Timber
Oh!

General
Timber is one of the oldest building
materials, along with stone, earth and
various vegetable materials.
Timber has remained until today the most
versatile and, in terms of indoor comfort
and health aspects, most acceptable
material.

Cont’d
Timber is an extremely complex
material, available in a great variety of
species and forms, suitable for all
kinds of applications.

Classification of Timber
There are two main groups of timber
producing trees:
 Softwoods
Hardwoods

Softwoods
Softwoods are coniferous trees and
the timber is not necessarily 'soft'.
They are 'evergreen‘, do not shed
their leaves in autumn.
 Their general characteristics
are:
 Straight
 round but slender
 tapering trunk
 The crown is narrow and rises
to a point.
 It has needle like or scale-like
shaped leaves.
 The bark is course and thick

Hardwoods
Hardwood trees are:
 Broadleaf and generally
deciduous.
 Their timber is not
necessarily hard.
 The general
characteristics are: Stout
base which divides into
branches to form a wide,
round crown.
 The leaves are broad
 The bark may be smooth
or course and varies in
thickness and colors.

Timber for construction
Timber for building construction is divided
into two categories:
 Primary timber
Secondary timber

Primary timbers
Primary timbers are:
 Slow-grown
 Aesthetically appealing
 Have considerable natural resistance to
biological attack, moisture movement and
distortion.
 Expensive and in short supply.

Secondary timbers
Secondary timbers are:
Mainly fast-grown species
Low natural durability
With appropriate seasoning and
preservative treatment, physical properties
and durability of secondary timbers can be
greatly improved.

Advantages of timber
Timber is suitable for construction in all
climatic zones, and is unequaled by any
other natural or manufactured building
material in terms of versatility, thermal
performance and provision of comfortable
and healthy living conditions.

Cont’d
Most species have very high strength:
weight ratios, making them ideal for most
constructional purposes, particularly with a
view to earthquake and hurricane
resistance.
Timber is compatible with traditional skills
and rarely requires sophisticated
equipment.

Cont’d
The production and processing of timber
requires less energy than most other
building materials.
Timber provides good thermal insulation
and sound absorption, and thicker
members perform far better than steel in
fire.

Cont’d
Demolished timber structures can often be
recycled as building material, or burnt as
fuel wood, the ash being a useful fertilizer.

Disadvantages of timber
 Fire risk of timber members and
timber products with smaller
dimensions.
 High costs and diminishing
supplies of naturally resistant
timber species.
 Failure of joints between timber
members due to shrinkage or
corrosion of metal connectors.

Susceptibility of cheaper, more abundantly
available timber species to fungal decay
(by moulds and rot) and insect attack (by
beetles, termites, etc.).

Fungi attacked timber

Timber growth

Section of a trunk

Annual or growth rings
In temperate climates
there are two distinctive
growth seasons, spring
and summer ~ the spring
growth is rapid and is
shown as a broad band
whereas the hotter, dryer
summer growth shows up
narrow.
In tropical countries the
growth rings are more
even and difficult to
distinguish.

Description of terms
• Bark:
The outer layer,
corklike and provides
protection to the tree
from knocks and
other damage.

Cont’d
• Bast:
The inner bark, carries
enriched sap from the
leaves to the cells
where growth takes
place.

Cont’d
 Cambium:
Layer of living cells
between the bast and
the sapwood.

Cont’d
 Heartwood:
Mature timber, no
longer carries sap,
the heart of the tree,
provides the strength
of the tree. Usually a
distinctive darker
color than the
sapwood.

Cont’d
 Sapwood:
New growth, carries
the raw sap up to the
leaves. Usually lighter
in color than the
heartwood, especially
in softwoods.

Cont’d
 Medullary rays:
Food storage cells
radiating from the
medulla ~ provides a
decorative feature
found in quarter cut
timber.

Cont’d
 Pith or medulla:
The center of the tree,
soft and pithy
especially in the
branches.

Seasoning of timber
Seasoning is the process by
which the moisture content of
timber is reduced to its
equilibrium moisture -MC
between 8 and 20 % by weight.

Cont’d
Time required for seasoning process
depends on:
 Timber species and age,
Time of harvesting,
Climate,
Method of seasoning, etc.

