Timber – Complete Study Notes for GATE, ESE & SSC JE

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Timber — Complete Study Notes

10 chapters · GATE, ESE (IES), SSC JE, State PSC, RRB JE

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Timber is a high-yield, exam-favourite topic in GATE Civil, ESE (IES) and SSC JE Building Materials. This chapter covers every stage of the material's life — from the structure of wood and botanical classification, through growth, felling and conversion, to seasoning, defects, preservation, mechanical properties and modern engineered wood products — with all formulae, IS code values, diagrams and exam-pattern analysis.

Learning Objectives

After studying this chapter you will be able to:

Identify and label the macroscopic and microscopic structural components of a tree trunk and explain the role of moisture (free water, bound water, FSP) in timber behaviour
Classify timber botanically (exogenous/endogenous, softwood/hardwood) and by durability class per IS 287
Determine the correct felling season and age, and compare conversion (sawing) methods by their effect on shrinkage and warping
Select the appropriate seasoning method (air, kiln, water, chemical, electrical) for a given application and target moisture content
Identify natural, fungal, insect, conversion and seasoning defects from their visual and behavioural signatures
Choose an appropriate preservative and application method (pressure vs non-pressure) per IS 401
Apply IS 883 grade stresses and adjustment factors, and compute MOR, MOE and the size factor for structural timber design
Distinguish special wood products (plywood, MDF, particle board, glulam, bamboo) and select the correct one for a given construction scenario

Exam Relevance

ESE★★★★★

GATE★★★★☆

SSC JE★★★★★

State PSC★★★★☆

💡 Mechanical properties (IS 883 grade stresses) and defect identification are the single highest-frequency Timber topics in GATE/ESE; Preservation and Seasoning are near-guaranteed in ESE descriptive papers and SSC JE MCQs.

Concept Map

Structure of Wood (macro + micro) → Moisture (Free, Bound, FSP)

↓

Classification (Botanical + Durability, IS 287)

↓

Growth & Felling → Conversion (Sawing methods)

↓

Seasoning (Air / Kiln / Water / Chemical / Electrical)

↓

Defects — Natural, Fungal, Insect, Conversion, Seasoning

Preservation (IS 401) — Pressure & Non-Pressure methods

↓

Mechanical Properties (IS 883) → Structural Design

↓

Special Wood Products — Plywood, MDF, Glulam, Bamboo

Where This Fits

Prerequisite: Building Materials — Overview, for general material property concepts used throughout this chapter.

Related: Building Materials — Detailed Notes, which places timber alongside other construction materials for comparison; the mechanical-properties methodology here (grade stresses, adjustment factors) mirrors the approach used for structural design of RCC and steel elsewhere in the syllabus.

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Ch 1 · Structure of Wood

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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

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

💡 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.

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Ch 1 · Formulas

Moisture Content

$$ MC\% = \frac{W_{green} - W_{oven\,dry}}{W_{oven\,dry}} \times 100 $$

Based on oven-dry mass, not initial (wet) mass

Fibre Saturation Point

$$ FSP \approx 25\text{–}30\% $$

Cell walls saturated, no free water in lumens; strength/shrinkage change only occurs below this point

Typical MC Ranges

$$ \text{Green: }50\text{–}200\%\;|\;\text{Air-dry: }12\text{–}18\%\;|\;\text{Kiln-dry: }6\text{–}15\% $$

Green timber MC varies widely by species and season

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Ch 1 · Solved Examples

GATE CE / ESE level

A timber sample weighs 760 g in its green (as-received) state and 600 g after oven-drying. Find its moisture content, and state whether the sample is above or below the fibre saturation point.

Approach hint

Use $MC\% = (W_{green}-W_{oven\,dry})/W_{oven\,dry}\times100$, then compare with FSP ≈ 30%.

$MC\% = \dfrac{760-600}{600}\times100 = 26.7\%$
Compare with FSP ≈ 25–30%: 26.7% is below the typical FSP value

✓ MC ≈ 26.7%, which is just below FSP — the sample is already losing bound water, so measurable shrinkage and strength gain have begun

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Ch 1 · Diagrams

Click any diagram to zoom

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.

