4. Classification of Timber
• Mode of Growth
a. Exogens (Exogenous) – These trees grow outward.
Coniferrous Trees
Deciduous Trees
b. Endogens (Endogenous) – These trees grow inwards.
5. Classifications of Timber
• Modulus of Elasticity
Group A: E = 12.5 GPa
Group B: E = 9.8 GPa to 12.5 GPa
Group C: E = 5.6 GPa to 9.8 Gpa
• Durability
High Durability
Moderate Durability
Low Durability
6. Classifications of Timber
• Seasoning characteristics
High refractory (Class A)
Moderately refractory (Class B)
Non refractory (Class C)
• Grading
Select-Grade
Grade I
Grade II
7. Identifications of Timber
• General Properties
Color – a darker color in wood indicates greater durability.
Odor – it is present only on freshly cut trees.
Hardness – is the ability of wood to withstand indentations caused by
harder bodies.
Density – densest woods are generally the strongest.
8. Identifications of Timber
Grain - Depending on the actual alignment, the grain may be straight,
spiral, interlocked, wavy or irregular.
• Spiral Grain
• Interlocked Grain
• Wavy Grain
Texture - In hardwoods, the texture depends upon the size and
distribution vessels and rays. In softwoods, it is determined by the size
and distribution of tracheid.
9. Identifications of Timber
Luster- is the ability to reflect light beam pointedly.
Workability- the relative case in which wood is shaped cut and fastened
together than the others.
Warping- is the general term used to describe any variation from a true
surface.
Moisture content- is a percentage of the mass of water over the mass
of wood fibre in a piece of timber.
Specific Gravity- is the ratio found by dividing the weight of a
substance by the weight of an equal volume of pure water.
10. Structure of Softwoods
• Tracheids - These are hollow, needle-shaped units closely packed together
so that a cross-section through them resembles a honeycomb.
• Storage Tissues (Rays and Wood Parenchyma Cells) - They are too
small to be seen individually and are not of much in identification.
• Resin Canals - these canals are not cells but are cavities in the wood lined
by the parenchyma cells.
• Growth Rings - these are normally distinct in softwoods.
11. Structures of Hardwoods
• Pores or Vessels – these are tubular elements running in a vertical direction and
serve to conduct water and mineral nutrients.
• Inclusions – the pores or vessels are normally open to enable conduction but
many times, particularly in the heartwood, they become filled with inclusions of
various types.
• Parenchyma or Soft Tissues - these are storage tissues and consist of small,
thin-walled, rectangular or brick-shaped cells. These are vertically aligned.
• Rays - are groups of horizontally aligned parenchyma cells running in a radial
direction and their function is horizontal conduction and storage.
12. Structures of Hardwoods
• Fibres – These are vertically-aligned, thick-walled elements with pointed,
tapering ends which make up the bulk of the weight of the wood.
• Intercellular cells – These are long tubular cavities seen in the horizontal or
vertical direction.
13. Felling and Conversion
• Felling and Transporting the Timber
Felling is done by a clean saw cut as near to the base as possible.
Transporting the log to saw mill is done either by road transport or
dragging (by elephants) to the nearest river and floating them down in
rafts.
14. Felling and Conversion
• Sawing
Methods of Sawing
Ordinary or Flat Sawing
Tangential Sawing
Radial Sawing
Quarter or Rift Sawing
Combination Sawing
15. Moisture of Timber
• Moisture Content - It is one of the most important characteristics of
timber that affects its use on the site performance.
• Equilibrium Moisture Content (EMC) - moisture content of wood
changes in response to the temperature and humidity of its surroundings.
• Fibre Saturation Point (FSP) – when the cell cavities are empty but the cell
walls still retain their bound water.
16. Moisture of Timber
• Shrinkage and Swelling - Shrinkage occurs as moisture content decreases,
while swelling takes place when it increases.
