Timber – Complete Chapter-wise Study Notes

Timber (Wood) is an important topic in Building Materials for GATE ESE SSC JE State PSC RRB JE. Questions focus on structure of wood, classification of trees, seasoning methods, defects, preservation techniques, and mechanical properties with numerical values.

These notes cover all 10 chapters thoroughly — from the microscopic structure of wood cells to IS code provisions, standard timber tests, and special products like plywood and particle board. Every table contains exact IS code values for direct exam use.

1. Structure of Wood 2. Classification of Trees 3. Growth & Felling 4. Conversion of Timber 5. Seasoning 6. Defects in Timber 7. Preservation 8. Mechanical Properties 9. Special Wood Products 10. IS Codes & Quick Revision

1Structure of Wood ▶

Macroscopic Structure

A cross-section of a tree trunk reveals concentric layers that are critical exam topics:

Pith (Medulla): Central soft spongy core; first-formed wood; weakest part; present at the very centre of the trunk
Heartwood (Duramen): Inner darker-coloured, harder, denser core; dead cells; impregnated with resins, tannins, oils; more durable; resistant to decay; used for structural purposes
Sapwood (Alburnum): Outer lighter-coloured living wood; conducts sap (water + nutrients); less durable; susceptible to fungi and insects; width = 25–75 mm typically
Cambium Layer: Thin (1–2 cell thick) meristematic layer between sapwood and bark; responsible for growth in diameter (girth); produces wood (xylem) inward and bark (phloem) outward
Inner Bark (Phloem/Bast): Conducts manufactured food (sugars) from leaves down to roots
Outer Bark (Cortex): Dead protective layer; cork cells; protects against mechanical injury and pathogens
Annual Rings (Growth Rings): Concentric rings visible in cross-section; one ring = one year of growth (in temperate climates); formed by alternating early wood (spring) and late wood (summer/autumn)
Medullary Rays: Radial lines running from pith to bark; horizontal transfer of nutrients; prominent in oak (silver grain); plane of maximum strength

Fig 1 – Cross-section of a tree trunk: concentric zones from pith to outer bark. Heartwood (dark) is durable and used structurally; sapwood (light) is living and susceptible to decay. Cambium produces new wood inward and bark outward.

Microscopic Structure

Tracheids: Long (1–3 mm), narrow (10–50 µm) dead cells with tapered ends; constitute 90–95% of softwood volume; provide mechanical support and longitudinal water conduction; pits on cell walls allow lateral flow
Wood Fibres (Libriform Fibres): Shorter, thicker-walled dead cells in hardwoods; primary mechanical support; less porous than tracheids
Vessels (Pores): Characteristic of hardwoods only; wide-diameter (up to 500 µm), thin-walled tubular channels for water conduction; ring-porous (vessels concentrated in early wood, e.g. teak, oak) vs diffuse-porous (uniform distribution, e.g. birch)
Wood Parenchyma: Living cells; food storage and radial transport; ray parenchyma forms medullary rays
Cell Wall Layers: Middle lamella (lignin binding adjacent cells) → Primary wall → S1 (outer secondary) → S2 (main layer, most cellulose, controls mechanical properties) → S3 (inner secondary)
Cell Wall Components: Cellulose (40–50%, long crystalline microfibrils, provides tensile strength), Hemicellulose (20–30%, matrix polysaccharide), Lignin (20–30%, amorphous, phenolic polymer, binds and stiffens cell wall), Extractives (resins, oils, tannins, colouring — responsible for heartwood properties)

Chemical Composition of Wood

Component	Percentage (%)	Role
Cellulose	40–50	Tensile strength; longitudinal along microfibrils
Hemicellulose	20–30	Matrix; bulking agent; hygroscopic
Lignin	20–30 (hardwood) / 25–30 (softwood)	Rigidity, compression strength; cementing agent
Extractives	0.3–10	Colour, odour, durability, natural preservation
Ash (minerals)	0.1–1.0	Inorganic residue; silica, calcium, potassium

Moisture in Wood

Free water: Water in cell lumens (cavities); removed first during drying; removal causes no dimensional change
Bound water (Hygroscopic water): Water adsorbed in cell walls (between cellulose microfibrils); removal causes shrinkage and strength increase
Fibre Saturation Point (FSP): Moisture content at which cell walls are fully saturated but no free water exists in lumens; FSP ≈ 25–30% (approximately 30% for most species); critical moisture content below which shrinkage and strength change occur significantly
Equilibrium Moisture Content (EMC): MC at which timber neither gains nor loses moisture at given temperature and RH; EMC = f(RH, temperature); used for seasoning targets

MC (%) = [(W_green − W_oven_dry) / W_oven_dry] × 100 FSP ≈ 30% | Green wood MC: 50–200% | Air-dry MC: 12–18% | Kiln-dry MC: 8–15%

⚠ Key: All mechanical properties improve as MC decreases BELOW the FSP. Above FSP, properties are essentially constant. This is why seasoning is critical for structural timber.

2Classification of Trees & Timber ▶

Botanical Classification

Exogenous Trees (grow from outside)

Growth occurs by addition of concentric rings on the outside (between cambium and sapwood)
Annual rings visible in cross-section
Commercially important timber comes from these trees
Two sub-types: Conifers (softwoods) and Dicotyledonous (hardwoods)

Endogenous Trees (grow from inside)

Growth occurs from within (palms, bamboo, coconut)
No distinct annual rings
Fibres scattered throughout cross-section
Generally not used for structural timber (exception: bamboo in informal construction)

Fig 2 – Botanical classification of trees: Exogenous trees yield structural timber. Conifers (softwoods, gymnosperms) lack vessels; Dicots (hardwoods, angiosperms) have vessels. Note: "hardwood/softwood" is a botanical, not physical, classification.