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Ch 1 · Quick Quiz

3 questions · tap an option to reveal answer

Question 1

Which layer produces new wood inward and new bark outward?

The cambium is a thin (1–2 cell) meristematic layer that produces xylem (wood) inward and phloem (bark) outward, driving growth in girth ✓

Question 2

What happens to timber strength as moisture content decreases ABOVE the fibre saturation point?

Above FSP, only free water (in cell lumens) is being removed, which causes no dimensional change and no strength gain; change only begins below FSP ✓

Question 3

Which cell type conducts water and is found ONLY in hardwoods, not softwoods?

Vessels are wide, thin-walled conducting tubes unique to hardwoods (angiosperms); softwoods (gymnosperms) rely on tracheids alone for both support and conduction ✓

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Ch 2 · Classification of Trees & Timber

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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)

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.

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Ch 2 · Key Reference Values

Durability Classification (IS 287)

$$ \text{I}>120\;|\;\text{II}=60\text{–}120\;|\;\text{III}=20\text{–}60\;|\;\text{IV}<20 $$

Life expectancy in years, in-ground contact without treatment

years

Softwood vs Hardwood Density

$$ \rho_{softwood}=300\text{–}600,\;\rho_{hardwood}=400\text{–}900 $$

kg/m³ typical range; exceptions exist (balsa is a very light hardwood)

kg/m³

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Ch 2 · Applied Scenario

"Which species/class?" — common ESE descriptive style

A railway department needs sleepers for a bridge with no maintenance access for over a century, and wants to avoid any preservative treatment. Which durability class, and which specific species, best fits this requirement?

Approach hint

"No treatment, >100 years" points directly to the highest IS 287 durability class.

Untreated service life > 120 years requires Durability Class I
Class I species: Teak, Deodar, Sal, Shisham, Sisso
Sal (density 800–900 kg/m³) is specifically favoured for railway sleepers due to its exceptional hardness and load-bearing capacity

✓ Class I durability; Sal is the classic choice for railway sleepers, with Teak and Deodar as alternatives

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Ch 2 · Diagrams

Click any diagram to zoom

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.

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Ch 2 · Quick Quiz

3 questions · tap an option to reveal answer

Question 1

Which of the following is botanically a HARDWOOD but is physically one of the softest woods known?

Balsa is an angiosperm (hardwood by botanical classification) but is famously one of the physically softest, lightest commercial woods — a classic exam trap ✓

Question 2

Per IS 287, Durability Class I timber has an untreated service life of:

Class I (Teak, Deodar, Sal, Shisham, Sisso) exceeds 120 years untreated — the highest IS 287 durability band ✓

Question 3

Microscopically, what is the key structural feature that distinguishes hardwoods from softwoods?

Hardwoods (angiosperms) have vessels for water conduction; softwoods (gymnosperms) rely entirely on tracheids and lack vessels ✓

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Ch 3 · Growth & Felling of Trees

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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/Ring-barking: 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.

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Ch 3 · Key Reference Values

Best Felling Season (India)

$$ \text{October–February} $$

Minimum sap flow; lowest post-felling fungal risk

—

Ideal Felling Age

$$ \text{Hardwood: }50\text{–}100+\text{yr}\;|\;\text{Softwood: }30\text{–}50\text{yr} $$

Age at which heartwood development is maximised

years

Girdling Duration Before Felling

$$ 1\text{–}2\text{ years} $$

Ring of bark removed; tree pre-dries naturally before felling

years

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Ch 3 · Applied Scenario

Descriptive-style reasoning question

A timber merchant has two identical logs of the same species and age — one felled in July (monsoon season) and one in December (winter, dry season). Which log will season faster, and which is at greater risk of blue stain or fungal attack immediately after felling?

Approach hint

Think about sap flow and ambient moisture at each time of year, and the conditions fungi need to grow (MC > 20%, warm, humid).

