UNIT 2 DEFECTS IN TIMBER

Structure
2.1 Introduction
Objectives
2.2 Definitions .
2.2.1 Properties
2.2.2 Drawbacks
2.3 Classification of T i r
. 2.3.1 Exogenous T m
2.3.2 Endogenous Trees
2.4 Structure of a Tree
2.4.1 Macrostructure
2.4.2 Microstructure
2.5 Defects in Timber
2.5.1 Defects due to Conversion
2.5.2 Defects due to Fungi
2.5.3 Defects due to Ingects
2.5.4 Defects due to Natural Force
2.5.5 Defects due to Seasoning
2.6 Qualities of Good Timber
2.7 Decay of Timber
2.8 Summary
2.9 Answers to SAQs

2.1 INTRODUCTION - - ---

As you b o w , Timber is one of the important construction materials. Wood is used as

structural elements in buildings, widely for doors, windows and partitions and find large
I use through secondary wood products like plywood, particle boards and laminated boards,
etc. Both hard and soft woods are in use. As such defects in this u)nstruction material for
buildings will affect the structural safety. aesthetic aspects and even functional use
including frre protection.
I
Objectives
At the end of this unit, you should be able to:
I
understand the classification of timber, and
h o w their properties and the defects in timber along with extensive details
coucening the causes for the defects.
,
2.2 DEFINITIONS
Let us understand the ferm timber. The word 'timber' is &rived from an old English word
. 'timnbrian' which means to build. Timber, thus denotes wood which is suitable for building
or carpentry or various other Engineering purposes. The Timber is further termed as:
1) Standing Timber
This indicates timber contained in a living tree.
2) RoundTimber
n ~ i indicates
s timber which is obtained after felling a uee.
3) Converted Timber
This indicates timber which is sawn and cut h t o suitable commercial sizes.
2.2.1 Properties
Now,let us understand the properties of timber:
Defects in Building Timber or wood, as a building material, possesses a number of valuable proprties such as :
i) low heat conductivity,
ii) amenability to mechanical working,
iii) small bulk density, and
iv) relatively high strength, etc.
2.2;2 Diawbacks
Timber, nas its own drawbacks such as:
i) susceptibility to decay and inflammability,
ii) fluctuations in properties due to changes in moisture content,
iii) variations in strength in length and across fibres, etc.
These shortcomings of timber require careful consideration while making use of it.

2.3 CLASSIFICATION OF TIMBER

Classification of Trees
dere, let us classify the trees for the Engineering purposes. Trees are classitied according
to their mode of growth as below:
Trees
I 1
Exogenous Endogenous
1 I
Conifers Deciduous
I I
Soft wolod Hard wood
2.3.1 Exogenous Trees
These ttees increase in bulk by growing outwards and distinct consecutive rings are
formed in the horizontal section of such a tree. These rings are known as annular rings
because one such ring is added every year and these rings are useful in predicting the age
of tree. Timber which is mostly used for Engineering purposes belongs to this category.
i) Conifers :These are alsoknown as ever-green trees and leaves of these trees
I do no fall till new ones are grown. As these trees bear cone-shaped fruits, Uley
are given the name conifers. These trees yield soft woods.
i Deciduous : The trees are also known as broad leaf trees and leaves of these
trees fall in autumn and new ones appear in spring season. Timber for
Engineering purposes is mostly derived from deciduous trees. These trees
yield hard woods. .
iii) Soft woods and hard woods: Soft woods form a group of ever-green trees.
Hard woods form a group of broad leaf trees. It is quite likely that some
variety of soft wood may prove to be stronger than some variety of h,ud wood.
Examples of soft woods are chir, deodar, fir, kail, pine, spuce, etc and those of
hard woods are babul, mahogany, oak, sal, teak, etc.
iv) Generally hard wood is preferable for structural components. Soft woods are
also used extensively in the building industry for structural components
joinery and less sophisticated purposes like partitions, pelmets, etc. Many
products of secondary manufacture like plywood, particle boards, roofing
sheets and laminated boiud. have also come into wide use.
2.3.21 Endogenous Trees
Thesh trees grow inwards and fibrous mass is seen in their longitudinal sections. Timber
from these trees has very limited Engineering applications. Examples of endogenous trees
are b;lmboo, cane, pAm, etc.