17. Moisture of Timber
• Timber Drying :
Natural Drying or Air Drying
Artificial Drying or Kiln Drying
18. Mechanical Properties
• Tensile Strength – Timber is stronger in tension along the rain but it’s quite
difficult to determine this because of the difficulties in conducting test.
• Compressive Strength – The strength along the grain is important for
columns, props, and post.
• Shear Strength – Shear strength is important in the case of the beam and
slabs.
• Bending Strength or Flexural Strength – This refers as the strength of the
timber as a beam.
19. Mechanical Properties
• Cleavability – High resistance for cleavage is important for nailing and screwing
while low splitting strength is important for used as firewood.
• Brittleness – is use to describe the property of suffering little deformations before
breaking.
• Torsion Strength – is used to determine the torsion strength of the timber and
the specimen is loaded up to failure.
• Hardness – is important in case of timber for paving blocks flooring bearings and
other similar purposes.
• Stiffness - This property is important to determine the deflection of a timber under
a load
20. Defects in Timber
Defects caused in timber may be basically classified into two categories:
Internal Defects – due to factors affecting the growing tissue
or the tree.
External Defects – due to external agents or subsequent
treatment of timber.
21. Classification of Defects in Timber
• Knots - are dark, hard pieces occurring as signs of branches broken or cut
off.
According to size (diameter)
Pin Knot – not more than 6.5mm
Small Knot - more than 6.5mm but not more than 20mm
Medium Knot- more than 20mm but not more than 40mm
Large Knot- more than 40mm
22. Classification of Defects in Timber
According to form
Round or Oval Knot- a knot cut more or less at right angles to its
long axis so that the exposed is round or oval.
Splay or Spike Knot- a knot cut approximately parallel to its long axis
so that the exposed section has an elongated shaped.
23. Classification of Defects in Timber
According to quality
Live Knot- a knot whose fibers are completely intergrown with those
of the surrounding wood.
Sound Knot- a live knot free from decay.
Tight Knot- a live or dead knot held firmly in place
Dead Knot- a knot whose fibers are not intergrown with those of
the surrounding wood.
24. Classification of Defects in Timber
According to quality
Loose Knot- a dead knot which is not held firmly in place.
Knot Hole- a hole left as results of the removal of a knot.
Decayed Knot- a knot softer than the surrounding wood, and
containing decay.
25. Classification of Defects in Timber
• Shakes- or cracks are caused by the rupture of tissues resulting in partial or
complete separation of the fibers along the grain.
Star Shakes
Heart Shakes
Cups Shakes
Ring Shakes
Radial Shakes
26. Classification of Defects in Timber
• Rind Galls- these are typical curved swelling formed
upon the trunk of the tree by successive
layer of the sapwood.
• Upsets- are clipping or buckling of fibers caused by
crushing of fibers when the trunk is felled
violently over a rocky surface.
27. Classification of Defects in Timber
• Twisted or Spiral Grain- fibers or wandering hearts
are caused by the prevalent wind turning or
twisting the tree at its young age constantly in
one direction.
• Wind Cracks- are shakes on the outside of a log due
to the shrinkage of the exterior surface caused
by the atmospheric influences.
28. Defects Due to Seasoning
• Bow- a curvature of the timber in the direction of its
length.
• Cup- a curvature in the transverse direction of the
timber.
• Twist- a spiral distortion along the length of the
timber.
• Spring- a curvature of the timber in its own plane.
29. Defects Due to Seasoning
• Case Hardening- it is due to the unequal drying of
the exterior surfaces under compression and the
interior surfaces under tension due to rapid drying.
• Split- separation of the fibers along the grain and
extends from one end of the plank to the other.
• Honey Combing- separation of the fibers in the
interior due to drying stresses.
30. Defects Due to Seasoning
• Collapse - the cells of timber are flattened due to excessive shrinkage.
31. Defects Due to Manufacturing or Use
• Chipped or Torn Grain- a defect caused by the
breaking away of timber below the surface of
the dressing by the action of the planning tool.