Softwood vs Hardwood – Detailed Comparison

Property	Softwoods (Conifers)	Hardwoods (Dicots)
Botanical name	Gymnosperms	Angiosperms
Leaves	Needle-like, evergreen	Broad, may be deciduous
Cell types	Tracheids only (no vessels)	Vessels + fibres + parenchyma
Annual rings	Distinct (conifers)	May be indistinct (tropics)
Physical hardness	Generally softer (exceptions: yew)	Generally harder (exceptions: balsa)
Density	Lower (300–600 kg/m³)	Higher (400–900 kg/m³)
Durability	Lower (more resinous)	Generally higher
Workability	Easier to work	Harder to work
Growth rate	Faster	Slower
Indian examples	Deodar, Chir pine, Blue pine, Spruce, Fir, Cedar	Teak, Sal, Shisham, Neem, Bamboo, Mango, Oak
Uses	Light construction, packing, paper pulp, scaffolding	Heavy construction, furniture, flooring, railway sleepers

Classification by Durability (IS 287)

Class	Life (years)	Examples
Class I – High Durability	> 120	Teak, Deodar, Sal, Shisham, Sisso
Class II – Moderate Durability	60 – 120	Haldu, Poon, Mango, Neem
Class III – Low Durability	20 – 60	Bamboo, Spruce, Chir pine, Sissoo (sapwood)
Class IV – Perishable	< 20	Fir, Poplar, Rubber wood, Willow

Commercially Important Indian Timbers

Timber	Type	Properties	Uses
Teak	Hardwood	Density 640–720 kg/m³; Class I; very durable; self-oiling; resists termites; seasoning: good; slight shrinkage	Furniture, doors, floors, boat building, railway sleepers
Sal	Hardwood	Density 800–900 kg/m³; Class I; very hard; heavy; difficult to work; ring-porous	Railway sleepers, construction beams, bridges
Shisham (Rosewood)	Hardwood	Density 750–850 kg/m³; Class I; elastic; strong; good bending; seasons well	High-quality furniture, sports goods, plywood
Deodar (Indian Cedar)	Softwood	Density 560–640 kg/m³; Class I; aromatic; resinous; durable; easy to work	Building construction, bridges, railway sleepers, shingles
Chir Pine	Softwood	Density 480–560 kg/m³; Class III; resinous; straight grain; easy to work	Light construction, packing cases, paper pulp
Neem	Hardwood	Density 560–720 kg/m³; Class II; hard; durable; insect repellent	Furniture, agricultural implements
Bamboo	Endogenous grass	High tensile strength; light; fast-growing; hollow; splits easily	Scaffolding, roofing, basket work, paper pulp, composite

⚠ Teak is the most exam-frequent timber — remember: Class I durability, density 640–720 kg/m³, self-oiling (due to extractives), resists acids, suitable for all exposures without treatment.

3Growth & Felling of Trees ▶

Factors Affecting Timber Quality

Soil: Rich, deep, well-drained soil → wider annual rings → faster growth → possibly lower density; rocky/poor soil → narrower rings → denser, stronger timber
Climate: Temperate climates produce distinct annual rings (used for age determination); tropical climates produce multiple or indistinct rings; uniform temperature → uniform growth
Age: Older trees (50–100 years) yield stronger, more durable heartwood with narrower annual rings; very old or very young trees yield inferior timber
Species: Inherent genetic factors determine density, grain, texture, extractive content, and natural durability
Position in tree: Base of trunk → highest density and strength; top → lower density; outside of annual rings (latewood) → denser than early wood
Rate of growth: Slow-grown timber (narrow rings) → denser, stronger; fast-grown → less dense, weaker (for most species)

Correct Time for Felling

Best season: Winter or dry season (October–February in India) when sap flow is minimum; less moisture in wood → less weight to transport, less likelihood of fungal attack after felling
Best age: When tree has reached maximum heartwood development; typically 50–100+ years for hardwoods, 30–50 years for softwoods
Indicators of maturity: Tree ceases to grow rapidly; bark becomes rough and scaly; leaves become small and sparse; crown flattens
Time of day: Early morning, when sap pressure is lowest

Methods of Felling

Manual felling: Axe or saw; controlled by rope guides; traditional method; time-consuming
Mechanical felling: Chain saws, feller-bunchers; faster; modern logging
Controlled directional felling: Notch cut on desired fall side; back cut on opposite side; hinge wood guides fall direction

After Felling – Immediate Processing

Girdling (ring barking): Removing a ring of bark 1–2 years before felling → sap drying occurs naturally; reduces drying time after conversion; used for certain species
Log treatment: End sealing (paint, wax) to prevent end-checking (splits from rapid end drying); debarking to prevent insect attack
Floating (river floating): Logs floated downstream to mills; reduces transport cost; some soaking may improve durability (waterlogged timber) but may leach extractives
Seasoning immediately: Should begin promptly after conversion to prevent blue stain, fungal attack, and checking

💡 Girdling/Blasting method: Girdling 1–2 years before felling kills the tree gradually, allowing natural pre-drying through living transpiration. Reduces drying time and defects after conversion. Common for certain tropical hardwoods.

4Conversion of Timber ▶

Definition and Importance

Conversion is the process of sawing or cutting felled timber (logs) into commercially useful sizes and shapes for construction and other purposes. The method of conversion affects the quality, structural properties, and appearance of the final timber product.

Methods of Conversion

Fig 3 – Timber conversion methods: (a) Plain/flat sawn – all cuts parallel, maximum yield but more warping; annual rings at 0–45° to face; (b) Quarter sawn – radial cuts, minimum shrinkage and warping, better for structural use; annual rings at 45–90° to face; (c) Tangential same as plain sawn.