2.4 STRUCTURE OF A TREE

Now, after the classifications, we should know the structure of a tree how it has forn~ed
and q e term. in its formation. '(
A tree basically consists of three parts, viz., trunk, crown and roots. Defects in Timber

From the visibility aspect, the stn~~ture

of a tree can be divided into three categories:
i) Macrostructure
ii) Microstructure
2.4.1 Macrostructure
The stnrcture of wood visible to the naked eye or at a small magnification is called
macrostructure. Figure 2.1 shows the macrostructure of exogenous tree.

M&Ilary
lqrr
s q ward

Figure 21: Cross-section of an Exogenous Tree

i) Pith: The inner most central portion or core of the tree is called the pith or
medulla. It varies in size and shape for different types of trees. It consists
entirely of ccllular tissue and it nourishes the plant in its young age. When the
plant becomes old, the pith dies up and decays and sap is then transmitted by
the woody fibres depusited round the pith.
ii) Heart Wood: The inner annual rings surrounding the pith is known as heart
wood. It is usually dark in colour. As a matter of fact, it indicates dead
portion of tree and as such, it does not take active part in the growth of the
tree. But it imparts rigidity to tree and hence, it provides strong and durable
timber for various engineering purposes.
iii) Sap Wood: The outer annual rings between heartwood and cambium layer is
known as sapwood. It is usually light in colour and weight. It indicates recent
growth and it contains sap. The annual rings of sap wood are less sharply
defmed than those of heart wood. It takes active part in the growth of tree and
sap moves in upward direction through it. Sap wood is also known as
a1burnum.
iv) Cambium Layer: The thin layer of sap between sap wood and inner bark is
known as cambium layer. It indicates sap which has yet not been converted
into sapwood. If the bark is removed for any reason, the cambium layer gets
exposed and the cells cease to be active resulting in the death of the tree.
v) Inner Bark: The inner skin or layer covering the cambium layer is known as
inner bark. It gives protection to cambium layer from any injury.
vi) Outer Bark: ?be outer skin or cover of the tree is known as outer bark. It is
the outermost protective layer and it sometimes contains cracks and fissures.
It consists of cells of wood fibre and is also b w n as cortex.
vii) Medullary Rays: The thin radial fibres extending from pith to cambium layer
are known as medullary rays. The function of these rays is to hold together the
annual rays of heart wood and sap wood. These rays are sometimes broken
and in some varieties of trees, they are not very prominent.
2.4.2 Microstructure
The structure of wood apparent only at great magnifications is called microstructure.
When studied under a microscope, it becomes evident that wood consists of living and
dead cells of various sizes and shapes.
The moisture content plays an important part in the use of timber. For example, the
moisture content of a door frame or door shutter shoqd be appropriate for the service
conditions. There are an average of 15% for exteriorjoinery and an average varying from
10% to 12% for interior joinery.
Each species of timber has a certain natural durability which is normally greater and never
less for the heartwood than the sap wood. Durability in this context is a measure of the
natural resistance of fungal decay and not to insect attack. It is seen that the heart wood of
titnber classified as very durable can be expected to have an approximate life in excess of
25 years even if in contact with ground. In general, larger pieces would have longer life.
It is important to realise that even species with low natural durability will not decay if the
moisture content in service is kept below 20%. Even a piece of timber given the
descriptbn of perishable will not decay if the details and service conditions cause it to
have a moisture content in service below 20%.
The sap wood of almost all timbers not just soft woods is either perishable or non-durable.

2.5 DEFECTS IN TIMBER

You have understood the tree, its classification and structure. Now let us study the Defects
in timber.
As h u m body has many defects, timber also have defects. These are grouped into the
following five divisions:
1) Defects due to conversion
2) Defects due to fungi
3) Defects due to insects .

