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TOPIC: 1. TIMBER
INTRODUCTION: Before we begin talking about building houses, we think that students should know about where the timber comes from; the environment in which the tree has grown and the functions of a tree while it is still alive.
This topic teaches students how a tree grows, the parts of a tree, how a tree is converted into timber, the seasoning and shrinkage of timber, timber defects, common timber sizes and the names of timber used in the construction of a house.
OBJECTIVES:
1.1. Students should be able to state the names of the parts of a tree and their functions.
1.2. Students should know how a tree can be converted into timber.
1.3. Students must be able to define "seasoning of timber" and know why it is necessary.
1.4. Students should know what happens to timber during the process of drying.
1.5. Students must be able to identify the different defects and their causes.
1.6. Students must know the commercial names and sizes of the timber used for buildings .
1.7. Students should be able to name some common timber from their area and its characteristics.
1.8. Students must be able to name all the technical names of the timber used in a house construction.
METHODS: In teaching it is widely known that lessons are more effective when they also contain practicals as well as objects shown to students, in order to give them a better understanding of the subject.
1.1. After writing down the parts of a tree, prepare a cross section of a log and show the different parts and their colours.
1.2. We explain the different ways a log can be converted to timber. When possible organize an excursion to a sawmill and let the manager explain the process.
1.3., 1.4. After introducing this topic on the blackboard, prepare a piece of completely wet timber and let students take measurements. Let them measure it again after a few weeks and check if it has changed in size and shape.
1.5. Prepare some timber with defects as shown in this book and display it in the classroom. Explain to students how some defects can be prevented.
1.6. Prepare some pieces of timber in the sizes shown in the book and display them in the classroom.
1.7. Prepare some samples of common timber used in furniture and building work and describe their characteristics as well as their applications.
1.8. When possible, take the students to a building and show them all the parts of a house and name them.
NOTE: It is advised to go through each single topic first in the classroom and discuss it with the students. Students also should use their Trade Theory book to draw and write down all important things. To make work easier and faster, you can photocopy the drawings from the teachers book, cut them out and glue them in the students book.
Afterwards prepare a worksheet with questions which students can fill in during their private or nightstudies and be assessed later.
The Leaves contain a green pigment called chlorophyll. It utilises the energy of sunlight to combine carbon dioxide with mineral salts into food called carbohydrate.
The food (carbohydrate) is carried by the inner bark (sap-wood) to all parts of the tree. Trees grow each season by adding a new layer each time onto the old layer. The lines left behind during the seasons growth are called annual or growth rings.
Figure
The tree consists of three main parts: crown, trunk and roots.
a) Crown
- holds leaves together
- adds height
and spreads new growth of twigs
- manufactures food for trees
b) Trunk
- supports crown
- produces timber
-
helps to transport water to leaves
- stores food for trees
c) Roots
- anchor
- collect water and mineral
salt
- prevent soil erosion
Figure
The wood is made up of countless tube-like cells packed closely together and joined end to end.
The parts of a tree, as seen from a cross section.
a) Pith - the original tree sapling
b) Heartwood - inactive part which gives strength to the tree
c) Summer growth - tree stop, growing in dry season (PNG)
d) Spring growth - tree grows in wet season (PNG)
e) Sapwood - it is less valuable than heartwood, it offers less resistance to decay and attack by fungi and insects.
f) Cambium - thin layer of cells which forms new cells towards wood and bark.
g) Bark - protects and covers inner layer called Bast. Bast carries food from leaves to cambium.
h) Medullar rays - store and pass food horizontally
Cross section
Longitudinal
section
Single pass live sawing
Double pass live sawing
Back sawing (Tangential cut)
Quarter sawing (Radial
cut)
The sawn timber has to be stacked for drying until its moisture content goes down to 15 percent.
Drying is by evaporation: the air is able to absorb moisture, and moving air dries more effectively.
An open, well-drained site should be selected, the foundations solidly laid with provision for cross and end ventilation, and at least 450 mm above the ground.
The boards should be stacked 25 or 50 mm apart to form vertical flues.
Each layer of boards is separated by spacing strips set from 600 to 1200 mm apart to enable fresh air to enter from the side. The spacing strips have to be set in a vertical row.
The quality of the lumber depends a lot on how it was stacked for seasoning.