• Chip Mark- indentation on the finished surface
of the timber caused by the chips or other small
pieces of the timber being carried around on the
planning knife edges.
32. Defects Due to Manufacturing or Use
• Wane- the original round surface of a tree which
remains on finished plank.
• Boxed Heart- the pith at the center fully
enclosed within one piece in the process of
conversion.
33. Evaluation of Defects of Timber
• Knots- tend to weaken timber in tension but may improve the strength in
compression.
• Checks, Splits and Shakes- these reduce the shearing resistance of the wood.
• Compression Wood- increase density and shrinkage along the grain and
decreased shock resistance.
• Tension Wood- it has high longitudinal shrinkage tending it to warp and split.
• Sap Rot- this results in decreased shock resistance and increased absorption.
• Slope of Grain- this lowers the tensile strength parallel to the grain and the
modulus of rupture which increases in the case of radial slope of grain.
34. Evaluation of Defects of Timber
• Loose Grain- weaker than closed grain timber.
• Wane- this reduce the mechanical properties and volume.
• Worm Hole- deep, large worm holes spoil the appearance and reduce the
mechanical properties of timber.
• Pith Pockets- affect the appearance and reduce the mechanical properties of
timber.
• Boxed Heart- in cut sizes of timber, it does not affect the quality appreciably.
• Discoloration of Heartwood- it does not affect the mechanical properties and this
defect can be ignored in preservative treated timber.
35. Evaluation of Defects of Timber
• Mould- this essentially spoils the appearance of the wood.
• Sap Stain- which the stain is the most important, does not affect the
strength properties of timber.
36. Decay of Timber
• Decay or Rot - is usually caused by the presence of sap or by the alternate
dry and wet condition of the timber.
Forms of Decay
Wet rot
Dry rot
Due to fungal attack
Due to insects
37. Decay of Timber
Wet Rot - This is a decomposition of sap and
fibers of the living tree set up by the access of
water through wounds in the barks.
Dry Rot - This is a decomposition of converted
timber induced by the commonest and most
destructive dry rot fungi or weeping fungus.
38. Decay of Timber
Due to fungal Attack – A fungus starts its life as a tiny seed or spore, one
of millions produced at the fruiting stage.
o Fungi are system plant organisms which live on and attack timber causes
it to rot or decay.
o Fungi can only exist in following condition:
Air for their respiration
Moisture
Suitable Temperature
Food Supplies
39. Decay of Timber
Due to Insects – damage to timber by insects is quite considerable
particularly in warm country.
o Types of Insects:
Termites
Beetles
Boarers
43. Seasoning of Timber
• OBJECTIVES OF SEASONING
-To reduce movement of timber, to reduce the tendency to split.
-To make it immune from attack by insects.
-To increase strength, durability, workability and resilience.
-To make the timber receptive to finish like preservatives, paints, and
varnish.
-To reduce weight and minimize cost of transportation.
-To make the timber burn readily, if used as a fuel.
44. Seasoning of Timber
• Principles of seasoning
Three factors controlling this process:
Relative Humidity
Rate of Air Circulation
Temperature
45. Seasoning Characteristics of Timber
• Class A (Highly refractory woods) – These timbers are slow and difficult
to dry if the final product is to be free from defects, particularly cracks and
splits.
• Class B (Moderately refractory woods) – These timbers show a moderate
tendency to crack and split during seasoning.
• Class C (Non-refractory woods) – These timbers can be rapidly seasoned
to be free from defects even by applying high temperatures in industrial kilns.
46. Methods of Seasoning
• Natural Seasoning - is a slow process and the reduction in moisture content to the
desired level may not be attainable by air seasoning alone.
• Artificial or Kiln Seasoning - is a quicker method of seasoning timber t the
desired moisture content under highly controlled conditions.
Progressive kiln
Compartment kiln
• Electrical Seasoning - in this method high frequency alternate electric current is
passed through timber.
