| Product | Description / use |
|---|---|
| Veneers | Thin sheets of wood (commonly about 0.4–6 mm) obtained by rotary cutting, slicing or sawing — for plywood and decorative facing |
| Plywood | Odd number of veneers glued with grains of adjacent layers at right angles — strong in both directions, resists splitting and warping; types: commercial (interior), BWR/BWP (boiling water resistant/proof), marine plywood — panelling, furniture, formwork (shuttering plywood) |
| Blockboard | Core of wooden strips (blocks) glued edge to edge between veneers — doors, shelves, tabletops |
| Laminboard | Core of thin strips (laminae) — more stable than blockboard |
| Particle board (chipboard) | Wood chips/particles bonded with resin under heat and pressure — cheap furniture, partitions (low moisture resistance) |
| Fibreboards | Wood fibres bonded — hardboard (high density), insulation board (low density), medium density fibreboard (MDF) — smooth, machinable — furniture, panelling |
| Glued laminated timber (glulam) | Several layers of dimensioned timber glued together with grains parallel — large beams, arches, portal frames |
| Laminated veneer lumber (LVL) | Veneers glued with grains parallel — structural beams, headers |
| Cross-laminated timber (CLT) | Layers of lumber glued with alternating grain directions — structural wall and floor panels for mass timber buildings |
| Impreg timber | Wood impregnated with resin (e.g. phenol formaldehyde) and cured — improved dimensional stability and durability |
| Compreg timber | Impreg timber compressed under heat and pressure — very dense, strong — tool handles, bearings |
| Flush doors | Core (blockboard/particle board) faced with plywood or veneers |
| Bamboo mat board, bamboo composites | Eco-friendly alternatives |
Measurement of timber
FormulaVolume of timber
Sawn timber:
Round logs — quarter girth formula (commonly used in trade):
( = mean girth; = length.) It underestimates the true volume of a circular log (allowing for waste in squaring).
Worked examples
Worked ExampleExample 1 — moisture content
A timber sample weighs 5.00 kg when green and 4.20 kg when oven-dry. Find its moisture content.
Solution.
Worked ExampleExample 2 — volume of a log
Find the volume of a log 5 m long with a mean girth of 2.0 m by the quarter girth formula, and compare with the true cylindrical volume.
Solution. Quarter girth: True volume: radius m → (quarter girth gives about 78.5%)
Worked ExampleExample 3 — plywood layers
Why is plywood made with an odd number of veneers with adjacent grains at right angles?
Solution. Cross-graining equalises strength and shrinkage in both directions, preventing splitting and warping; an odd number keeps the construction symmetrical about the central ply so that stresses balance and the board stays flat.
Frequently tested points
- Exogenous (softwoods, hardwoods) vs endogenous (bamboo, palm).
- Softwoods: conifers, distinct rings, resinous, light (deodar, pine); hardwoods: broad-leaved, strong (teak, sal, shisham).
- Heartwood — dead, durable; sapwood — living, less durable; cambium forms new wood; medullary rays radial.
- Seasoning reduces moisture (well-seasoned about 10–15%); fibre saturation point ~25–30%.
- Kiln seasoning — quick and controlled (compartment and progressive kilns).
- Quarter and radial sawing reduce warping; flat sawing is economical.
- Shakes: heart, cup (ring), star, radial; knots reduce strength.
- Dry rot — damp unventilated conditions; wet rot — alternate wetting and drying.
- Seasoning defects: bow, cup, twist, checks, splits, honeycombing, case hardening.
- Preservatives: creosote (oil type), CCA/ASCU (water-borne); pressure impregnation — Bethell (full-cell), Rueping and Lowry (empty-cell).
- Large timber sections char and retain strength in fire.
- Strength much higher parallel to grain; tangential shrinkage > radial > longitudinal.
- Plywood: odd number of veneers, cross-grained; glulam, LVL (parallel grains), CLT (cross layers); compreg = compressed impreg.
- Quarter girth formula .
Common MistakeCommon mistakes
- Calling all softwoods physically soft and all hardwoods hard (the terms are botanical).
- Confusing dry rot (occurs in damp, unventilated places) with decay caused by dryness.
- Assuming timber strength is the same in all directions.
Revision SummaryChapter summary
- Timber comes mainly from exogenous trees, classified as softwoods and hardwoods; heartwood is the useful durable part.
- Good timber is sound, straight-grained, defect-free, dense and well seasoned by natural or artificial methods.
- Conversion methods affect appearance and stability; natural, fungal, insect, seasoning and conversion defects reduce quality.
- Preservatives (creosote, CCA) applied by surface or pressure methods protect timber from decay and insects; large sections resist fire by charring.
- Indian timbers such as teak, sal, shisham and deodar have characteristic uses, and engineered wood products (plywood, MDF, glulam, LVL, CLT) extend timber applications.