Reasons for seasoning
Seasoning makes the timber suitable for
the environment and intended use.
The MC of timber needs to be reduced for
the following reasons:

Cont’d
• Makes the timber more resistant to
biological decay, increases its strength,
stiffness and dimensional stability, and
reduces its weight (and consequently
transportation costs).

Cont’d
• Seasoned timber show fewer tendencies
to warp, split or shake.
• Seasoned timber although lighter will be
stronger and more reliable.

Cont’d
• The sap in timber is a food for fungi and
wood parasites. Remove the sap and the
wood will be less attractive to these
dangers!
• For construction grade timber the timber
must be below 20% MC to reduce the
chances of fungi infestations.

Cont’d
• Dry and well seasoned timber is stronger.
• Dry and well seasoned timber is easier to
work with and consequently safer
especially machine working.
• Timber with higher moisture content is
difficult to finish i.e. paint, varnish, etc.

Methods of seasoning timber
There are two main ways of seasoning
timber:
 Air seasoning
 Kiln seasoning
Both methods require the timber be
stacked and separated to allow the full
circulation flow of air, etc. around the
stack.

Air seasoning
Air seasoning is done
by stacking timber
such that air can pass
around every piece.
Protection from rain
and avoidance of
contact with the
ground are essential.

Kiln seasoning
There are two main methods used in kiln
seasoning:
 Compartmental
 Progressive
Both methods rely on the controlled
environment to dry out the timber. and
require the following factors:

Cont’d
Kiln seasoning require the following factors:
 Forced air circulation by using large fans,
blowers, etc.
Heat of some form provided by piped
steam.
Humidity control provided by steam jets.
The amount and duration of air, heat and
humidity again depends on species, size,
quantity, etc.

Compartmental kiln
A compartment kiln is a single enclosed
container or building, etc. The timber is
stacked and the whole stack is seasoned
using a program of settings until the whole
stack is reduced to the MC required.

Progressive kiln
A progressive kiln has the stack on trolleys
that ‘progressively’ travel through
chambers that change the conditions as it
travels through the varying atmospheres.
The advantage of this system, although
much larger, has a continuous flow of
seasoned timber coming off line.

Cont’d
Kiln seasoning achieves
accelerated seasoning in
closed chambers by
heating and controlling air
circulation and humidity,
thus reducing the time by
50 to 75 %, but incurring
higher costs.
An economical alternative
is to use solar heated
kilns.

Preservative treatment
Seasoning alone is not always sufficient to
protect secondary timbers from fungal
decay and insect attack. Protection from
these biological hazards is effectively
achieved by preservative treatments with
certain chemicals.
There are many chemicals, used singly or
in combination, which preserve timber
against insect and/or fungal attack.

Properties of timber
Timber, as a natural material is variable! The
disadvantages of variability are overcome by
selection or grading processes and by the
application of safety factors in structural
calculations.
Strength
Durability
Permeability
Fire resistance

Strength
The structural strength of timber is a measure of
its ability to resist external forces, such as
compression, tension and shear.
The density is reliable indicator of many
structural and mechanical properties.
There is a particularly strong relationship
between density and compressive strength,
bending strength and hardness.

Cont’d
Density ranges from an average of 160 kg/m3 to
1040 kg/m3.
There is a marked difference in strength
properties depending upon whether they are
measured parallel to or perpendicular to the
grain of the timber. The tensile strength of most
timbers parallel to the grain is three to four times
the compressive strength. The tensile strength
parallel to the grain can be thirty times as high
as perpendicular to it, while for compressive
strength the ratio is of the order of six to one.

Cont’d
For most timbers:
(Tensile strength
parallel to the grain) = 3 to 4 times (the compressive strength)
(The tensile strength
parallel to the grain) =30 times as high as perpendicular to it)
(The compressive strength
parallel to the grain) =6 times as high as perpendicular to it)

Factors affecting strength
The strength of a piece of timber is affected by
characteristics such as
 Knots
 Direction and slope of grain (diagonal or sloping
grain reduces strength, particularly bending and
stiffness),
 Moisture content (generally timber is more
flexible when wet but increases in strength as it
dries):
The strength of timber is broadly related to
its density.

Durability
The durability of timber is a measure of its
resistance to attack by insects and fungi.
The most effective means of preventing
fungal attack is to ensure that the moisture
of timber remains below 22% when there
is not enough moisture for the fungus to
survive. Insect attack is often associated
with fungal decay.