The December log is felled when sap flow is minimum and ambient humidity is lower, so its initial (green) moisture content is lower
The July log is felled during peak sap flow and high humidity, giving it a higher starting MC and warmer, wetter surrounding conditions favourable to fungi
Fungi require MC > 20% plus warmth and moisture — the July log satisfies these conditions for longer before it can be seasoned

✓ The December log seasons faster (lower starting MC); the July log carries a materially higher risk of blue stain and fungal attack before seasoning can begin

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Ch 3 · Quick Quiz

3 questions · tap an option to reveal answer

Question 1

The best season for felling timber in India is generally considered to be:

Winter/dry season felling coincides with minimum sap flow, giving lower green MC, less weight, and lower fungal risk after felling ✓

Question 2

Girdling (ring barking) is carried out:

Girdling removes a ring of bark 1–2 years before felling, causing gradual natural pre-drying through living transpiration ✓

Question 3

Within a single tree, which position generally has the HIGHEST density and strength?

The base of the trunk carries the highest structural load through the tree's life and develops the highest density and strength ✓

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Ch 4 · Conversion of Timber

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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.

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.

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Ch 4 · Key Reference Values

Scantling Area Limit

$$ A \leq 5400\ \text{mm}^2 $$

e.g. 50×50 mm to 75×75 mm cross-sections

mm²

Veneer Thickness

$$ t = 0.4\text{–}6\ \text{mm} $$

Sliced, peeled or sawn from the log; used for plywood manufacture

Ring Orientation — Quarter Sawn

$$ 45^\circ\text{–}90^\circ \text{ to board face} $$

Plain sawn: 0°–45° to board face

degrees

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Ch 4 · Applied Scenario

"Which sawing method?" — common SSC JE / ESE style

A flooring contractor wants boards for a high-traffic corridor that must resist cupping and show a decorative silver-grain figure, and the client is willing to pay a premium. Which sawing (conversion) method should be specified?

Approach hint

Match "minimum cupping + silver grain visible + premium acceptable" against the plain-sawn vs quarter-sawn comparison table.

Cup warping is driven by tangential shrinkage, which is highest in plain sawn boards
Quarter sawn boards have annual rings at 45–90° to the face, giving low (radial) shrinkage and minimal cupping
Quarter sawing also exposes medullary rays as a decorative silver-grain figure — exactly what is requested

✓ Quarter sawn (radial) — lower shrinkage/warping and exposed silver grain, at the cost of higher waste and price, matching both requirements

🪚

Ch 4 · Diagrams

Click any diagram to zoom

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.

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Ch 4 · Quick Quiz

3 questions · tap an option to reveal answer

Question 1

In quarter-sawn timber, the annual rings meet the board face at an angle of:

Quarter sawing cuts radially outward from the pith, so rings intersect the face at 45–90° ✓

Question 2

Which sawing method gives the maximum timber yield from a log but is more prone to warping?

Plain/flat sawn uses all-parallel cuts for maximum log recovery, but its higher tangential shrinkage causes more cup warping ✓

Question 3

A "slab" in timber conversion terminology refers to:

A slab is the outer, bark-bearing side cut removed first when squaring a log — mostly waste or low-grade material ✓

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Ch 5 · Seasoning of Timber

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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

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

💨

Ch 5 · Key Reference Values

Kiln Seasoning Duration

$$ 2\text{–}10\text{ days} \to MC = 6\text{–}15\% $$

Temperature schedule 40–80°C

days

Air Seasoning Duration

$$ \text{Hardwood: }1\text{–}3\text{ yr}\;|\;\text{Softwood: }3\text{–}6\text{ mo} $$

Final MC 12–18%, depends on ambient EMC

—

Stacking Dimensions

$$ \text{Sticker }25\times25\text{mm}\;|\;\text{Platform} > 450\text{mm} $$

Sticker spacing per layer; platform height above ground

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Ch 5 · Solved Example

GATE CE / ESE level

A batch of green Sal timber weighs 1200 kg at MC = 70%. Find (a) the oven-dry mass, and (b) the mass of water that must be removed to bring the timber down to a service MC of 15%.