• Chemical Seasoning - If the outer layers of timbers are treated with such
solutions, the vapor pressure of the outer layers is reduced and a vapor pressure
gradient is set up.
47. Preservation of Timber
• Preservatives Types:
Type 1 (Oil Type) – coal tar creosote with or without admixture of
petroleum, coal tar, fuel oil or other suitable oils having high boiling range.
Type 2 (Organic solvent type) – consist of toxic chemical compounds
dissolved in a non- aqueous solvent like volatile oils, spirits, naphtha etc.
Type 3 ( Water-soluble [leachable] ) – these preservatives are inorganic
salts soluble in water.
Type 4 ( Water-soluble [Fixed] ) – These consists of the various salt
described above with the addition of fixative salt, usually sodium or
potassium dichromate.
48. Preservation of Timber
• Characteristics of a Good Preservative
It should be strongly toxic to insects and fungi
It should penetrate readily into the timber.
It should not be soluble in water nor should it change in composition under
ordinary climatic condition.
It should cover a large surface area with a comparatively small quantity of
preservative.
It should have no destructive influence on the strength of the timber.
It should be harmless to human beings.
49. Preservation of Timber
• Characteristics of a Good Preservative
It should not render the timber treated with it more inflammable.
It should not be non-corrosive to metals.
It should be pleasant in color and give an attractive appearance to the wood
work.
It should be highly resistant to water and dampness.
It should be economical and available readily.
50. Preparation of Timber for Treatment
• Methods of Treatment
• Surface Application • Full Cell or Bethel Process
• Soaking • Empty Cell Process
• Hot and Cold Process • Choice of Treatment
• Boucherie Process
• Diffusion Process
• Pressure or Pneumatic Process
51. Fire-proofing of Timber
• Timber about a particular thickness, cannot support self-sustained combustion
and continue to burn only when external heat is applied.
• Timber cannot be made fire proof but can only be made fire resistant.
• It is supposed that the fire-proofing chemicals act in one of the following ways:
The melting point of the chemicals is low so that it melts and forms a
barrier to the supply of oxygen to the inside.
The chemical decomposes under heat, yielding non-inflammable gases that
dilute the inflammable gases. This retards the ignition of inflammable point.
The chemical vaporizes at sufficiently low temperature, absorbing sufficient
heat that the temperature of wood does not rise to decomposition point.
52. Availability and Uses of Timber
• Common Timber
Teak - The sapwood is pale yellowish or greyish-white in color and is
distinguished from heartwood. The heartwood is golden brown and turns a
dull deeper brown on exposure.
Deodar - The sapwood is white to creamy white and is distinct from the
heartwood which is light yellowish brown turning to pinkish brown on
exposure.
Chir - The sapwood is white to creamy white and is distinct from the
heartwood which is light yellowish brown turning to pinkish brown on
exposure.
53. Availability and Uses of Timber
• Common Timber
Kail - The yellowish in white to greyish sapwood is white to pale-yellowish
white in color and is distinct from the heartwood which is light pinkish-red
to light red.
Sisso - Sapwood of sisso is pale yellowish or greyish white and is distinct
from the heartwood which is golden brown or dark brown in color.
Sal - The sapwood is pale-yellowish or brownish white and is distinct from
the heartwood which is brown or reddish brown.
Mango - This is brown in color.
54.
55.
56.
57.
58. Availability and Uses of Timber
• Advantages of Timber
In terms of specific strength.
Timber can be used for both load bearing structures and non-load bearing
instruments.
It can be easily converted to any shape and size.
With the advent of superior adhesives and other forms of timber
connectors.
The wastage in timber construction is less because all wastes can be put one
use or the other.
59. Availability and Uses of Timber
• Advantages of Timber
It has high resale and salvage value.
Timber has better insulating properties than any other materials.
Durability of timber can be very high, if properly seasoned and preserved.
Timber constructions are light in weight and can be made economical.