A waterproof covering should be placed over the stack to protect it from sun and weather.
The time taken for air seasoning depends upon several factors, such as type of timber, size of boards, time of the year, site on which the stack is built, and the method of stacking. Roughly it takes about twelve month for each 20 mm timber-thickness.
Figure
Shrinkage takes place during the process of drying or seasoning of the timber. All timber will shrink, regardless of the type or the method by which it is seasoned. Some timber shrinks more than others.
There are two stages in the drying process of green timber. Firstly the removal of the "free water" from the cell cavities and secondly, the drying of the "combined moisture" from the cell walls. It is during this second stage that timber shrinks; no shrinkage occurs during the removal of the free water.
When the moisture is drawn from the cell walls, the cell walls themselves become smaller, consequently the board becomes smaller.
cell filled
cell cavity dry
cell dry - shrinkage
The cells do not shrink to any extend in their length, hence there is very little shrinkage in the length of a board when it is dried. Similarly the medullary rays do not shrink very much in their length, and as they run across the timber (in a quarter sawn board) they tend to prevent the layers of growth rings from shrinking towards the centre of the pith. Most shrinkage, therefore takes place in the direction of the growth rings, at right angles to the medullary rays. This is called tangential shrinkage. Tangential shrinkage is about double radial shrinkage and about hundred times as great as longitudinal shrinkage.
The sapwood, containing more moisture, will shrink more than the heartwood.
Figure
Owing to the fact that the outer surface of a log are in contact with the air they dry more quickly, and therefore shrink before the inner layers of the wood. This cause splitting or surface cracks. It is therefore advisable to "break down" the log into commercial sizes as soon as possible after felling.
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. 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.
a) During growth
- Shake is called a partial or complete separation between adjoining layers of wood, due initially to causes other than drying. The three types are Heart shake, Cup shake and Star shake.
Starshake
Heartshake
Cupshake
- Knots can be caused by a branch or limb being cut through the process of sawing up the log. Knots are classified in two groups: live knot and dead knot. Live knot is left by a branch when the tree is felled.
Live knot
- Dead knot is left by branches that have been cut off or broken before felling, and which cannot be relied on to remain in position in the piece.
Dead knot
- Gum pocket is a cavity which has contained or contains gum.
Gum pocket
b) During or after drying process
- Bow is a deviation from the flat, the piece being arched.
Bow
- Twist is a spiral distortion along the length of a piece of timber.
Twist
- Cup is a curvature across the width of a piece of timber.
Cup
c) Defects through insects
- Termites or white ants attack timber structures and are a serious problem in Papua New Guinea.
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.
Types of termite damage
Left unchecked termites eat the wood, leaving the outer shell intact. Often their presence is not evident until serious damage has occurred
Common trade names:
a) Log - A trunk with branches off.
b) Plank - Pieces 38 mm to 100 mm (1½" to 4") thick and 150 mm (6") or over wide.
c) Boards - Pieces 10 mm to 38 mm (3/8" to 1½") thick and 75 mm (3") and over wide.
d) Battens - Pieces 19 mm to 38 mm (¾" to 1½") thick and from 25 mm to 75 mm (1" to 3") wide.
e) Strips - Under 19 mm (¾") thick and up to 75 mm (3") wide.
f) Moulding - Shaped timber, can be plank, boards, strips etc...
Plank
Battens and Stripes
Boards
Mouldings
Manufacturing terms:
a) Rough Timber - Timber with sawn surface.
b) Dressed Timber - Machine planed timber.
E.g. DIS - Dressed one side
DD (or D2S) - Double dressed or dressed two
sides.
DAR (or D4S or PAR) - Dressed all round, dressed four sides or planned
all round.
c) Milled Timber - A machine shaped or moulded timber. E.g. tongue and groove, rebated dowels etc. .