Approach hint

Rearrange $MC\% = (W_{wet}-W_{dry})/W_{dry}\times100$ to get $W_{dry} = W_{wet}/(1+MC/100)$, then apply it again at the target MC to find the target wet mass.

Oven-dry mass: $W_{dry} = \dfrac{1200}{1+0.70} = \dfrac{1200}{1.70} = 705.9$ kg
Water present in green state: $1200 - 705.9 = 494.1$ kg
Target wet mass at 15% MC: $W_{15\%} = 705.9 \times 1.15 = 811.8$ kg
Water to be removed: $1200 - 811.8 = 388.2$ kg

✓ Oven-dry mass ≈ 705.9 kg; approximately 388.2 kg of water must be removed to reach 15% MC

💨

Ch 5 · Diagrams

Click any diagram to zoom

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.

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Ch 5 · Quick Quiz

3 questions · tap an option to reveal answer

Question 1

Kiln seasoning typically achieves the required moisture content in:

Kiln (artificial) seasoning is dramatically faster than air seasoning, typically taking 2–10 days to reach 6–15% MC ✓

Question 2

Fungal decay of timber becomes a risk when moisture content exceeds approximately:

Wood-decay fungi need MC > 20% (plus suitable temperature and oxygen) to establish and grow, per IS 1141 ✓

Question 3

Case hardening in kiln-seasoned timber is caused by:

When the surface dries and shrinks faster than the core, residual stresses lock in, leaving the shell in compression and the core in tension — this is case hardening ✓

🐛

Ch 6 · Defects in Timber

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Classification of Defects

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

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.

🐛

Ch 6 · Key Reference Values

Fungal Growth Conditions

$$ MC > 20\%,\; T = 25\text{–}35^\circ C,\; O_2 \text{ present} $$

Removing any one condition prevents fungal growth

—

Knot Size Limit (IS 1331)

$$ \text{Grade 1} \leq \tfrac{1}{4}w,\;\text{Grade 2} \leq \tfrac{1}{3}w $$

w = member width; larger knots downgrade the timber

—

Slope of Grain Limit

$$ \text{Grade 1} \leq 1{:}15,\;\text{Grade 2} \leq 1{:}12 $$

Diagonal grain steeper than 1:15 is rejected for Grade 1 structural use

—

🐛

Ch 6 · Applied Scenario

Common ESE descriptive style

During a site inspection, a teak beam that was struck heavily during felling and transport shows no visible cracks, but the site engineer suspects it may be structurally compromised. Which defect is most likely present, why is it especially dangerous, and how should it be checked?

Approach hint

Think of the defect specifically associated with impact/compression during felling that leaves no visible external sign.

Impact or compression during felling/transport is the classic cause of upsetness (compression failure of fibres)
Upsetness is dangerous precisely because the timber appears externally sound while its tensile and bending strength are severely reduced
Detection requires closer inspection: careful bend/load testing, or examining for faint wavy/crimped grain lines under raking light — a simple surface check is not reliable

✓ Upsetness — the most dangerous timber defect precisely because it is invisible externally; verify with a load/bend test or detailed grain inspection before using the beam structurally

🐛

Ch 6 · Diagrams

Click any diagram to zoom

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

🐛

Ch 6 · Quick Quiz

3 questions · tap an option to reveal answer

Question 1

A "cup shake" is a separation that occurs:

Cup (ring) shake is a separation along the curved (tangential) direction of an annual ring, distinct from heart shake which is radial ✓

Question 2

Which timber defect is considered the MOST dangerous because it is invisible externally?

Upsetness (compression failure of fibres) leaves timber looking externally sound while its tensile and bending strength is severely reduced ✓

Question 3

The Common Furniture Beetle (Anobium) typically attacks:

Anobium attacks sapwood of essentially any species under dry conditions, leaving round exit holes 1.5–2 mm dia and fine bore dust (frass) ✓

🛡

Ch 7 · Preservation of Timber

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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

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).