Permeability
Permeability is an important factor in the
treatment of timber with chemicals such as
preservatives and flame retardants.
Permeability varies enormously between
species although the sapwood of all
specious is more permeable than the
heartwood.

Fire resistance
Fire resistance is an important
consideration in using timber. Although
wood is used as a fuel, large sections of
timber are difficult to ignite and the
charcoal produced on the surface provides
protection for the wood underneath.

Defects of timber
Since timber is a natural product,
developed through many years of growth
in the open air, exposed to continual and
varying climate conditions, it is prone to
many defects.
Defects cannot be corrected and therefore
each individual piece must be inspected
before use and judged on its own merits.

Cont’d
Defects can be caused during growth,
during drying, through insects, through
fungi or during subsequent handling or
machining, and each should be known, so
that imperfect pieces can be detected and
rejected.

1. Shrinkage
When timber is seasoned
and its moisture content
(MC) is reduced below
the Fiber Saturated Point
(FSP) continued drying
will cause dramatic
change such as increase
in strength but also
distortion and shrinkage.

2. Cupping
Because of the varying
shrinkage rates tangential
boards tend to cup because of
the geometry of the annual
rings shown on the end
grain. It can be seen that
some rings are much longer
than the others close to the
heart. Therefore they will be
more shrinkage at these parts
than the others cupping is the
result.
In square section timber cut
from the same place,
diamonding is the result.

3.Knot
Knots are the result of the
trees attempt to make
branches in the early growth of
the tree. They are the residue
of a small twig, shoot, etc. that
died or was broken off by man
or an animal in the wood or
forest. The tree subsequently
continued its growth over this
wood.
The knot may be live, sound,
or tight or if it has become
separated and is contained in
residue of bark, dead.

4. Splits
A separation of the wood
fibers along the grain
forming a fissure that
extends through the
board from one side to
the other.
It is usual in end grain
and is remedied by
cutting away the defected
area. All boards should
have an allowance so
that some end grain may
be cut away because of
possible shakes or splits.

5. Checks and end checking
A separation of the
fibers along the grain
forming a fissure
which shows up on
one face or at the end
grain but does not
continue through to
the other side.

6. Wind or Twisting
Spiral or corkscrew
distortion in a longitudinal
direction of the board.
Due to the board being
cut close to the centre of
the tree which has spiral
grain. The board is of not
much use but small
cuttings may be obtained
from it with careful
selection.

7. Bow
Bowing is concave/convex distortion along the length of the board. It
is a seasoning and or storage defect caused by the failure to support
the board with stickers at sufficient intervals. The boards own
weight and probably those above it bears down and the resultant
bow is inevitable. This defect can and should be avoided by careful
use of stickers supporting the board at the correct width.

8. Spring
Spring is concave/convex distortion along the
length of the board again but this time the
distortion is in the flat plane of the board.
Boards with this defect may have been cut from
near the heart of the board and is the result of
growth stresses being released on conversion.
Useable timber may be recovered from these
boards by cutting a straight edge and re
sawing. The grain direction however may not be
satisfactory for aesthetics and care should be
taken for placing the possible short grain figure
where stability is required.

9. Shakes
Shakes are separation of the fibers along the
grain developed in the standing tree, in felling or
in seasoning. They are caused by the
development of high internal stresses probably
caused by the maturity of the tree.
The shake is the result of stress relief and in the
first place results in a single longitudinal crack
from the heart and through the diameter of the
tree.

10. Defects through insects
Termites or white ants attack timber
structures and are a serious problem. The
species that causes the damage live in the
ground. Precautions involve treating
timber with a preservative or avoiding
direct timber contact with the ground.

CONVERSION OF TIMBER
As soon as possible after felling the tree
should be converted into usable timber.
There are two main methods of converting
timber:
Through and through (or Plain) and
Quarter also referred to as rift sawn.

Through and through
Through and through
 produces mostly
tangentially sawn timber
and some quarter sawn
stuff.
 Tangential timber is
prone to cupping but it is
stronger when placed
correctly. Because of this
it is used extensively in
the construction industry
and especially for beams.

Quarter sawn
Quarter sawn
 Is far more expensive
because of the need to
double (or more) handle
the log. There is also
more wastage.
 It is however more
decorative and less prone
to cup or distort.
Note also there are two
ways of sawing the
quarter.