FOREST REGIONS OF THE WORLD
MAJOR TIMBER AREAS
SOME MAJOR COMMERCIAL TIMBER:
|
TIMBER |
COLOUR AND CHARACTERISTICS |
PROVINCE |
USES | |
| |
SAPWOOD |
HEARTWOOD |
MAP No. 3 REFERENCE NUMBER | |
|
AMBEROI |
White |
White to cream. Ripple marks. |
General |
Veneer, plywood, interior finish, formwork, concrete shuttering, packing cases, low quality matches. Floaters for rafting sinker logs. |
|
BASSWOOD |
White |
Pale yellowish cream. Tension wood. |
General |
Moulding, veneer, boards, lining, joinery, interior finish, match splints, match boxes, shutering, turnery, dowels, pattern making, packing cases, furniture, cabinet work, weatherboards, shingles, drawing-boards. |
|
BEECH, P.N.G. |
Creamy-yellow |
Pink-brown to red-brown. |
4 |
General construction, flooring, furniture, scantling, panelling, tool handles, joinery, window frames and sills, turnery, sliced veneer, bridge decking, sleepers, boat building. |
|
BEECH, WAU |
White |
Pale brown to golden brown. |
3, 4, 7, 8, |
Furniture, veneer, plywood, moulding, fine finishing, boat building (including decking) canoes, panelling, cladding, flooring, lining, turnery, carving, joinery, cabinet work. |
|
BEECH, |
Pale Yellowish Brown. |
Pale brown to honey-brown Siliceous Greasy nature Aromatic |
3, 11, |
Furniture, cabinet work, drawer slides and sides, face veneer, plywood, boat building, light framing, moulding, wall panelling, interior trim, panelling, linings, cappings, joinery, window & door frames, brush handles, turnery, bobbins, clog soles, pattern making. |
|
OAK, P.N.G. |
Pale brown |
Brown. Tannin odour |
3,4, |
Plywood, sliced veneer, casks, interior fittings, cabinet making, general construction, furniture, panelling, flooring, shingle & shakes boat building, lining, joinery. |
|
OAK P.N.G. |
Pale brown |
Brown or pinkish - Tannin odour. |
1,3,4,5, |
Plywood, sliced veneer, casks, interior fittings, cabinet making, general construction, furniture, panelling, flooring, boat building tool handles, shingles & shakes, joinery, lining. |
|
OAK, WHITE TULIP |
Straw to pale yellow |
Pale brown to cream. Ripple marks. |
2,7, |
General construction, bridge and wharf superstructure, interior finish, cabinet work, joinery, moulding, furniture, panelling, veneer, tool handles, cladding, lining, steps, hand rail. |
|
ROSEWOOD |
White to pale yellow |
Red-brown golden brown or blood-red Ripple marks Fragrant odour. |
1,2,7,9, |
Furniture, cabinet work, joinery, boat building, rifle, butts, veneer, plywood, flooring, decorative panelling native artifacts, knife handles, carpenters levels drawing instruments, turnery, musical instruments. |
|
TAUN |
Pink |
Pinkish-brown or blood-red brown. Pink and white deposits. Contains saponin |
General |
General construction, panelling, railing, domestic flooring, veneer, joinery furniture, cabinet work, boat building, spars, mouldings, interior finish, dowels, door and window frames, fibreboard billiard tables, tool handles. |
|
WALNUT, P.N.G. |
Pale yellow. |
Grey-brown with black streaks. (Tension wood) |
1,2,6,7,10 |
Decorative veneers, plywood, furniture, joinery, cabinet work, interior finish, light construction, panelling, shop fitting, moulding, flooring, turnery, cladding, lining, carving, artifacts. |
|
CEDAR, PENCIL |
Pale pink. |
Brown or pale pink-brown (Siliceous) Contains saponin |
1,6,8,10, |
Light construction, interior finish, moulding, furniture, boat building, joinery, dowels, cabinet making, panelling, veneer, flooring, turnery, carving. |
|
CEDAR, JAVA |
Pink or pale red-brown |
Red-brown of beef red. Dark coloured, deposits. |
3,7,10, |
General construction, interior finish, joinery, flooring, furniture, blacksmiths charcoal, paving blocks in temperate climates, interior trim, lining, cover moulds, rails, scotia, piles, posts, poles, decking, sleepers, billiard cue butts. |
