All about timber as a construction material, from conversion of wood, seasoning, defects, deterioration and preservation, and wood products

1. TIMBER

2. CONTENTS
 Conversion of Wood
 Seasoning of Wood
 Deterioration and preservation of timber
 Special Timber products

3. CONVERSION OF WOOD
Wood conversion is a phrase used in reference to turning a
log into a pile of boards, planks. This is done using a sawmill.
Before the wood is actually used in construction, certain
operations such as felling of trees, sawing them to logs ,
poles and beams etc. become essential.

4. HOW IS WOOD CONVERTED
 FELLING;
This is a process of cutting down individual trees. The person
cutting trees is a feller. There are two factors considered before
felling:
i. Maturity. Trees become mature only after certain years
when there is a good balance established between there
heartwood and sapwood. In mature trees growth rate is
reduced considerably and they have achieved height and
girth of standard characteristic for their species. only such
trees should be felled for conversion. Young trees are
immature and give mostly sapwood and old trees give
poor quality as the heartwood starts decaying with age.

5. CONT’N
i. Time of felling. Spring and summer are always bad
seasons for felling trees because spring is a season for
active growth where cells are rich in sap and summer is
bad because the trees loss sap at a much faster rate
during this period after felling. This fat drying is likely to
cause development of a large of shrinkage defects.
The best time for felling is autumn and beginning of winter.
There is no growth of the tissues in living cells and also the
shrinking defects are minimal with favorable working
conditions. Branches and bark are removed

6. CONT’N
 It is important to take into account the taper (the
difference in size between the two ends of the logs)the
size of the taper affects how a log needs to be cut in order
for the best use to be made of its timber.
 The size of the planks that are needed should also be put
into consideration.
 Rot or cracks in the log and the roundness of the log.

8. SAWING OF TIMBER
Sawn timber is timber that is cut from logs into different
shapes and sizes. Sawn timber is generally cut into varying
rectangular widths and lengths, but may also be wedge
shaped.
 After their felling trees are usually cut diametrically in
convenient lengths. These are called logs. Collectively
called round timber. The logs are first air seasoned for
some period and then further cut into transportable parts.
 The main methods of sawing of logs are;
 Through and through sawing and
 Selective sawing

9. TYPES OF CUTS/BOARDS
 Tangential boards/cuts (crown, plain
or flat sawn) are used extensively for
beams and joists. They are stronger
when placed correctly edge up with
the load in the tangential axis.
These type of boards suffer from
'cupping' if not carefully
converted, seasoned, and stored
properly. Annual growth rings form an
angle less than 45 degrees.

10. CONT’N
 Radial boards (radial, figured or quarter
sawn) are typically cut on 'the
quarter' and produce a pattern of the
medullary rays especially in quartered
oak. Such timber is expensive due to the
multiple cuts required to convert this
board. The radial face of the board is
slightly stronger and stiffer than the
tangentially face but the cross section and
condition of the timber has more effect
on strength.
Annual growth rings form an angle greater
than 45 degrees.

11. THROUGH AND THROUGH SAWING
In this method, the log is cut lengthwise into pieces of
suitable dimensions, one after the other, without giving it
any rotating during sawing. All the cut surfaces are thus
essentially parallel. Timber cut using this method is used for
garden furniture, fencing posts, and for making pallets.

12. CONT’N
ADVANTAGES
 This method is very cheap method because there is no
need to double the log as compared to quarter method.
 Plain method is a very fast method compared to the
tangential method.
 Very little waste produced since the cuts are parallel.
DISADVANTAGES
 Planks contain hardwood and sapwood.
 Planks are not very strong.
 Planks are prone to warp, split and twist.
 No distinctive grain pattern

13. SELECTIVE SAWING
In this method, the log is rotated in different directions
after one or more successive cuts in a particular direction.
There are two methods in selective sawing namely;
i. Quarter Sawing and
ii. Tangential

14. QUARTER SAWING
In this method, the log is cut into quarters and it
produces radial boards which are cut in a such away
that each board has one edge from the center of the
log and the other edge from the peripheral part of the
log. The process consists of;
 Cutting the log into two halves by a central cut
 Each of the two halves is given one cut again. This
results in four parts of the log.
 Boards are then cut from each of the four parts by
parallel cuts.
 Heartwood and sapwood can be separated using this
method and it is used on hard woods.
 Timber cut this way is used for floorboards and
furniture