🛡

Ch 7 · Key Reference Values

CCA (Bethell) Pressure

$$ p = 10\text{–}14\ \text{kg/cm}^2 $$

Applied after initial vacuum removes air from cells

kg/cm²

CCA Retention

$$ 8\text{–}16\ \text{kg/m}^3 \text{ (above ground)},\;16\text{–}25\ \text{kg/m}^3 \text{ (ground contact)} $$

Higher retention required where in permanent ground/damp contact

kg/m³

Creosote Retention (Sleepers)

$$ 100\text{–}160\ \text{kg/m}^3 $$

Per IS 1477, for railway sleeper application

kg/m³

🛡

Ch 7 · Solved Example

GATE CE / ESE level

A batch of utility poles requires CCA treatment for ground-contact service, using the IS 401 retention range of 16–25 kg/m³. If each pole has a volume of 0.08 m³, find the minimum and maximum mass of CCA salt required per pole.

Approach hint

Retention (kg/m³) is a mass-per-volume figure — multiply directly by the pole's volume.

Minimum: $16 \times 0.08 = 1.28$ kg per pole
Maximum: $25 \times 0.08 = 2.00$ kg per pole

✓ Between 1.28 kg and 2.00 kg of CCA salt is required per pole to satisfy the IS 401 ground-contact retention range

🛡

Ch 7 · Diagrams

Click any diagram to zoom

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.

🛡

Ch 7 · Quick Quiz

3 questions · tap an option to reveal answer

Question 1

The full-cell pressure process is also known as the:

The Bethell process is the classic full-cell method: initial vacuum, fill with preservative, apply pressure, then final vacuum — giving maximum retention ✓

Question 2

CCA pressure treatment (Bethell process) typically applies a pressure of:

10–14 kg/cm² is the standard IS 401 pressure range applied after the initial vacuum step in the Bethell process ✓

Question 3

The Boucherie process is a non-pressure preservation method that works by:

The Boucherie (sap replacement) process caps one end of a green log with a preservative reservoir; the hydraulic head drives preservative in as sap is pushed out the other end ✓

💪

Ch 8 · Mechanical Properties of Timber

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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

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
$MOR = \frac{3PL}{2bh^{2}}$ 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 ($\sigma_{c||}$): Much higher than perpendicular; fibres act as columns; buckling controlled by slenderness; typical values 35–60 MPa (air-dry)
Perpendicular to grain ($\sigma_{c\perp}$): Much lower; cell walls crush; typical 5–12 MPa; governs bearing under beams, bolts, washers
Ratio $\sigma_{c||}/\sigma_{c\perp}$ ≈ 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 $E_{\perp}$: 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 ($K_1$): Strength values at 12% MC; for wet service (MC > 18%), multiply by $K_1 = 0.7$–$0.8$ depending on property
Duration of load factor ($K_2$): Long-term load → apply $K_2 = 0.5$–$0.8$; short-term (wind/earthquake) → $K_2 = 1.25$
Size factor ($K_3$): For beams deeper than 300 mm; $K_{3} = \left(\frac{300}{d}\right)^{0.11}$
Creep factor: For sustained deflection; long-term deflection = short-term × (1 + creep factor)
Temperature factor ($K_4$): For T > 40°C; reduce strength by $K_4$; 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

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	$\sigma_{c\|\|}$
Compression ⊥ grain	50×50×150	IS 1708 Pt 8	$\sigma_{c\perp}$
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	$\tau$ // 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	$MC\% = (W_{wet}-W_{dry})/W_{dry} \times 100$
Density	50×50×50 clear wood	IS 1708 Pt 1	Density at known MC

💪

Ch 8 · Formulas

Modulus of Rupture (centre-point load)

$$ MOR = \frac{3PL}{2bh^{2}} $$

P = load, L = span, b = width, h = depth

N/mm²

Modulus of Elasticity (from bending)

$$ MOE = \frac{PL^{3}}{4bh^{3}\delta} $$

δ = mid-span deflection at load P

N/mm²

Size Factor (IS 883)

$$ K_{3} = \left(\frac{300}{d}\right)^{0.11} $$

Applied to beams deeper than 300 mm; d = actual depth (mm)