For aesthetic and decorative appearance.
60. Wood-Based Product
• Veneers - thin sheet of wood, 0.4 mm to 0.6 mm
in thickness obtained by different knife cutting
process.
Rotary veneers
Cutting sliced veneers
• Plywood - is made by gluing together veneers.
61. Wood-Based Product
• Plywood
Plywood grades and sizes
Boiling water proof (BWP) grade
Boiling water resistance (BWR) grade
Warm water resistance (WWR)
Cold water resistance (CWR) grade
63. Wood-Based Product
• Plywood
• Dimensions (sizes of plywood)
(cm x cm) (cm x cm)
240 x 120 180 x 90
240 x 90 150 x 120
210 x 120 180 x 90
210 x 90 120 x 120
189 x 120 90 x 90
64. Tolerance of plywood
Dimension Nominal size Tolerance
Length Up to and including 120 cm
Above 120
+ 3 mm
- 0 mm
+ 6 mm
- 0 mm
Width Up to and including 90 cm
Above 90 cm
+ 3 mm
- 0 mm
+ 6 mm
- 0 mm
Thickness Up to and including 5 mm
6 to 9 mm
Above 9 mm
± 10%
± 7%
± 5%
65. Wood-based Products
• Advantages of plywood
It has a good strength both across as well as along the grain.
Plywood has better splitting resistance due to grains in adjacent layers
running at the right angle and nailing and screwing even closed to the edges
in safe.
Can easily be bending to a curvature limited by the tensile and the
compressive strength of the other plies.
The face piles made at different heartwoods show great aesthetic variety in
grain pattern and color.
66. Wood-based Products
• Hardboard - manufactured from raw materials like
wood waste, veneer manufacturing, furniture making.
Sizes of hardboards
Medium hardboard
Normal hardboard
Tempered hardboard
67. Wood-based Products
• Thickness of Hardboards
Type (mm) Nominal thickness
(mm)
Tolerance
Medium hardboard 6
8
10
12
± 0.5
± 0.7
± 0.7
± 0.9
Standard hardboard 3, 4, 5
6
9
± 0.4
± 0.5
± 0.7
68. Wood-based Products
• Width and Length of Hardboards
Type Width (m) Tolerance on
width,
Max. (mm)
Length (m) Tolerance on
length max.
(mm)
Medium
hardboard
1.2 ± 3 1.2, 1.8, 2.4 ± 3
Standard
hardboard
1.2 ± 3 3.0, 3.6, 4.8 ± 3
Tempered
hardboard
1.2 ± 3 5.5 ± 3
69. Wood-based Products
• Particle boards - is a board manufactured
from particles of wood or other lingo
cellulose materials.
• Block board – these are boards having a
core made up strips of wood each not
exceeding 25 mm.
Grade 1
Grade 2
71. Wood-based Products
• Other boards
• Batten board – it is a board having a core made
up of strips of wood usually 8 cm wide each
laid separately or glued or otherwise joined to
form a slab.
• Lamin board – it is a board having core strips,
each not exceeding 7 mm in thickness.
72. Wood-based Products
• Adhesives
Synthetic Resins – it has a composition consisting substantially of
phenolic or amino plastic resins including a hardening agent, fortifier, filler
or extender.
Four types of synthetic resin
Boiling Water Proof (BWP)
Boiling Water Resistant (BWR)
Warm Water Resistant (WWR)
Cold Water Resistant (CWR)
73. Wood-based Products
Classification According to their Uses
Gap filling adhesive – suitable use in situation where the surfaces to
be joined may or may not be in continuous contact.
Close contact adhesive – where the surfaces to join can be brought
into closed contact by means of adequate pressure.
74. Wood-based Products
• Anima Glue – it is generally obtained from hides and bones of animals.
• Vegetable Glue – it is made from natural grains and starch-bearing roots,
potato, corn, wheat, etc. with acid like Sodium Hydroxide (NaOH).