15. CONT’N
Tangential sawing
In this method, the cut is made tangential to the annual rings
and requires a lot of turning of the log.
 Heartwood and sapwood are separated using this method
 Timber from this method is used for furniture and veneers
Two varieties are;
i. Slash cut. (crown, plain or flat sawn) are used extensively
for beams and joists. These are the boards that have been
cut in a direction tangential to the annual rings. In such
boards, the annual rings make an angle of less than 45 with
a face. They give a plain appearance. They are stronger
when placed correctly edge up with the load in the
tangential axis. These type of boards suffer from 'cupping'
if not carefully converted, seasoned, and stored
properly. Annual growth rings form an angle less than 450

16. CONT’
i. Rift cut (vertical or edge grain). These are boards in
which the annual rings make an angle of more than 45 to
the face. Such boards are obtained making parallel cuts
through the central parts of the log traversing through
the annual rings.
 Quality floor boards are prepared from rift sawn timber
because it wears well and shrinks less.

17. ADVANTAGES
 The planks are very strong
 The planks have an attractive grain pattern (Flame figure)
 Heartwood and sapwood are separated
DISADVANTAGES
 The planks are prone to swelling and shrinking
 Costly and slow
 Lots of waste

18. REASONS FOR CONVERSION OF WOOD
 To reduce the wood to workable size
 Small planks are easier to dry out than large logs

19. DRYING AND SEASONING OF WOOD

20. SEASONING
This is the process of removing the moisture content from
wood to minimize structural problems when used in
construction or to provide less smoke and more uniform
combustion when used as firewood.
Seasoning is relevant as it improves certain properties of
timber that make it of purpose in different construction
work for example durability, strength, hardness and
weight reduction.

21. OBJECTS OF SEASONING
 Reduction in weight- freshly cut logs which may have
50 to 100% moisture content.
 Increase in strength- there is higher quantity of wood
tissue in dry wood than in green wood hence the lower
the moisture content the higher the strength.
 Improvement in workability. Green timber is more
difficult to work compared to seasoned timber.
 Freedom from shrinkage defects- timber if not properly
seasoned will shrink and wrap.
 Longer life of durability- less insect and fungal attack
due to reduced moisture.

22. METHODS OF SEASONING
There are mainly two types of seasoning used while drying
timber.
 Natural seasoning;
- Air seasoning
-Water Seasoning
 Artificial seasoning

23. AIR SEASONING
This is a traditional method for
drying wood and is also the
longest
Stacks should be protected from
rain, direct wind and direct
scorching heat and free from
ground level debris and built on
dry ground.
Natural Seasoning

24. PREPARATION FOR NATURAL SEASONING
 Stack ground. It should be leveled, free from debris and
on a dry land. It may take a few centimeters from the
ground level
 Stack Pillars. These are constructed at regular intervals
out of bricks or masonry or concrete and may be of 30-
50cm height from the ground level. The top surfaces
should be flat and level with each other.
 Stack proper. These are made of sawn timber
shapes(sleepers, planks, poles). One stack should have
timber of one shape and same length and width.

25. WATER SEASONING
This gives good results with logs of freshly cut trees.
Logs are placed in running water and the sap from the cells
is washed out.
Timber logs can also be placed in stagnant water if running
water is not available.
Natural seasoning

26. ADVANTAGES OF NATURAL SEASONING
 It is highly economical that is air and water are readily
available.
 It requires little supervision.
 It is applicable to thicker timber parts as well as thin
sections

27. DISADVANTAGES OF NATURAL SEASONING
 It is a very slow process because it is dependable on
nature.
 It keeps valuable land and timber blocked for longer
periods.
 Moisture content cannot be brought below a certain
limit.
 Seasoning is not always uniform in all sections of
timber.

28. ARTIFICIAL SEASONING
This is where timber is subjected to drying by use of
artificial means. Its dived into several methods;
 Kiln Seasoning
 Microwave Seasoning
Others include;
 Chemical Seasoning
 Seasoning by boiling
 Electrical Seasoning

29. KILN SEASONING
The is the most common and effective commercial
process for drying wood and accelerates the process of
removing moisture through the use of external energy.
The drying takes 2-3days
Kiln seasoning is divided into 2 methods
 Progressive
 Compartmental

30. PROGRESSIVE METHOD
 In the progressive kiln, timber enters at one end and
moves progressively through the kiln much as a car
moves through a tunnel.
 Progressive kilns may be further subdivided into
natural draft kilns in which heated air is allowed to rise
through the material by natural convection, and forced
draft kilns in which fans are employed to force the air
through the wood.
 A progressive kiln has the stack on trolleys that
‘progressively’ travel through chambers that change
the conditions as it travels through the varying
atmospheres.