4) Defects due to natural forces

5) Defects due to seasoning
Now, lem us h o w various types of defects under each category:
2.5.1 Defects due to Conversion
During the process of converting timber to Commercial form, the following defects may
OCCUT:

i) Chip mark
ii3 Diagonal grain
iit) - Tom grain
iV) Wane
Chip Mpk: This defect is indicated by the marks or signs placed by chips on the finished
surface bf timber. They may also be formed by the parts of a planing machine.
Diagonal Grain: This defect is formed due to improper sawing of timber. It is indicated
by diagonal mark on straight grained surface of timber.
-= - -
Tom Gtain: This defect is caused when a small depression is formed on the finished
surface of timber by falling of a tool or so.
Wane: ?his defect is denoted by the presence of original rounded surface on the
manufautured piece of timber.
2.5.2 Defects due to Fungi
Fungi ate minute microscopic plant organisms. They attack timber only when the
followidg two conditions are satisfied simultaneously.
i) The moisture content of timber is above 12-158
ii) There is presence of air and warmth for the growth of fungi.
If any of the above condition is abser~t,decay of wood due to fungi would not occur.
Hence, &y wood having moisture content less than 12-1496 will remain sound for
centuries. Similarly, wood submerged in water win not be attacked by fungi because of
absence of air. Following defects are caused in timber by fungi
i)i Blue stain
ii) Brown rot *