—

Compression Ratio (∥ to ⊥ grain)

$$ \sigma_{c||}/\sigma_{c\perp} \approx 5\text{–}10 $$

Compression parallel is far higher than perpendicular

—

Long-term vs Short-term Strength

$$ f_{long} \approx 0.5\text{–}0.6 \times f_{short} $$

Duration-of-load (creep) effect, per IS 883 factor $K_2$

—

💪

Ch 8 · Solved Examples

GATE CE / ESE level

A clear-wood beam specimen (50 mm × 50 mm cross-section, 760 mm span) fails under a centre-point load of 12 kN in a static bending test. Compute the modulus of rupture.

Approach hint

Use $MOR = 3PL/(2bh^2)$ with P in N, L, b, h in mm.

$P = 12{,}000$ N, $L = 760$ mm, $b = h = 50$ mm
$MOR = \dfrac{3 \times 12{,}000 \times 760}{2 \times 50 \times 50^{2}} = \dfrac{27{,}360{,}000}{250{,}000}$
$MOR = 109.4$ N/mm²

✓ MOR ≈ 109 MPa — consistent with a dense hardwood such as Sal (typical range 90–110 MPa) tested as clear (defect-free) wood

A structural timber beam has an actual depth of 450 mm. Find the IS 883 size factor $K_3$ to be applied to the tabulated grade stress.

Approach hint

Apply $K_3 = (300/d)^{0.11}$ directly with d = 450 mm.

$K_3 = (300/450)^{0.11} = (0.667)^{0.11}$
$\ln(0.667) = -0.405$; $-0.405 \times 0.11 = -0.0446$
$K_3 = e^{-0.0446} \approx 0.956$

✓ $K_3 \approx 0.956$ — a small (~4.4%) reduction in grade stress for the deeper-than-standard beam

💪

Ch 8 · Diagrams

Click any diagram to zoom

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.

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.

💪

Ch 8 · Quick Quiz

3 questions · tap an option to reveal answer

Question 1

For centre-point loading in a standard static bending test, the modulus of rupture is given by:

$MOR = 3PL/2bh^2$ is the standard formula for centre-point loading on a simply supported beam ✓

Question 2

Among timber's mechanical properties, which is MOST sensitive to changes in moisture content below FSP?

MOR shows the steepest strength gain as MC drops below FSP, while tension perpendicular to grain is the least sensitive property ✓

Question 3

Timber's long-term (sustained load) strength is approximately what fraction of its short-term strength?

Due to creep under sustained load, timber's long-term strength drops to about 50–60% of its short-term (instantaneous) strength — captured by IS 883's duration-of-load factor $K_2$ ✓

🏗

Ch 9 · Special Wood Products

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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.

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

🏗

Ch 9 · Key Reference Values

Glulam Design Stress

$$ f_{glulam} \approx 1.15\text{–}1.25 \times f_{solid} $$

Higher than solid timber of same grade due to redistribution of defects

—

Glulam Char Rate

$$ 0.65\text{–}0.8\ \text{mm/min} $$

Char layer insulates structural core in fire

mm/min

Bamboo Tensile Strength

$$ 200\text{–}350\ \text{MPa} $$

Outer fibres, parallel to culm axis

MPa

🏗

Ch 9 · Applied Scenario

"Which product?" — common SSC JE / ESE style

A boat builder needs plywood for a hull that will be continuously exposed to seawater and requires the most waterproof, durable adhesive bond available. Which plywood grade should be specified?

Approach hint

Compare MR, BWR and marine/BWP grades on adhesive type and intended exposure.

MR grade (UF adhesive) is for interior, non-wet use only — unsuitable
BWR grade (PF adhesive) resists boiling water but is intended for semi-exterior use
Marine plywood (IS 710) uses PF adhesive with preservative treatment, specifically rated for permanent water/marine exposure

✓ Marine plywood per IS 710:2010 — PF-bonded with preservative treatment for continuous marine/watercraft exposure

🏗

Ch 9 · Diagrams

Click any diagram to zoom

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.