31. CONT’N

32. COMPARTMENTAL METHOD
A compartment kiln is a single enclosed container or
building, etc. The timber is stacked as described above
and the whole stack is seasoned using a program of
settings until the whole stack is reduced to the MC
required.
Compartment kilns differ from progressive kilns in that
the timber is loaded into the kiln and remains in place
throughout the drying process.
Compartment kilns are usually smaller than progressive
kilns, and because of their construction the temperature
and humidity conditions within them can be closely
controlled.

33. CONT’N

34. SOLAR KILN
This method combines the speed of kiln
seasoning with the low energy of air
drying. Solar kilns have single-thickness
windows on the south side of the
structure that work as collectors to trap
the sun’s energy.
Heat collectors, made from black metal
are attached near the top of the
window sashes. Various methods force
the heated air to circulate through the
kiln to dry the wood. Some solar kilns
have insulation to retain heat at night

35. MICROWAVE SEASONING
Microwave seasoning uses pulsed energy directed into
timbers to drive out moisture in a manner that will not
cause seasoning degrade.
This method also provides advantages such as high speed
and high quality and is well suited for seasoning lumber,
blocks, veneer, chips, paper and wood-based composite
materials.
Areas in the wood with the most moisture absorb the
most energy resulting in even temperature during the
drying process and a uniform moisture content. These
factors enhance quality and reduce timber checking and
warping.

36. OTHER METHODS OF ARTIFICIAL SEASONING
 Chemical /Salt Seasoning:
In this method, timber to be seasoned is treated with a
chemical solution like sodium chloride, nitrate or urea, the
purpose of the solution is that it reduces the vapour pressure
on the surface of application.
Chemical seasoning enables timber to dry fast from inside. It
is advantageous in that it prevents risk of cracking of the
outer shell

37. CONT’N
 Seasoning by boiling:
This is a quick method of removing sap from within the
cells, the timber to be seasoned is immersed in water and
water is heated to the boiling point. It is kept boiling for 4
to 5 hours. The sap is washed out by this process and on
placing such treated timber in air, it dries quickly but at
the cost of strength and elasticity of fibers, so it is rarely
used.

38. CONT’N
 Electrical Seasoning:
It is of theoretical importance only. Dry wood is a non-
conductor of electricity but when an Alternating current
is passed through a green piece of timber, heat generated
is enough to dry out moisture of the cells which do
conduct some electricity, in this way, the timber pieces
are dried quickly.
The technique is costly in that it requires high electricity
consumption and also causes damage to the cell walls

39. ADVANTAGES OF ARTIFICIAL SEASONING
 The quality of seasoned timber is highly satisfactory
DISADVANTAGES
 It’s a costly method because it involves expensive
equipment.
 It needs expert handling for good quality seasoning.

40. DETERIORATION AND PRESERVATION

41. DETERIORATION AND PRESERVATION OF
TIMBER.
Deterioration of timber
Timber is subjected to deterioration from many causes and
its strength eventually becomes so weakened that it must be
removed and replaced.
The chief factors of this deterioration are decay, mechanical
abrasion, fires etc.

42. CAUSES OF DECAYS
 Insects
A few insects are known to
actually spoil huge quantities of
timber within a short time.
Termites are the common insects
that attack timber/wood
Other insects which attack timber
are: pole-borers and marine
borers.

43. CONT’N
 Fungi
Decay of timber is attributed to
small plants and organisms that
live on and spread within the
timber.
A special type of organism called
fungus starts growing on the
timber, especially when it is
placed on a moist ground or is
used in a damp situation.

44. THE THREE MAIN TYPES OF FUNGI THAT AFFECT
TIMBER
 The white rot: this type of fungus destroys the lignin
part of the cell by consuming it and leaving behind the
fluffy light colored cellulose part in rotten form.
 The brown rot: unlike white rot, it eats up and
destroys the cellulose part of the cell tissue leaving
behind a brownish lignin part in rotten form.

45. CONT’N
 Mold
it’s a cotton like growth of fungus spread over various parts
on the surface of the board. It may not spoil the inner parts
of the portions of the wood but its appearance on the
surface is also quite displeasing
 The blue stain.
It’s a fungus that lives on sugars and starching the cell tissue.
Such a fungus causes staining or discoloration of the original
board.