Comparison – Plain Sawn vs Quarter Sawn

Property	Plain (Flat) Sawn	Quarter (Radial) Sawn
Annual ring orientation	0–45° to board face	45–90° to board face
Shrinkage (width)	High (tangential shrinkage)	Low (radial shrinkage)
Warping tendency	High (cup warping)	Low
Timber yield from log	High (max. recovery)	Lower (more waste)
Appearance	Pronounced grain pattern; cathedral grain	Straight uniform grain; silver ray figure
Medullary rays	Less visible	More visible (decorative silver grain)
Cost	Lower	Higher
Best use	General carpentry, floorboards	Structural beams, decking, high-quality flooring
Wear resistance	Lower	Higher (end grain fibres more upright)

Standard Timber Sections (IS 1200, IS 4891)

Scantling: Cross-section area ≤ 5400 mm² (e.g., 50×50 mm to 75×75 mm)
Batten: Width < 150 mm, thickness 50–75 mm
Plank: Width ≥ 150 mm, thickness < 75 mm
Deal: 50–100 mm thick × 225–300 mm wide
Balk: Square or nearly square section ≥ 175 mm × 175 mm
Board: Width ≥ 100 mm, thickness < 50 mm
Slab: Side cut of a log including bark on one or both faces
Veneer: Thin sheet (0.4–6 mm) sliced, peeled or sawn from log; used for plywood

💡 Quarter sawn is structurally superior because: (1) less shrinkage and warping, (2) annual rings perpendicular to width provide better resistance to surface wear, (3) medullary rays exposed on face (silver grain) for aesthetics, (4) more uniform texture. Disadvantage: higher waste and cost.

5Seasoning of Timber ▶

Definition and Purpose

Seasoning is the controlled removal of moisture (free water and bound water) from freshly cut (green) timber to reduce its moisture content to a level suitable for the intended use. IS 1141:1993 covers seasoning of timber.

Why Seasoning is Necessary

Reduce weight: Green timber MC = 50–200%; seasoned = 12–18%; significant reduction in density for transport
Increase strength: All mechanical properties improve as MC drops below FSP (30%); strength may increase by 2–4× for some properties
Improve durability: Dry timber resists fungal growth (fungi need MC > 20% to survive)
Improve workability: Dry timber takes paint, polish, glue, and nails better
Reduce shrinkage in service: Pre-dried timber shrinks less when installed, reducing warping, checking, and splitting in situ
Improve fire resistance: Dry timber ignites at higher temperature (moisture acts as heat sink); moisture content affects charring rate
Allow treatment: Preservatives penetrate dry timber more effectively

💡 Required MC for use: Timber in buildings: 12–15%; Exterior joinery: 14–18%; Interior joinery: 8–10%; Furniture: 8–10%; Railway sleepers: 15–20% (IS 1141).

Methods of Seasoning

1. Natural (Air) Seasoning

Timber stacked in open sheds with good ventilation; stickers (spacers 25×25 mm) placed between layers for air circulation
Timber on platform > 450 mm above ground to prevent ground moisture
Stack ends painted or sealed to prevent end-checking
Stacks oriented to prevailing wind; cover top with sloping roof
Duration: 1–3 years for hardwoods; 3–6 months for softwoods
Final MC: 12–18% (depends on ambient conditions)
Advantages: Simple, inexpensive, no equipment, large volumes possible, minimal cell damage
Disadvantages: Very slow, large storage area needed, MC depends on climate (EMC), risk of fungal attack if MC remains > 20%, cannot achieve low MC in humid climates

2. Artificial (Kiln) Seasoning

Timber placed in a heated chamber (kiln) with controlled temperature, humidity, and air circulation
Progressive kilns: timber moved from wet end to dry end as drying proceeds
Compartment kilns: entire charge dried together with changing schedule
Duration: 2–10 days (vs months for air seasoning)
Temperature: 40–80°C (low-temperature schedule for refractory species)
Final MC: 6–15% as required
Advantages: Fast, any MC achievable, kills insects and fungi in wood (sterilisation), year-round, consistent quality, small floor area
Disadvantages: High capital and operating cost, requires skilled operators, small batches, possible cell collapse if schedule too aggressive, case-hardening if surface dries too fast

3. Water (Boiling/Immersion) Seasoning

Timber immersed in running water or stagnant pond for 2–4 weeks; sap leaches out
After removal, normal air drying proceeds more rapidly
Can also use steam treatment (boiling in large tanks)
Advantage: Reduces drying time; removes water-soluble extractives (reduces staining)
Disadvantage: Loses some natural preservatives/extractives; MC still requires air/kiln drying afterward; not suitable for all species

4. Chemical Seasoning

Timber coated or immersed in hygroscopic salt solution (e.g., urea, common salt) which withdraws moisture from wood by osmosis
Less common; used for special applications
Advantage: Can pre-condition timber for treatment
Disadvantage: Residual chemicals may cause corrosion of metal fasteners

5. Electrical Seasoning

High-frequency alternating current (dielectric heating) or resistance heating passed through timber
Water molecules align and generate heat → rapid uniform drying from inside out (unlike conventional kiln which dries outside-in)
Very fast (hours); very uniform MC; no case-hardening
Very expensive; impractical for large commercial volumes

Fig 4 – (a) Correct air drying stack: stickers 25×25 mm between layers for air circulation; platform > 450 mm above ground; ends sealed to prevent end-checking; sloping roof for rain. (b) Comparison of all seasoning methods with durations and achievable MC.

Defects Caused by Incorrect Seasoning

Defect	Cause	Prevention
Checking / End checking	Rapid surface drying; stress concentration at ends	Seal ends with paint/wax before seasoning
Case hardening	Surface dries too fast → compressive stress in core	Control kiln temperature/humidity schedule; reconditioning
Honeycombing	Internal tension cracks when case-hardened shell restrains core shrinkage	Proper kiln schedule; reconditioning steam treatment
Collapse (washboarding)	Cell cavities buckle under capillary tension during rapid drying; occurs in wet timber above FSP	Low temperature predrying; reconditioning
Warping	Uneven drying across grain; differential shrinkage	Proper sticker placement; weight/clamp stacks; quarter-sawing
Blue stain	Fungal attack (non-structural) in green timber at MC > 20%	Rapid seasoning; fungicide treatment; prevent prolonged green exposure

6Defects in Timber ▶

Classification of Defects

Defects in timber reduce its strength, durability, and utility. IS 1141 classifies them into five categories:

Fig 5 – Common defects in timber: Knots, Shakes, Warp types, Cross-grain, Decay, Insect damage, Rind Gall, and Upsetness. Each reduces structural integrity differently.