iii) Dry rot

iv) Heart rot
v) Sapstain
vi) Wet rot
vii) White rot .
Hlue Stain: Sap of wood is stained to bluish col~urby the action of certain type of fungi. Defects in Thnber ,
Brown Hot: The term rot is used to indicate decay or disease of timber. Certain types of
fungi remove cellulose compounds from wood and hence, wood assumes the brown
bolour. Illis is kl~owna$brown rot.
Dry Rot: Certain types of fungi feed on wood and during feeding, they attack on wood
and convert it into dry powder form. This is known as dry rot. Following are to be noted:
a) Dry rot occurs at places where there is no free circulation of air such as
improperly ventilated basements, rooms, etc.
b) Unseasoned soft woods and sap wood are easily attacked by dry rot.
c) If timber is not properly stored after being felled down, it is liable for the
attack of dry rot.
d) It is not necessary to have damp conditions for the development of dry rot.
e) Dry rot is also caused by charring, painting and tarring the unseasoned.timber.
Heart Rot: This is formed when a branch has come out of a tree. In such a case, heart
wood is exposed to the attack of atmospheric agents. Ultimately, the tree becomes weak
and it gives out hollow sound when struck with a hammer.
Sap Stain: Certain types of fungi do not bring about the complete decay of timber. But
they feed on cell contents of sap wood. In doing so, sap wood loses its colour. This is
known ilS sap staiii and it generally occurs where moisture content goes beyond 20% or SO.
Wet Rot: Some varieties of fungi cause chemical decomposition of wood of the timber
and in doing so, they convert timber into a greyish brown powder. This ig known as wet
rot. The important facts to be remembered are:
a) Alternate dry and wet conditions favour the development of wet rot.
b) If unseasoned or improperly seasoned timbers are exposed to rain and wind,
they become easily liable for the attack of wet rot.
C) To prevent wet rot. well-seasoned timber should be used for exterior work or
for underground work and it should be covered by tar or paint for pfotection
against moisture.
White Rot: This defect is just opposite of brown rot. In this case, certain types of fungi
attack lignin of wood and wood assumes the appearance of a white mass consisting of
cellulose compounds.
2.5.3 Defects due to Insects
Following are the insects which are usually responsible for the decay of timber:
i) Beetles
ii) Marine borers
iii) Termites
Beetles : 'Ihese are small insects and they cause rapid decay of timber. They form
pin-holes of size about 2 mm diameter in wood. They attack the sap wood of all species of
hard wood. Tunnels are formed in all directions in sap wood by the larvae of these beetles.
The timber is converted into fine flour-like powder. They usually do not disturb the outer
shell or cover. Hence, timber piece attacked by beetles may look sound till it completely
fails.
a) Ambrosia Beetles - Pinhde Borer: The standing trees or recently felled logs of
several soft woods can be attacked by Pinhole borer. The attack ceases and dies when
the timber is converted and dried. Attack is more common in sap wood than
heartwood. The defect in timber takes the form of circular holes or short tunnels 0.5 to
3 mm dia mainly across the grain. The holes are dark stained and contain no bone dust.
b) House Longhorn Beetles (HYWTRUPES BAJULUS): House Longhorn Beetles
are a risk to soft wood in roof voids or pitched or flat roofs where there is sufficient
warmth. They can attack dry soft wood.
c) Furniture Beetles (ANOBIUM PUNCTATUM) :Furniture Beetles is the most
common insect attack in some places. This can attack dried sap wood and even dried
heart wood.
Defects ill Huildi~~gs d) W o 4 Wasps (SIRICIDAE): Wood Wasps attack standing trees and logs. Thc at1;lck
cease$ and dies when the timber is converted and dried. The tunnels ii,r~lledarc
circulhr and are filled with tightly packed bone dust.
Marine Borers: These are generally found in salty water. Most of the varieties of marine
borers do [lot feed on wood. But they make holes or bore tunnels in wood for taking
shelter. Tlhe diameter and length of these holes may go as high as 25 rnni and
60 lnm respectively. The wood attacked by marine borers loses colour and strength. It may
be noted #at no timber is completely immune from the attack of marine borers.
Termites4 These are popularly known as 'white ants' and they are found in abundance in
tropical a)d sub-tropical countries. These insecb live in a colony and they are very fast in
eating awgy the wood from core of the cross section. They make tunnels inside the timber
in differedt directions and usually do not disturb the outer shell or cover. Hence, timber
piece attatked by termites may look sound till it completely fails. Very few gtKd timbers
such as teik, sal, etc; can resist the attack of white ants. Such timbers have certaii~
chemicals in their composition and the smell of these chemicals is not favourable for
termites.
2.5.4 Defects due to Natural Force
The main tlatural forces responsible for causing defects in timber are two, nan~ely
i) abnormal growth, and
ii) rupture of tissues.
Following,defects are caused by these forces:
i) Burls
ii) Chemical stain
iii) , C w s e grain
iv) Dead wood
v) , Knots
vi) Shakes
vii) Twisted fibres
viii) Upsets
ix) Water stain
X) Wind cracks
Burls: T h a e are also known as 'excrescences' and they are particularly forined when a
tree has received shock or injury in its young age. Due to such injury, the growth of tree is
completely upset and irregular projections appear on the body of timber.
Chemical Stain: Wood is sometimes discoloured by the chemical action caused to it by
some external agency. This is known as chemical stain.
Coarse Gnain: If a tree grows rapidly annual rings are widened. It is know11 as coarse
grained tiqber and such timber possesses less strength.
Dead Wo4d: Timber which is obtained from dead standing trees contains dead wood. It
is indicated by light weight and reddish colour. t
Knots: Thdse are the bases of branches or limbs which are broken or cut off from the tree.
The portio~lfrom which the branch is removed receives nourishment from the stem for a
pretty long time and it ultimately results in the formation of dark, hard rings which are
known as knots. As continuity of wood fibres is broken by knots, they forn~a source of
weahess. Figure 2.2 shows a typical knot.
Knots are classified on the basis of their size and form. The following Tables 2.1,2.2 show
the classifidation of knots on the basis of their size and form and quality.
Table 2.1 : Classification of Knots on the Basis of their Size

SI.No Type of Knot Size

11 Pin knot Diameter upto 6.5 rnm
2) Small knot Diameter between 6.5 and 20
31 Medium knot Diameter between 20 mm and 40 m m
4) Large knot Diameter greater than 40 mm
 Meets in Timber

\
Knot
Figure 22 :Typical Knot
I
Table 2.2 :Classification of Knots on the Basis of Form and Quality
I
I S1 No. Type of Knot Remarks