46. PRESERVATION OF TIMBER
This is carried out mainly to prevent timber from decaying.
The main objective of such a treatment is to ensure a longer,
trouble free life of timber.

47. TYPES OF METHODS USED FOR PRESERVING
TIMBER
 Application of some chemical substances (called the
preservatives) on the surface of the timber.
 Injection of the preservatives into the body of timber.
 Construction of protective covering or shields around
timber used in construction.

48. WOOD PRESERVING CHEMICALS
 The Oil-Soluble Salts
Such compounds are soluble only in oils. The most
commonly used wood preservative, coal tar creosote oil,
 Water soluble Salts
Such salts make an easy solution with water.
Among the water soluble salts are included zinc chloride,
copper sulphate , sodium fluoride , sodium fluosilicate ,
sodium dinitrophenoxide and compounds of arsenic.

49. CONT’N
 Volatile Base Salts
These make solutions with substances like petroleum. The
creosote petroleum blends are typical examples of this
category

50. METHODS OF APPLICATION
Preservatives are used by different methods depending upon
the extent of preservation required. These methods include;
 Brush application
 Dipping
 Open tank

51. BRUSH APPLICATION
In this method , timber is given one or two coats of the
preservative with the help of a brush. This is used for
painting coal tar at the ends of beams that are embedded in
the walls or base of poles and posts that go to the ground
with coal tar .
The method is quite cheap at the same time, it is not very
effective.

52. DIPPING METHOD
The timber part to be treated is made to dip in the
preservative and kept immersed in it for various periods
from a few hours to few days .
It is used when organic preservative solvents are to be
applied.

53. OPEN TANK APPLICATION
In this method , the timber is kept immersed in a suitable
metallic tank of proper size till a proper saturation is
obtained .
The tank containing the timber is then heated at about
700 - 800c for several hours this ensures deeper
penetration of the preservative into the timber.
After this, the timber is allowed to cool within the tank in
the presence of preservative.

54. SPECIAL TIMBER PRODUCTS

55. SPECIAL TIMBER PRODUCTS
Veneers
A veneer is essentially thin sheets of timber with a thickness
varying between 0.4mm to 6mm. Its also sometimes called a
ply.
This is the basic component where most wood products are
prepared from and easily.

56. MANUFACTURE OF VENEERS
They are obtained from a suitable type of timber logs using 3
methods
 Sawing
 Slicing
 Rotary cutting

57. SAWING METHOD
This is the earliest method and its done in this way;
 Firstly, the logs are cut into smaller units called
flinches.
 Each flinch is then subsequently swan carefully to
obtain veneers of required thickness

58. SLICING METHOD
Horizontal slicing.
Here the log is held in
position by a mechanical
system and the cutting edge
is set into a horizontal
motion
Vertical slicing
Here the log is held in
position by a mechanical
system and the cutting edge
is set into a vertical motion

59. ROTARY CUTTING METHOD
Its a modern method by which quality of veneers can be cut
quickly.
A special wood log is kept in one position as it rotates about
a sharp knife(cutter)

60. USES OF VENEERS
 Veneers are mainly used in the manufacture
of plywood, laminated board and batten
boards.
 Used in interior decorations

61. PLYWOOD
Plywood is a flat panel made by bonding together, under
pressure, a number of thin layers of veneers(or plies).

62. STRUCTURE OF PLYWOOD
In the simplest type of plywood the
board consist of 3 layers of plies
arranged in a such a manner that;
1. One ply that lies in the middle
called the central ply.
2. Two plies each lying on the
either sides of the central play.
The sides of the plier are called
the free ply and the back ply.

63. PROPERTIES OF PLYWOOD
 It has a better resistance to shrinkage defects like
swelling and warping.
 The plywood is free from splitting defects
 It has uniform mechanical properties in both the
directions
 It has better strength than the ordinary wood of equal
thickness
 It is light in weight and easy to handle and convenient
for using.

64. MANUFACTURE OF PLYWOOD
Commercially manufacture of plywood involves the following
stages
1. Veneering
2. Bonding
3. Stacking
4. Pressing

65. VENEERING
This involves obtaining thin sheets or
plies or veneers from a selected type of
timber.
Veneers may be obtained by sawing,
slicing, rotary cutting or peeling.
The thickness of the veneers varies from
0.4mm to 6mm and veneers so obtained
are sorted and dried to a moisture
content level below 10%

66. BONDING
This stage consists of applying adhesive or glue over the
surface of the veneers
Animal glues, vegetable glues and synthetic resin glues are
variously used for bonding the plies together.