A. Defects Due to Natural Causes

Knots

Definition: Cross-sections of branches embedded in the trunk; formed where branch joined the trunk
Live (tight/sound) knot: Firmly attached; continuous grain; less serious
Dead (loose/encased) knot: Surrounded by bark; grain not continuous; may fall out; more serious
Effect: Reduces tensile and bending strength significantly; stress concentration; slope of grain around knot reduces strength by up to 50%
Knot size limits (IS 1331): Structural timber: knot diameter ≤ 1/4 of member width for Grade 1; ≤ 1/3 for Grade 2
Types by position: Face knot (on wide face), edge knot (on narrow face – more critical in bending), margin knot (at corner)

Shakes

Cup shake (Ring shake): Separation along annual ring in curved (tangential) direction; occurs due to frost, wind movement, or fungal activity; serious defect
Heart shake: Cracks radiating from pith outward; due to heart rot beginning at pith; occurs in old trees
Star shake: Multiple radial cracks radiating from pith in star pattern; due to extreme cold or sudden temperature change
Side/Radial shake: Crack on surface; due to rapid drying or sudden blow
Upshot shake: Caused by wind sway or felling impact; fibres partially ruptured

Other Natural Defects

Cross grain (Diagonal grain): Fibres not parallel to the longitudinal axis; due to spiral growth; seriously reduces strength; classified as: diagonal grain (slope of grain > 1:15 rejected)
Rind gall: Abnormal curved swelling on bark surface caused by healed over wound; irregular grain below
Burr: Large wart-like excrescence; irregular grain; decorative (veneer) but structurally poor
Upsetness: Compression failure of wood fibres due to excessive wind sway or blow during felling; wavy/crimped grain visible; most dangerous as it appears externally sound; occurs especially in beams subject to high compressive stress; reduces all strength properties
Resin pockets: Pockets or cysts filled with resin; weaken structure at that point; mainly in conifers
Reaction wood: Compression wood (conifers, bottom side of leaning stem) or Tension wood (hardwoods, upper side); abnormal properties; high longitudinal shrinkage, brittle
Water stain: Iron-tannate discolouration from contact with iron in water; structural effect minimal

B. Defects Due to Fungi

Conditions for fungal growth: MC > 20%, temperature 25–35°C, O₂ presence; absence of any one prevents growth
Brown (Destructive) rot: Fungi digest cellulose and hemicellulose; leaves brown crumbly mass of lignin; most serious; severe strength loss; example: Serpula lacrymans (dry rot)
White rot: Fungi digest both cellulose and lignin; leaves white fibrous residue; strength also greatly reduced
Soft rot: Surface softening; bacteria-assisted; tunnels in S2 layer; common in wet environments
Dry rot: Misnomer — actually requires moisture to start; Serpula lacrymans can conduct water long distances; forms grey mycelium sheets; appears dry when advanced; serious in buildings
Wet rot: Coniophora puteana; only in persistently wet conditions (MC > 40%); doesn't spread dry
Blue stain: Fungi that live on sapwood sugars; stain only — no structural damage; often treated with fungicide to prevent further growth
Sap stain: Discolouration of sapwood from fungi or bacteria

C. Defects Due to Insects

Insect	Timber Attacked	Damage Pattern	Detection
Termites (White ants)	All timber; prefer sapwood; avoid heartwood of durable species	Hollow out wood from inside; mud galleries on surface; internal galleries	Hollow sound when tapped; mud tubes on surface
Common Furniture Beetle (Anobium)	Sapwood of all species; dry conditions	Round exit holes 1.5–2 mm dia; fine bore dust (frass)	Fresh bore holes + cream powder (frass)
Death Watch Beetle	Hardwood heartwood; decayed wood; old structures	Round holes 3 mm dia; coarser frass; rhythmic tapping	Larger holes; biscuit-shaped frass
House Longhorn Beetle	Sapwood of softwoods; roofing timbers	Oval exit holes 5–10 mm; cream-coloured frass; can destroy entire sapwood	Large oval holes; powdery cream frass
Lyctus (Powder Post) Beetle	Sapwood of large-pored hardwoods (oak, ash)	Fine talcum-powder frass; round holes 1–2 mm; complete destruction of sapwood	Very fine powder; tiny holes
Marine Borers (Teredo, Limnoria)	Timber submerged in sea water	Torpedo-shaped tunnels through cross-section; rapid destruction	Extensive tunnelling below surface; external appearance intact

D. Defects Due to Conversion (Manufacturing)

Warp: Distortion from true plane; types: Cup (across width), Bow (along length), Twist/Wind (diagonal), Crook/Spring (along width in plan)
Torn grain: Fibres torn by saw instead of cut cleanly; surface roughness
Chip mark: Marks made by saw teeth or planer blades
Raised grain: Undulation of surface after planing; latewood stands above early wood
Diagonal grain: Saw cuts not parallel to grain direction

E. Defects Due to Seasoning

(See Chapter 5 — Checking, Case-hardening, Honeycombing, Collapse, Warping, Blue stain)

⚠ Upsetness is the most dangerous defect because it is not visible externally — the timber appears sound but has microscopically ruptured fibres with severely reduced tensile and bending strength. Always check timber that may have been subjected to impact or compression during felling.