1) Dead knot The fibres of knot are not properly interconnected with
those of surrounding wood. Hence, it can be easily
separared out from the body of wood. It is not safe to use
wood with such a knot for engineering purposes.
2) Decayed knot It is also known as an unsound knot and it is formed by
the =%on of fungi on wood.
3) Live knot It is also known as a sound knot. It is free from decay and
cracks. It is thoroughly fixed in wood and hence, it canaot
be separated out from the body of wood. Presence of such
h o t s makes wood difficult to plane. However, wood
containing such h o t s can be used for engineering
purposes.
4) Loose knot It indicates preliminary stage of dead knot. The fibres of
this knot are not firmly held in the surrounding wood.
5) Round knot Cross-section of this type of knot is either round or oval.
It is obtained by cutting the knot at right angles to its long
axis.
6) Tight knot It indicates prelimiuary stage of live knot.
The fibres of knots are firmly held in the surrounding
wood.

Rind Galls : Rind means bark and gall indicates-abnonnalgrowth. Hence peculiar curved
swellings found on the body of a tree are known as Rind Galls as shown in Figure 2.3
below. They develop ;It points from where branches are improperly cut off or removed.

FPgore 2.3: R i d Cab

t Shakes: llese are cracks which partly or completely separate the fibres of wood.
Following are7thedifferent varieties of shakes:
a) Cup Shakes:These are caused by the rupture of tissue in a circular direction as shown
in Figure 2.4. It is a curved crack and it separates partly one annual ring from the
A

Cup slmkes

Fipre2.4: Cop ShnLcs

23
Defects in Buildings other. It develops due to non-uniform growth. It may not prove to be harmful, if it
covers only a portion of ring.
b) Heart Shakes: These cracks occur in the centre of cross-section of tree and they
extend from pith to sap wood in the diiection of medullary rays as shown in Figure
2.5. These cracks occur due to shrinkage of interior part of tree which is approaching
rnabbrity. Heart shakes divide the tree cross-section into two to four parts.

shaker

c) Ring Shakes: When cup shakes cover the entire ring, they are known%.s ring shakes
as shown in Figure 2.6.

Ring shake

d) Star Shakes: These are cracks which extend from bard towards the sap wood. ?hey
are usually codined upto the plane of sapwood. They are wider on the outside ends
and narrower on the inside ends as shown in Figure 2.7. ?hey are usually formed due
to extreme heat or frost.

e) Radbl Shakes: These are similar to star shakes. But they are fine, irregular and
num&ous. They usually occur when the tree is exposed to sun for seasoning after
being felled down.?hey run for a short distance from bark towards the centre, then
follow direction of annual ring and ultimately nm towards pith. Figure 2.8 shows
radial shakes.
Twisted Pibres: These are also known as wandering hearts and they are caused by
twisting of young trees by fast blowing wind The fiwes of wood are twisted in one
 Radial
shake

Fipre 28 :Radial Shakes

direction as show11 in Figure 2.9. Timber with twisted fibres is unsuitable for sawing. It
can however be used for posts and poles in an unsawn candition.

FSgure 29 :Twisted Nbm

Upsets: These are also known as ruptures and they indicate wood fibres which are injured
by crushing or conlpression. Figure 2.10 shows a timber piece with this defect. Upsets are
mainly due to improper felling of tree and exposure of tree in its young age to fast blowing
wind.

Egure 2.10 :Upsets

Water Stain: WQod is sometimes discoloured when it comes into contact with water.
This is known as water stain and this defect is usually found in converted timber.
Wind Cracks: If wood is exposed to atmospheric agencies, its exterior surface shrinks.
Such a shrinkage results in cracks as shown in Figure 2.1 1. These are known as wind
cracks.

,Wind cracks

W p m 211 :Wmd CReks

c 2.5.5 Defects due to Seasoning

Following defects occur in seasoning process of wood.
i) Bow
ii) Case-hardening
 iii) Check
iv) Collapse

) Honey-combing
vii) Radial shakes
viii) Split
ilr) Twist
X) wacp
Bow: The defect is indicated by the curvature fonned in the direction of length of timber
as shown in Figure 2.12.