67. STACKING
This consists of arranging the glued plies around a core ply in
such a manner that grain in adjacent plies runs at right
angles to each other.

68. PRESSING
.
The stacked plies are either hot pressed or cold pressed
depending upon the type of adhesive used in bonding.
Pressing is in 2 ways;
 Cold pressing is applied when vegetable glues have
been used

69. CONT’N
Hot pressing is applied when
synthetic resins have been used.
In hot pressure, the material is
raised to a temperature where glue
is perfectly liquefied and then the
pressure varying between 7-
14kg/cm2 is applied through
hydraulic presses.

70. MANUFACTURE OF PLYWOOD
Processes

71. ADVANTAGES OF PLYWOOD(WHY PLYWOOD?)
 Dimensional stability
 Strength and stiffness/weight ratio
 Split resistance
 Panel shear or shear through thickness capacity
 Resistance to concentrated loads
 Resilience, impact and fatigue resistance
 Thermal insulation with low thermal mass
 Chemical resistance
 Sound reflectance
 Workability
 Aesthetics

72. TYPES OF PLYWOOD
 Exterior type
This is where water proof glue is used for bonding at the
manufacturing stage and such plywood is suitable for
external applications in building construction e.g. doors,
windows.
Such plywood is obtained from pine, spruce, mahogany,
Douglas fir etc.

73.  Interior Type
This uses common glues are used.
Such plywood is mostly used in the
interior where highly aesthetic finish is
required.
Its used in ceilings, wall lining, internal
furniture, interior cladding etc.

74. GENERAL USES OF PLYWOOD
 Plywood is extensively used in construction and furniture
making such as doors, windows, tables, etc.
 It can be used as roof and wall materials.
 Plywood is used in sheathing and flooring.
 Used for wall and ceiling tile backings
 Used for making table reels and floor coverings
 Can be used for truck bodies, streamlined trains and boat
interiors
 Shuttering boxes are made of shuttering plywood.

75. LAMINATED TIMBER
This another type of processed wood that consists of suitably
selected wood sheets that have been glued together in a
such a way that grains of all the sheets are parallel
longitudinally.

76. LAMINATED TIMBER(CONT’N)
It comes in a wide variety of shapes and appearances and
those are;
 Industrial glade laminated timber. Here small voids
are allowed but one face is made free of loose knots
and holes. It is used in garages and industrial plants.
 The Architectural grade. This has a very smooth
external surface in which all the knots have been
removed and voids so created filled with clear wood
tissue
 The premium grade. This is manufactured with great
care and is used where highly appealing and smooth
external finish is required.

77. FIBERBOARDS
These are made by pressing together
fibrous materials such as fiber of wood,
cane and even vegetable matter.
The fiber mass is first heated and then
pressed hard in that hot state.
On cooling, a rigid material of light of light
density and good utility is obtained.
Its thickness varies between 3mm to 10mm

78. USES OF FIBERBOARDS
They are used as;
 Insulating boards for both heat and sound insulation
 Partition wall panels
 Parts of flushed doors
 Finished flooring
 Low slope roofing

79. IMPREG AND COMPREG TIMBERS
This is a class of processed timber products where advanced
technology is used.
In this class, the veneers are treated with suitable types of
resins.
The type of resin used, the method of method of treatment
and the type of raw material used gives rise to a variety of
such treated timbers

80. CONT’N
 In impreg timber, the veneers are immersed in resins,
commonly phenol formaldehyde at ordinary temperature
and pressure.
Once the tissue is saturated and the resin is set, the
consolidated mass is cured at a raised temperature(150c).
It’s then finally given proper finishing treatment and is
marked under trade names.

81. CONT’N
 Compreg timber. This a variety of impreg timber that is
prepared by curing the resin impregnated timber at a high
temperature and high pressure.
This type of curing results into a timber of high density and
strength that is quite durable.

82. ADVANTAGES OF IMPREG AND COMPREG
TIMBER
 Very beautiful in appearance
 Almost impervious outer surface
 Resistance to acids and solutions
 High strength, low shrinkage and much increased
life(durability)

83. REFERENCES
 Building materials by S.K.Duggal
 Wikipedia
 Materials for Construction and Civil Engineering by
Mr.Clara
 Materials in Construction Technology by G.D.Taylor