7Preservation of Timber ▶

Purpose of Preservation

Extend service life of timber by protecting against biological agents (fungi, insects, marine borers) and environmental factors
Allow use of less naturally durable (Class III/IV) species for Class I/II applications after treatment
Economic: low-durability species are faster-growing and cheaper; treatment is economical vs using only Class I species
IS 401:2001 – Code of Practice for Preservation of Timber

Types of Preservatives

1. Oil-Based Preservatives

Preservative	Composition	Properties	Uses
Coal Tar Creosote	Distillation product of coal tar (>200 compounds including polycyclic aromatics)	Dark brown/black; strong odour; excellent fungicide and insecticide; not leachable; paintable after some time; not suitable for indoor use	Railway sleepers, telegraph poles, marine piling, dock timbers, fencing
Anthracene Oil	Heavier fraction of coal tar distillation	Similar to creosote; slightly less effective; less penetrating	Similar to creosote applications
Carbolineum	Petroleum-based, darkened with coal tar	Less penetrating than creosote; brown colour	Fences, poles

2. Water-Soluble Preservatives

Preservative	Composition	Properties	Uses
CCA (Copper Chrome Arsenic)	CuSO₄ + K₂Cr₂O₇ + As₂O₅ in water	Green colour; odourless; paintable; fixed in wood; excellent broad-spectrum; durable above ground and in ground contact; toxic (arsenic — use restricted in some countries)	General construction, fencing, decking, piles, utility poles (most widely used)
CCB (Copper Chrome Boron)	CuSO₄ + K₂Cr₂O₇ + H₃BO₃	No arsenic; similar to CCA; good for wood in contact with humans	Playground equipment, indoor use
Boric acid / Borax (Tim-bor)	Na₂B₄O₇ + H₃BO₃	Colourless; odourless; low toxicity to mammals; effective against insects and fungi; leachable (not for ground contact)	Interior timber, furniture, sash, dry conditions
Zinc chloride	ZnCl₂ solution	Colourless; hygroscopic; effective above ground only; paintable; not for ground contact; corrosive to metal	Interior joinery, furniture
Sodium fluoride	NaF solution	Colourless; odourless; low toxicity; effective insecticide; leachable	Interior joinery; against insects

3. Organic Solvent Preservatives

TBTO (Tributyltin oxide): Effective fungicide and insecticide; dissolved in light petroleum or white spirit; leaves surface paintable and clean; used for joinery, windows, doors
Pentachlorophenol (PCP): In organic solvent; excellent broad-spectrum; good penetration; restricted in many countries (toxic)
Lindane (γ-BHC): Insecticide against wood-boring beetles; dissolved in white spirit; applied by brush
Copper naphthenate: Green oil-soluble; good for exterior use; paintable after drying

Fig 6 – Timber preservation methods: (a) Full-cell (Bethell) pressure process — vacuum removes air, then preservative under 10–14 kg/cm² pressure; maximum penetration and retention; (b) Non-pressure methods — decreasing effectiveness: Brushing → Soaking → Hot-Cold Open Tank → Sap displacement → Diffusion.

Methods of Applying Preservatives (IS 401)

Pressure Methods (Best Penetration and Retention)

Full-cell (Bethell) process: Initial vacuum (removes air from cells) → fill with preservative → apply hydraulic pressure (10–14 kg/cm²) → final vacuum (removes surplus); maximum retention; used for CCA treatment of railway sleepers, piles
Empty-cell Lowry process: No initial vacuum → fill with preservative under pressure → release pressure → back pressure of compressed air drives out excess; less retention than Bethell; lighter treatment; used for poles, sleepers
Empty-cell Rueping process: Initial air pressure (3–4 kg/cm²) into timber → fill preservative → increase pressure → release → trapped air drives out excess; less preservative used; economical; good for poles

Non-Pressure Methods

Brushing/Spraying: Surface application; 2–3 coats; superficial penetration; simplest; used for maintenance and in-situ treatment of installed timber
Soaking/Steeping: Immersion in preservative bath for 24 hours to 2 weeks; deeper penetration than brushing; cold soaking; suitable for permeable timber
Hot-and-cold open tank: Heat timber in preservative (90–95°C) for 2–4 hours → remove to cold bath → preservative absorbs as timber cools; better penetration than cold soaking; used for creosote treatment
Sap replacement (Boucherie process): Fresh-cut green log; one end capped with cup connected to preservative; hydraulic head forces preservative in, driving sap out from other end; effective for permeable species (sapwood); cannot treat dry timber
Diffusion method: Wet/green timber treated with water-soluble preservative (e.g., borax); preservative diffuses into cell walls over weeks/months; slow but effective for green timber
Vacuum (Envelope/Blanket) method: Wrapping timber in plastic sheeting, applying vacuum → preservative drawn in; field application; used for in-situ treatment

Requirement of Good Preservative

Toxic to fungi, insects, and marine borers; non-toxic to humans and animals in service
Stable and durable — remains active for life of structure; not volatile, not leachable
Penetrating — should penetrate sapwood (and ideally heartwood); should not clog vessels
Should not reduce structural properties (strength) of timber
Should allow subsequent use of treated timber: finishing, painting, gluing if required
Economical and readily available
Should not cause corrosion of metal fasteners (exception: some water-borne preservatives like CCA may corrode non-galvanised steel)
Odourless or acceptable odour for intended use

⚠ Key exam values: CCA pressure treatment (Bethell process): pressure = 10–14 kg/cm²; temperature 15–20°C; retention = 8–16 kg/m³ for above-ground, 16–25 kg/m³ for ground contact (IS 401). Creosote retention for railway sleepers: 100–160 kg/m³ (IS 1477).