Rgwe 2.12 :Bow

Case-hardening: The exposed surface of timber dries very rapidly. It, therefore, shrinks
and is w&r compression. ?he interior surface which has not completely dried is under
tensiori. This defect is known as case-hardening and it usually occurs in timbers which are
placed at the bottom during seasoning.
Check A check is a crack which separates fibres of wood. It does not extend from one
end to the other.
Collapse: Due to uneven shrinkage, wood sometimes flattens during drying. This is
know as collapse.
Cup: 'Ibis defect is indicated by the curvature formed in the transkerse direction of timber
as shown in Figure 2.13

Honaycombing: Due to suess developed during drying, various radial and circular cracks
develjop in the interior portion of timber. Timber thus assumes honey-comb texture and the
defect so developed is known as honey-combing.
Radl3 shakes: These are radial shakes. They are explained earlier.
Splitt When a crack extends from one end to the other, it is known as a split.
M t : When a piece of timber has spirally distorted along its length, it is known as twist.
It is qhown in Figure 2.14.
Warp: When a piece of timber has twisted out of shape, it is said to have warped.
 Me& in Thnber

2.6 QUALITIES OF GOOD TIMBER

So far, we studied the various defects in timber and how the timber is affected. Now let us
know about good qualities of timber.
Following are the characteristicsor qualities of a good timber.
Appearance
A freshly cut surface of timber should exhibit hard and shining appearance.
Colour
The colour of timber should preferably be dark. Light colour usually indicates timber with
low strength.
Defects
A good timber should be free from serious defects such as dead knots, flaws, shakes, etc.
Durability
A good timber should be durable. It should be capable of resisting the actions of
fungi,insects, chemicals, physical agencies and mechanical agencies. If wood is exposed
to the actions of acids and alkalies for a prolonged period, it is seriously damaged. Weak
alkali aid acid solutions usually do not affect wood to a considerable extent.
Elasticity
This is the property by which timber returns to its original shape when load causing its
deformation is removed. This property of timber would be essential when it is to be used
for hows, carriage shilft~,sports g a d s , etc.
Fibres
Timber should have straight fibres.
Fire Resistance
Timber is a bad conductor of heat. Heat conductivity of wood is low and it depends on
various factors such as porosity, moisture content, surrounding temperature, orientation of
fibres, bulk density, etc.

A good timber should be hard, i.e. it should offer resistance when it is being penetrated by
another body. Chelllicals present in heart wood and density of wood impart hardness to
timber.
Mechanical Wear
A good timber should not deteriorate easily due to mechanical wear or abrasion. This
property of timber would be essential for places where timber would be subject to traffic,
e.g. wooden floors, pavements, etc.
Shape
A good timber should be capable of retaining its shape during conversion or seasoning. It
should not how or warp or split.
Smell
A gcml timber should have sweet smell. An unpleasant smell indicates decayed timber
Defects in Buildings Sound
A good timber should give out a clear ringing sound when struck. A dull heavy sound,
when struck, indicates decayed timber. The velocity of sound in wood is 2 to 17 times
greater Fhan that in air and hence, wood may be considered high in sound transmission.
Strength
A good timber should be sufficiently strong for working as structural member such as
joist, b e m , rafter, etc. It should also possess enough strength in direct and transverse
directions.