🏗

Ch 9 · Quick Quiz

3 questions · tap an option to reveal answer

Question 1

Plywood must always be constructed with an ODD number of plies because:

With plies at 90° to their neighbours, an odd count keeps the outer face and back plies grain-parallel, balancing stresses on both sides and preventing warping ✓

Question 2

MDF (Medium Density Fibreboard) has a typical density range of:

MDF (IS 12406) falls in the 600–800 kg/m³ range, denser than particle board but less dense than hardboard (> 800 kg/m³) ✓

Question 3

Compared to solid timber of the same grade, glulam's design stress is typically:

Laminating redistributes defects across many thinner layers, allowing glulam to be designed to 15–25% higher stresses than solid timber of the same grade ✓

📚

Ch 10 · Revision & Practice

IS codes, all formulae, quick-recall tables, mnemonics, PYQ pattern, mini quiz and practice questions

○ Not Started

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 — Timber

Moisture Content

$$ MC\% = \frac{W_{green}-W_{oven\,dry}}{W_{oven\,dry}}\times100 $$

FSP ≈ 25–30% (30% commonly used)

Modulus of Rupture (centre-point load)

$$ MOR = \frac{3PL}{2bd^{2}} $$

Standard static bending test

N/mm²

Modulus of Elasticity (from bending)

$$ MOE = \frac{PL^{3}}{4bd^{3}\delta} $$

δ = mid-span deflection at load P

N/mm²

Shrinkage

$$ S = \frac{D_{green}-D_{dry}}{D_{green}}\times100\% $$

Tangential S ≈ 2 × Radial S; Longitudinal S < 0.5%

Size Factor (IS 883)

$$ K_{3} = \left(\frac{300}{d}\right)^{0.11} $$

For beams deeper than 300 mm

—

Janka Hardness

$$ \varnothing 11.28\text{mm ball}\to\text{embed }d/2,\; A=1\text{cm}^2 $$

Load to embed the standard steel ball to half its diameter

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

GATE/ESE Question Frequency by Topic

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.

Mini Quiz

A green timber sample weighs 900 g and its oven-dry mass is 500 g. What is its moisture content?

$MC\% = (900-500)/500 \times 100 = 80\%$, well within the typical green-timber range of 50–200% ✓

Per IS 883, which species group carries the HIGHEST characteristic bending (MOR) stress?

Group A (Teak, Sal, Shisham) carries the highest tabulated bending stress at 18.0 N/mm² ✓

Which preservation process uses an initial VACUUM step before filling with preservative?

The Bethell (full-cell) process starts with an initial vacuum to remove air from the cells before the preservative is introduced under pressure ✓

Tangential shrinkage compared to radial shrinkage is approximately:

Tangential shrinkage is roughly 2× radial shrinkage — the key mechanism behind cup warping in plain-sawn boards ✓

Which of these is Durability Class I (life > 120 years) per IS 287?

Sal is Class I (Teak, Deodar, Sal, Shisham, Sisso); Poplar and Rubber wood are Class IV; Chir Pine is Class III ✓

Practice Questions

EasyState the moisture content formula and the typical moisture content range for green timber.

$MC\% = (W_{green}-W_{oven\,dry})/W_{oven\,dry}\times100$; green timber typically ranges from 50% to 200% depending on species and season of felling.

EasyWhy must plywood always be constructed with an odd number of plies?

Adjacent plies are bonded at 90° to each other; an odd ply count keeps the outermost face and back plies grain-parallel, balancing the panel on both sides and preventing warping.

MediumA timber merchant is offered two lots at the same price: one Class III kiln-dried Chir Pine, one Class IV air-dried Poplar, both for permanent outdoor decking without any preservative treatment. Which offers better long-term value, and why?