8Mechanical Properties of Timber ▶

Key Factors Affecting Mechanical Properties

Moisture content (below FSP): All properties improve as MC decreases; rule: for every 1% reduction in MC below FSP, strength increases ~1–6% depending on property; MOR approximately doubles from green to air-dry
Grain direction: Strength is maximum along the grain (longitudinal direction); shear and tension perpendicular to grain are weakest; slope of grain drastically reduces strength
Density: Higher density → generally higher strength; good predictor of strength for same species at same MC
Growth rate: Slow-grown → narrower rings → generally stronger; fast-grown → wider rings → less dense
Temperature: Higher temperature → lower strength; above 60°C, permanent degradation; below 0°C, strength increases
Duration of load: Wood creeps under sustained load; long-term strength = 50–60% of short-term strength (modulus of rupture); IS 883 applies a duration-of-load factor
Defects: Knots, shakes, cross grain reduce strength proportionally; dead knots more serious than live knots
Species: Inherent anatomical and chemical differences govern baseline properties

Fig 7 – Timber strength vs moisture content: all properties improve below FSP (≈30%); above FSP, strength is constant. MOR is most sensitive to MC change; tension perpendicular to grain is least sensitive.

Mechanical Properties – Definitions and Values

Bending (Flexural) Strength – Modulus of Rupture (MOR)

Most important property for structural timber (beams, joists, rafters)
Tested by standard beam loaded at centre or third-points
MOR = 3PL / (2bh²) for centre-point loading
Typical values (at 12% MC): Teak = 70–90 MPa; Sal = 90–110 MPa; Deodar = 50–65 MPa; Chir pine = 45–60 MPa
Knots and cross grain reduce MOR significantly; dead knots worst

Compression Strength

Parallel to grain (σ_c||): Much higher than perpendicular; fibres act as columns; buckling controlled by slenderness; typical values 35–60 MPa (air-dry)
Perpendicular to grain (σ_c⊥): Much lower; cell walls crush; typical 5–12 MPa; governs bearing under beams, bolts, washers
Ratio σ_c||/σ_c⊥ ≈ 5–10 typically

Tensile Strength

Parallel to grain: Highest strength property; 100–150 MPa for clear wood (rarely achieved due to defects and eccentricity)
Perpendicular to grain: Very low; 1–5 MPa; easily split; determines beam splitting failure at supports
Tension parallel to grain is the single highest strength value in wood

Shear Strength

Horizontal (parallel to grain): Low; 4–12 MPa; governs notched beams and cross-lap joints; "rolling shear" in plywood
Vertical (perpendicular to grain): High; rarely governs design
Shear failure is critical in short, deep beams loaded near supports

Modulus of Elasticity (E)

Parallel to grain E: typically 8,000–18,000 MPa (8–18 GPa) for structural species
Perpendicular to grain E⊥: much lower, ~E/50 to E/100
Important for deflection calculations in beams and columns
Teak: E ≈ 11,000 MPa; Sal: E ≈ 14,000 MPa; Deodar: E ≈ 9,500 MPa

Hardness (Janka Hardness)

Load required to embed a steel ball (11.28 mm dia, area = 1 cm²) to half its diameter
Measured in N or kgf; important for flooring, decking
Teak ≈ 4,500 N; Sal ≈ 8,000 N; Shisham ≈ 7,000 N; Deodar ≈ 3,500 N

Impact Strength (Toughness)

Energy absorbed per unit volume under impact loading
Tested by falling hammer; total energy to failure
Important for tool handles, sports goods, vehicle floors, scaffolding
Mango, ash, hickory — high toughness; generally higher toughness in timber with interlocked grain

IS 883:2016 – Design of Structural Timber in Building

Property	Species Group A (High-strength)	Species Group B (Medium)	Species Group C (Low-strength)
Bending (MOR) – N/mm²	18.0	12.0	8.0
Tension // grain – N/mm²	11.0	8.0	5.0
Compression // grain – N/mm²	11.0	8.0	5.6
Compression ⊥ grain – N/mm²	3.5	2.5	1.6
Shear // grain – N/mm²	1.05	0.84	0.56
Modulus of elasticity – N/mm²	12,600	9,800	5,600

⚠ IS 883 values are characteristic (grade) stresses for structural design, applicable to timber graded to Grade 1 and Grade 2 per IS 1331. Species Group A: Teak, Sal, Shisham; Group B: Deodar, Neem, Haldu; Group C: Chir pine, Spruce, Fir. These values are for dry (12% MC) conditions.

Modifications / Adjustment Factors (IS 883)

Moisture content factor (K₁): Strength values at 12% MC; for wet service (MC > 18%), multiply by K₁ = 0.7–0.8 depending on property
Duration of load factor (K₂): Long-term load → apply K₂ = 0.5–0.8; short-term (wind/earthquake) → K₂ = 1.25
Size factor (K₃): For beams deeper than 300 mm; K₃ = (300/d)^0.11
Creep factor: For sustained deflection; long-term deflection = short-term × (1 + creep factor)
Temperature factor (K₄): For T > 40°C; reduce strength by K₄; high temperature weakens timber
Grade stress factor: Grade 1 (better quality, fewer defects) uses full tabulated stresses; Grade 2 uses 65–80% of Grade 1 values

Strength in Different Directions – Anisotropy

Fig 8 – Wood is an orthotropic material with three principal directions: L (longitudinal, along grain), R (radial), T (tangential). Stiffness and strength are highest in L-direction. Tangential shrinkage ≈ 2× radial shrinkage, explaining cup warping of flat-sawn boards.