|
CEDAR RED |
Pinkish-white |
Dark red-brown Aromatic odour White/pink deposits. |
3 |
Interior finish, cabinet work, joinery, furniture, canoes, decorative wall panelling, face veneer, boat building, bend work, cigar boxes, clog soles, ornamental work, artifacts, louvred doors, pattern making, shingles, weatherboards. |
|
CHEESEWOOD, YELLOW |
Yellow |
Dark yellow Greasy nature Bitter taste |
General |
Light framing, moulding, interior joinery, windowsills, drawer slides, shingles, weatherboards, cladding. |
|
EBONY, BLACK |
White |
Black or black with brown strips |
9 |
Decorative sliced veneer, billiard cue butts, carving, musical instruments, brush-backs, marquetry, inlay, chessmen, ornamental work. |
|
EBONY, WHITE, P.N.G. |
White |
Ivory |
7 |
General construction, telegraph poles, heavy duty flooring. |
|
ERIMA |
Pale yellow to pale brown |
Pale brown grey-yellow, purplish cast. Foetid odour. |
7,8,10, |
Interior finish moulding, veneer, plywood, weatherboards shelving, cottins, canoes, carcassing, joinery, concrete shuttering, shingles, match boxes. |
|
FIG, P.N.G. |
White |
Pale yellow |
General |
Moulding, interior work, cladding, concrete formwork, fruit cases. |
|
GALIP |
Pale brown |
Pink-brown or red-brown |
1,10, |
General construction, moulding, interior, finish, veneer, utility furniture, flooring, panelling, door and window frames, joinery cabinet work. |
|
GUM, WATER |
Pale-brown |
Brown to red-brown |
General |
General construction, boat building, crossarms, sleepers, decking, stairs, steps, flooring, windowsills, furniture, joinery, piles, poles, posts, pallets, handles |
|
KAMARERE |
White top pink |
Red-brown |
10, |
General construction, cabinet work furniture, joinery, poles posts piles, interior trim, plywood, panelling, scanting, flooring, boat building, moulding, cladding, decking, lining, sleepers, tool handles |
|
KAPIAK |
Pale yellow |
Yellow-brown to gold-brown White deposits (Siliceous) |
General |
Light framing, interior trim, moulding, shuttering, crates, canoes. |
|
KWILA |
White or pale yellow |
Brown, dark brown or golden brown yellow deposits. leathery odour |
1,2, |
Heavy construction, flooring sills, boat building, bridge and wharf superstructure, decking, steps, truck bodies, crossarms, carving, turnery, posts, poles, furniture, bench and counter tops, switchboards, joinery, cabinet work, interior fittings, decorative veneer, panelling, artifacts, sleepers. |
|
MANGO |
Straw to pale brown |
Dark pink brown, yellow splashes. Tension wood. |
3,7, |
General construction, interior trim, moulding, attractive rotary veneer, plywood, furniture, interior joinery, cabinet work, flooring, decorative wall panelling, shelving. |
|
Stump |
100 x 100 |
150 x 150 | |
|
|
Bearer |
150 x 75 |
200 x 75 | |
|
|
Joist |
100 x 50 |
100 x 75 | |
|
|
Bottom-plate |
75 x 40 | |
| |
|
Studs |
75 x 40 | |
| |
|
Nogging |
75 x 40 | |
| |
|
Braces |
50 x 20 |
75 x 20 | |
|
|
Trimmer |
75 x 40 | |
| |
|
Jamb |
140 x 25 |
140 x 30 | |
|
|
Head |
140 x 25 |
140 x 30 | |
|
|
Sill |
140 x 45 | |
| |
|
Architrave |
25 x 50 | |
| |
|
Skirting |
50 x 25 |
75 x 25 | |
|
|
Flooring |
100 x 25 | |
| |
|
Cornerstud |
100 x 100 |
75 x 75 | |
|
|
Top-plate |
75 x 40 | |
| |
|
Ceiling joists |
100 x 50 |
100 x 75 | |
|
|
Rafter |
100 x 40 |
100 x 50 | |
|
|
Purlin |
75 x 40 |
75 x 50 |
50 x 50 | |
|
Beam or Ridge |
150 x 40 |
150 x 50 | |
|
|
Barge board |
150 x 25 |
100 x 25 |
150 x 30 |
200 x 30 |
|
Fascia board |
150 x 25 |
100 x 25 |
150 x 30 |
200 x 30 |
|
Weather board |
135 x 20 | |
| |
|
Tread |
255 x 30 | |
| |
|
Riser |
190 x 20 | |
| |
|
Handrail |
100 x 50 |
100 x 40 |
75 x 75 |
75 x 40 |
|
Stringer |
250 x 40 |
200 x 40 | |
|
Figure
Figure
Figure
 |
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