It should be uniform. Fibres should be firmly added. Medullary rays should be hard and
compact. Annual rings should be re~wlarand they should be closely located.
Toughqess
A good timber should be tough, i.e., it should be capable of offering resistance to shocks
due to vibrations. This property of timber would be essential when it is to be used for tool
handles, parts of motor cars and aeroplanes, etc.
Water Permeability
A good timber should have low water permeability which is measured by the quantity of
water filtered through or unit surface area of specimen of wood. Water permeability is
greater along the fibres than in other directions and it depends on initial nloisturc content,
character of cut, type of wood, width of annual rings, age of wood, etc.
WeatheHng Effects
A good timber should be able to stand reasonably the weathering effects. When timber is
exposed to weather, its colour normally fades and slowly turns grey. A good timber
should show the least disintegration of the surface under adverse weather coilditioils such
as drying and wetting, extreme heat ,and extreme cold, etc.
Weight
Tiinber Mth heavy weight is considered to be sound and strong.
Working Condition
Timber should be easily workable. It should not clog the teeth of saw 'and should be
capable Of being easily planed or made smooth.
It may be mentioned that the chief factors affecting strength of timber are as follows:
i) abnormalities of growth.
ii) faults in seasoning,
iii) invasion of insects,
iv) irregularities of grain,
v) moisture content,
vi) presence of knots, shakes, etc., and
vii) way in which a timber piece is cut from the log, etc.

2.7 DECAY OF TIMBER

Timber is said to be decayed when it is so deteriorated that it loses its value as an
engineering material. Various defects in timber have been mentioned earlier. When these
defects we in excess, timber decays and such timber is not used for engineering purpose.
Following are the various causes or situations which favour the early decay of timber:
1) Alternate dry and wet conditions.
2) Bad storage or stacking of timber.
3) Fungi which are responsible for developing diseases in timber such as blue
stain, brown rot, dry rot, heart rot, sap stain, wet rot and white rot.
4) Improper seasoning.
5) Insects such as beetles, marine borers, termites, etc.
 6) Keeping timber in contact with damp wall, damp earth, etc. Defects in Timber

7) Shocks or impacts rqeived cluing young age from natural forces such as fast
blowing wind, etc.
8) Use of timber without taking out sap wood from its structure.
9) Using seasoned timber without applying suitably preservative on its surface.
10) Using unseasoned wood with the application of protective coat of paint or tar.
SAQ 1
i) Enumerate various defects in timber.
ii) Which are the defects caused due to fungi?
iii) What are hots? How are they classified?
iv) What are the different types of shakes?
I v) What are the defects caused due to seasoning?

1
1 2.8 SUMMARY
Timber is subjected to various defects, like defects due to conversion, due to fungi, due to
insects, due to natural forces and due to seasoning. The timber used for engineering
purposes should be free from defects as far as possible. It is difficult to choose a timber
free from defects. However, care should be taken while selecting the timber for
engineeringpurposes.

2.9 ANSWERS TO SAQs

SAQ 1
i) Defects occurring in timber are grouped into the following five divisions:
a) Defects due to conversion
,

b) Defects due to fungi

c) Defects due to insects
d) Defects due to natural forces
e) Defects due to seasoning
Various types of defects under each category are:
a) Chip mark,diagonal grain, tom grain, are the defects due to
conversion.
b) Blue stain, brown rot, dry rot, heart rot, sap stain, wet rot and white rot are
the defects due to fungi.
c) Beetles, marmarhe borers and tennites are the defects due to insects.
d) Burls, Chemical stain, come grain, dead wood, knot, rind galls, shakes,
twisted fibres, upsets, water stain and wind cracks are the defects due to
natural forces.
e) Bow, case hardening, check, collapse, cup, honey-combing, radial shakes,
split, twist and wrap are the defects due to seasoning.
ii) The defects caused due to fungi are:
a) Blue stain
b) Brownrot
\ -
1)efect.sin Buildings d) Heart rot
e) Sap stain
f ) Wet rot
g) White rot
iii) These are the bases of branches or limbs which are broken or cut off from the
tree. The portion from which the branch is removed receives nourishment
from the stem for a pretty long time and it ultimately results in the formation
of dark, hard rings which are known as knots.
Knots are classified on the basis of their size ref Table 2.1 in the Text.
iv) The varieties of shakes are as follows:
a) Cup shakes
b) Heart shakes
c) Ring shakes
d) Star shakes
e) Radial shakes
v) Following defects occur in seasoning process of wood:
a) Bow f ) Honey - combing
b) Case-hardening g) Radial shakes
c) Check h) Split
d) Collapse i) Twist
e) Cup. j) Warp