Grading of Structural Timber (IS 1331:1975)

Grade 1 (Select Structural): Minimum defects; knot diameter ≤ 1/4 width; slope of grain ≤ 1:15; no shakes, decay, insect damage; maximum strength utilisation
Grade 2: Knot diameter ≤ 1/3 width; slope of grain ≤ 1:12; minor shakes permissible; for general structural use
Grade 3: For non-structural or light structural use; larger defects permissible
Visual vs Machine grading: Visual grading by trained inspector (IS 1331); Machine stress grading uses non-destructive testing (flatwise bending to measure E) for more accurate grade prediction

Standard Timber Tests

Test	Specimen Size (mm)	IS Code / Standard	Property Measured
Static Bending	50×50×760 (clear wood)	IS 1708 Pt 3	MOR, MOE, work to MOR
Compression // grain	50×50×200	IS 1708 Pt 7	σ_c\|\|
Compression ⊥ grain	50×50×150	IS 1708 Pt 8	σ_c⊥
Tensile strength // grain	Dumbbell, gauge 50×25	IS 1708 Pt 5	UTS parallel to grain
Shear strength // grain	50×50×50 notched	IS 1708 Pt 4	τ // grain
Impact Bending	50×50×700	IS 1708 Pt 15	Toughness (total energy)
Hardness (Janka)	50×50×150	IS 1708 Pt 9	Ball-embedding force
Cleavage	50×50×50 with groove	IS 1708 Pt 13	Resistance to splitting
Moisture content	Any; oven-dry method	IS 1708 Pt 2	MC% = (wet–dry)/dry × 100
Density	50×50×50 clear wood	IS 1708 Pt 1	Density at known MC

9Special Wood Products ▶

Overview

Engineered wood products overcome the limitations of solid timber (limited sizes, natural defects, anisotropy, warping). They redistribute defects and utilise wood fibre more efficiently.

Fig 9 – Special wood products: Plywood (cross-bonded veneers, IS 303), Particle Board (IS 12823), MDF (IS 12406), Hardboard (IS 1658), LVL (Laminated Veneer Lumber), Glulam (IS 14616). Glulam and LVL are structural; others primarily non-structural or light structural.

Plywood – Detailed Notes

Construction: Odd number (3, 5, 7, 9, ...) of veneer plies bonded with grain direction of adjacent plies at 90° to each other; face and back grains parallel; balanced construction
Veneer thickness: 1.5–4 mm per ply; total thickness: 3–25 mm standard
Adhesives: MR (moisture-resistant) grade: UF (urea formaldehyde) adhesive; BWR (boiling water resistant): PF (phenol formaldehyde); BWP (boiling waterproof): resorcinol formaldehyde or PF; Marine grade: PF with preservative treatment
IS 303:2018: Plywood for general purposes; BWP (exterior), BWR (semi-exterior), MR (interior)
IS 710:2010: Marine plywood; for watercraft, boat building, marine structures
IS 5509:2000: Resin-bonded plywood for packaging and general use
Advantages of plywood: Nearly equal strength in both directions; no warping; large panel sizes; efficient use of timber; good impact resistance
Sizes: Standard 2440×1220 mm (8×4 ft); also 2745×1220 mm (9×4 ft)

Fibre Boards

Product	Density (kg/m³)	Binder	IS Code	Key Uses
Hardboard	> 800	Lignin (own); no added binder	IS 1658	Wall lining, floor underlay, furniture back
Medium Board	350–800	Lignin; some synthetic	IS 1658	Panelling, pinboards
Softboard (Insulation Board)	< 350	Lignin; minimal	IS 1658	Thermal insulation, acoustic tiles, pinboards
MDF (Medium Density Fibreboard)	600–800	UF/MF resin, 8–12%	IS 12406	Furniture, cabinets, doors, skirting boards

Particle Board (Chipboard)

Made from wood chips, shavings, and flakes bonded with synthetic resin (UF most common) under heat and pressure
Three-layer board: fine particles on faces, coarser in core
Density: 600–750 kg/m³; lower strength than MDF or plywood
IS 12823:1990; available as flat-pressed and extruded types
Highly susceptible to moisture swelling; not suitable for exterior use unless specially treated
Uses: furniture, kitchen cabinets, shelving, floor underlay

Glulam (Glued Laminated Timber)

Structural member made by bonding (RF or MF adhesive) 2 or more layers of dressed dimension lumber with grain parallel; each lamination typically 19–45 mm thick
Advantages: Large sections possible; curved and tapered members; defects distributed; higher design stress than solid timber; dimensional stability; good fire resistance (char layer)
Design stress: 15–25% higher than solid timber of same grade due to redistribution of defects
IS 14616:1999; uses: large-span beams, arches, portal frames, bridges, sports halls
Fire performance: Glulam chars at ~0.65–0.8 mm/min; char layer insulates structural core; inherently better fire behaviour than steel (maintains strength longer)

Bamboo

Technically a grass (Poaceae family), not timber, but widely used as building material
Fastest-growing woody plant (some species: 90 cm/day); renewable in 3–7 years
Hollow culm; nodes provide shear transfer; very high tensile strength parallel to grain (200–350 MPa for outer fibres)
High strength-to-weight ratio; sustainable alternative to steel reinforcement in some countries
Susceptible to insect attack (especially powder post beetle), fungal decay, and splitting; requires treatment
IS 6874:2008: Bamboo for structural purposes; IS 8242: Bamboo culms for general purposes
Uses: scaffolding, formwork, flooring (bamboo composite), reinforcement (experimental), roofing, partition

10IS Codes, Key Values & Exam Quick Revision ▶

Complete IS Codes for Timber

IS Code	Title / Subject
IS 287:1993	Maximum Permissible Moisture Content of Timber — Recommendations
IS 401:2001	Code of Practice for Preservation of Timber
IS 399:1963	Classification of Commercial Timbers and Their Zonal Distribution
IS 883:2016	Design of Structural Timber in Building — Code of Practice
IS 1141:1993	Code of Practice for Seasoning of Timber
IS 1331:1975	Method of Grading of Timber — Structural Purposes
IS 1708 (Parts 1–18)	Methods of Testing of Small, Clear Specimens of Timber
IS 303:2018	Plywood for General Purposes — Specification
IS 710:2010	Marine Plywood — Specification
IS 1658:2013	Fibre Hardboard — Specification
IS 12406:2011	Medium Density Fibreboard — Specification
IS 12823:1990	Flat-Pressed Three-Layer Particle Board — Specification
IS 14616:1999	Glued Laminated Timber — Structural Requirements
IS 1477 (Pts 1 & 2)	Code of Practice for Painting Timber
IS 6874:2008	Bamboo for Structural Purposes
IS 8242:1976	Bamboo Culms — Structural Design
IS 1200 (Pt 14)	Method of Measurement — Woodwork and Carpentry
IS 4891:1988	Dimensions of Commercial Timber