Tangential boards
Tangential boards are the stronger boards
and when placed correctly, used for
beams and joists. These types of boards
suffer from 'cupping' if not carefully
seasoned, converted and used properly.

Radial boards
Radial boards are cut on 'the quarter' and
produce a typical pattern of the medullary
rays especially in quartered oak. Such
timber is expensive due to the multiple
cuts required to convert this
board. Quality floor boards are also
prepared from this rift sawn timber
because it wears well and shrinks less.

TIMBER COMPOSITIES
Timber is a natural composite which can
be used in its original or sawn sections.
Timber can also be converted into
particles, strands or laminates which
can be combined with other materials
such as glues to form timber
composite products.

Reasons for transforming timber
The principal reasons for transforming timber
into composite products include:
 to exceed the dimensional limitations of sawn
timber
 to improve performance; structural properties,
stability or flexibility
 to transform the natural material into a
homogenous product
 to utilize low-grade materials, minimize waste
and maximize the use of a valuable resource

Classification of Timber Composites
Timber composites can be divided into
three categories:
Layered composites
Particle Composites
Fiber composites

1.Layered composites
Layered composites are used to produce
both sections and sheets.
Since composites are often made from
relatively small sections, efficient use of
the source material can be maximized.

Types of layered composite
Layered composites can be classified into
three groups:
Parallel laminates
 Glued laminated timber( glulam),
 Laminated veneer lumber (LVL)
Cross laminates - Plywood
Sandwich panels

Glued laminated timber
Glued laminated timber (glulam) is formed
by gluing together a series of precision cut
small sections of timber to form large
cross section structural members of long
length.
The member can be straight or curved and
can be made with a variable section
according to the structural requirements.

Glue laminated lumber

Laminated Veneer Lumber
Laminated Veneer Lumber (LVL) is
manufactured from thin sheets or laminate
which are peeled from the log. The
veneers are glued together to provide the
required thickness and then cut into
structural sized sections. A proportion of
the laminates within a section may be laid
with the grain at right angles to balance
the movement characteristics of the
section.

2. Cross laminates
Plywood
The basic characteristics of plywood are
veneers bonded together, most frequently
with synthetic glues.
 In most plywood the grain of the wood in
each veneer is laid at right angles to the
adjoining one.

Cont’d
 Plywood usually contains an uneven
number of veneers so that the properties
are 'balanced' about the central veneer or
core.
 The core in some plywood may be a
double veneer. One of the outer veneers
may be a decorative.

Cont’d
The quality and durability of plywood
depends on both the timber species and
the adhesive used to bond it.
The quality of the face veneer may be of
particular significance if the plywood is to
be seen.

Plywood

Advantages of Plywood
 Plywood is dimensionally stable and can be
used for large uninterrupted surfaces.
 It is resistant to splitting and can be nailed or
screwed close to the edges of the panel.
 Plywood panels can, within limits, be bent
without cracking to form smoothly curved
surfaces.
 Common uses for plywood are sheathing,
paneling, floors and structural diaphragms,
concrete formwork, furniture and fittings.

Sandwich panels
Sandwich panels are built up of layers of
different materials. Normally the outside
layers are of high strength and stiffness
with a thicker core of lower strength
material.

Sandwich panels

Other types of layered composites
Other types of layered composites are
produced, for example cores of timber
strips produce boards by the name of
battenboard, blockboard or laminboard,
depending upon the width of the strips
used.

3.Particle Composites
Chipboard
 Chipboard is produced from dried and graded
chips mixed with resin which are formed into
boards by curing in a heated press. Board
thickness range from 6 - 25 mm, although
panels up to 70 mm thick can be produced.
 Chipboard has a wide variety of uses in building,
such as flooring and cladding. It is widely used
pre-painted or faced with decorative wood
veneers

Chipboard manufacturing

Cont’d
Fiberboard
Fibers are produced from chips of wood (mainly
from forest thinning) which are reduced to a pulp
by mechanical or pressure heating methods. In
wet process boards the pulp is mixed with water
and other additives, formed on a flat surface and
pressed at high temperature.
Types: Soft, medium or hard board

Hardboard manufacturing

COMMERSIAL SIZES OF TIMBER
Moldings
Battens and Stripes
Plank

Fire resistance and permeability