The Class III Chir Pine is the better choice: its untreated service life (20–60 years) is materially longer than Class IV Poplar's (< 20 years), even though both would ideally be preservative-treated for genuinely long-term outdoor service; the kiln-dried state is also an added durability advantage over air-dried since it starts service at a lower, more stable MC.

Medium500 kg of green Deodar timber at 90% MC must be dried to 12% MC before delivery. Compute the mass of water that must be removed.

Oven-dry mass = 500/(1+0.90) = 263.2 kg. Target wet mass at 12% MC = 263.2 × 1.12 = 294.7 kg. Water to be removed = 500 − 294.7 ≈ 205.3 kg.

HardExplain, from the cell-wall mechanism, why timber's mechanical properties change only below the fibre saturation point, and why the modulus of rupture (MOR) is the most MC-sensitive property while tension perpendicular to grain is the least sensitive.

Above FSP, only free water in the cell lumens is present; removing it causes no change to the cellulose microfibril structure in the cell wall, so no shrinkage or strength change occurs. Below FSP, bound water between the microfibrils is removed, drawing the microfibrils closer together (hydrogen bonding increases) and stiffening/strengthening the wall — this is the mechanism behind both shrinkage and strength gain. MOR (bending) is a composite property governed heavily by tension and compression along the grain, both of which depend strongly on the tightly-bonded, stiffened microfibril network that forms as bound water is lost — making it highly MC-sensitive. Tension perpendicular to grain instead depends mainly on the weaker lignin/hemicellulose matrix bonding between adjacent cells (not the microfibril network itself), which is far less affected by moisture-driven microfibril tightening — making it the least MC-sensitive property.

Learning Objectives

Exam Relevance

Concept Map

Where This Fits

Macroscopic Structure

Microscopic Structure

Chemical Composition of Wood

Moisture in Wood

Botanical Classification

Exogenous Trees (grow from outside)

Endogenous Trees (grow from inside)

Softwood vs Hardwood — Detailed Comparison

Classification by Durability (IS 287)

Commercially Important Indian Timbers

Factors Affecting Timber Quality

Correct Time for Felling

Methods of Felling

After Felling – Immediate Processing

Definition and Importance

Comparison – Plain Sawn vs Quarter Sawn

Standard Timber Sections (IS 1200, IS 4891)

Definition and Purpose

Why Seasoning is Necessary

Methods of Seasoning

1. Natural (Air) Seasoning

2. Artificial (Kiln) Seasoning

3. Water (Boiling/Immersion) Seasoning

4. Chemical Seasoning

5. Electrical Seasoning

Defects Caused by Incorrect Seasoning

Classification of Defects

A. Defects Due to Natural Causes

Knots

Shakes

Other Natural Defects

B. Defects Due to Fungi

C. Defects Due to Insects

D. Defects Due to Conversion (Manufacturing)

E. Defects Due to Seasoning

Purpose of Preservation

Types of Preservatives

1. Oil-Based Preservatives

2. Water-Soluble Preservatives

3. Organic Solvent Preservatives

Methods of Applying Preservatives (IS 401)

Pressure Methods (Best Penetration and Retention)

Non-Pressure Methods

Requirement of Good Preservative

Key Factors Affecting Mechanical Properties

Mechanical Properties – Definitions and Values

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

Compression Strength

Tensile Strength

Shear Strength

Modulus of Elasticity (E)

Hardness (Janka Hardness)

Impact Strength (Toughness)

IS 883:2016 – Design of Structural Timber in Building

Modifications / Adjustment Factors (IS 883)

Grading of Structural Timber (IS 1331:1975)

Standard Timber Tests

Overview

Plywood – Detailed Notes

Fibre Boards

Particle Board (Chipboard)

Glulam (Glued Laminated Timber)

Bamboo

Complete IS Codes for Timber

All Key Formulae — Timber

Critical Numerical Values – Direct Exam Use

GATE/ESE Question Frequency by Topic

Mnemonics and Quick Memory Aids

GATE/ESE Previous Year Question Patterns

Mini Quiz

Practice Questions

Related Topics & Continue Learning