All Key Formulae

MOISTURE CONTENT MC (%) = [(W_wet − W_dry) / W_dry] × 100 FSP ≈ 25–30% (≈30% commonly used) STATIC BENDING TEST (Centre-point loading, span L, section b×d) MOR = 3PL / (2bd²) MOE = PL³ / (4bd³ × δ) [δ = mid-span deflection at load P] JANKA HARDNESS Load to embed 11.28 mm dia steel ball to half diameter (area = 1 cm²) SHRINKAGE S = (D_green − D_dry) / D_green × 100 % Tangential S ≈ 2 × Radial S | Longitudinal S < 0.5% SIZE FACTOR (IS 883) for beams deeper than 300 mm K₃ = (300/d)^0.11

Critical Numerical Values – Direct Exam Use

Parameter	Value
Fibre Saturation Point (FSP)	≈ 25–30% (commonly quoted as 30%)
Green timber MC range	50–200%
Air-dry timber MC	12–18%
Kiln-dry MC	6–15%
MC for fungi to grow	> 20% (IS 1141)
Sticker size (air drying)	25×25 mm
Stack platform height	> 450 mm above ground
Class I timber life	> 120 years
Class II timber life	60–120 years
Class III timber life	20–60 years
Class IV (perishable) life	< 20 years
Teak density	640–720 kg/m³
Sal density	800–900 kg/m³
Deodar density	560–640 kg/m³
CCA pressure (Bethell)	10–14 kg/cm²
Creosote retention (sleepers)	100–160 kg/m³
Hardboard density	> 800 kg/m³
MDF density	600–800 kg/m³
Softboard density	< 350 kg/m³
IS 883 Group A — Bending stress	18.0 N/mm²
IS 883 Group B — Bending stress	12.0 N/mm²
IS 883 Group C — Bending stress	8.0 N/mm²
IS 883 Group A — Shear // grain	1.05 N/mm²
IS 883 Group A — MOE	12,600 N/mm²
Teak MOE	≈ 11,000 N/mm²
Long-term vs short-term strength ratio	50–60% (duration-of-load effect)
Tangential shrinkage : Radial shrinkage	≈ 2:1
Plywood standard size	2440×1220 mm (8×4 ft)
Knot limit Grade 1 (IS 1331)	≤ 1/4 of member width
Knot limit Grade 2 (IS 1331)	≤ 1/3 of member width
Slope of grain limit Grade 1	≤ 1:15
Slope of grain limit Grade 2	≤ 1:12
Glulam lamination thickness	19–45 mm
Glulam char rate	0.65–0.8 mm/min
Bamboo tensile strength (outer fibre)	200–350 MPa
Bamboo maturity for use	3–7 years

Fig 10 – GATE/ESE/SSC JE question frequency for Timber topics. Mechanical properties and defects have highest frequency; preservation and seasoning are regular ESE and SSC JE topics.

Mnemonics and Quick Memory Aids

Tree parts from inside out (PHCSIB): Pith → Heartwood → Cambium → Sapwood → Inner bark → Bark
Durability class mnemonic "Teak Deodar Sal Stays" → Class I. Poplar/Fir/Willow → Class IV.
Shakes types: "Cup-Heart-Star-Side" (CHSS) — Cup (along ring), Heart (radial from pith), Star (many radials), Side (surface)
Preservatives: "Creosote is Black, CCA is Green, Borax is Clear"
Plywood rule: Always ODD number of plies (3,5,7...); adjacent plies at 90°; face plies parallel
Shrinkage order: T > R > L (Tangential > Radial > Longitudinal)
"Below FSP all properties rise; above FSP they stabilise"

GATE/ESE Previous Year Question Patterns

Topic	Frequency	Type	Key Points to Focus
Mechanical properties / IS 883 values	Very High – GATE/ESE	NAT, MCQ	Group A/B/C stresses; MOE; effect of MC; MOR formula
Defects – identification and causes	Very High – all exams	MCQ, Descriptive	Knot types; Shakes; Upsetness definition; fungi conditions
Preservation methods and preservatives	High – ESE/SSC JE	MCQ, Descriptive	Bethell/Lowry/Rueping; CCA vs creosote; pressure values
Seasoning methods and defects	High – ESE/SSC JE	MCQ	Air vs kiln; FSP; MC values; case-hardening; honeycombing
Classification (Softwood/Hardwood)	Medium – SSC JE/State PSC	MCQ	Hardwood ≠ hard; vessels in hardwood; Indian timber classification
Plywood and wood products	Medium – ESE/SSC JE	MCQ	IS 303 grades; odd plies; MDF vs hardboard densities
Structure of wood / Micro structure	Medium – GATE	MCQ	Annual rings; FSP; cell wall components; heartwood vs sapwood
Conversion methods	Low – SSC JE	MCQ	Plain vs quarter sawn; warping tendency; ring orientation
Bamboo properties	Low – State PSC	MCQ	Tensile strength; endogenous; IS 6874

⚠ Most Common Exam Mistakes:
1. Confusing Hardwood (botanical = angiosperm) with physically hard wood — Balsa is hardwood but physically soft.
2. Stating properties change above FSP — they DON'T; change only occurs BELOW FSP.
3. Mixing up Bethell (full-cell, initial vacuum) with Rueping (empty-cell, initial air pressure).
4. Upsetness looks externally sound — always the most dangerous unseen defect.
5. Cup shake is along annual ring; Heart shake is radial from pith — commonly confused.
6. Tangential shrinkage ≈ 2× radial shrinkage (not equal) — important for warping analysis.