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What is a timber? Classification,Properties and uses of timber

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Introduction

Wood, a classic material extracted from nature's core, has been a fundamental part of building for ages. Whether it's the strong beams holding up old buildings or the modern design aspects in new architecture, wood showcases its flexibility and lasting appeal. This blog delves into the details of wood, examining its different types, the thorough testing it goes through, its natural qualities, and the various uses that make it an essential construction material.

Stacked wooden planks with a smooth texture, creating a geometric pattern. The word “Timber” is displayed above in bold black text.

Defination of timber

Timber means wood that builders use for construction work. The word "timber" comes from the Old English word Timbrian, meaning "to build." When a tree yields good-quality wood, foresters call it standing timber. After they cut down the tree, workers remove its branches and slice the stem into pieces of suitable length. They then load these pieces and transport them to the timber yard.

We refer to this form of timber as rough timber. Workers saw it into commercial sizes such as planks, battens, posts, and beams. People collectively call these cut pieces converted timber.

Early humans used timber as a building material. Many ancient civilizations built temples, palaces, and bridges with timber, and we can still see these structures today.

Classification of Timber

Experts generally classify timbers based on various criteria. First of all, they assess grain, texture, and hardness. Additionally, color, durability, and origin matter. Moreover, usage, treatment, and resistance are key. Furthermore, density, strength, and availability count. In particular, pest and moisture resistance is crucial. Therefore, selection depends on these. In summary, the list below highlights the most important ones.

  • Mode of growth
  • Modulus of elasticity
  • Durability
  • Grading
  • Availability

(i) Classification Based on Mode of Growth: On the basis of mode of growth trees are classified
as

  • Exogeneous
  • Endogeneous

(a)Exogenous Trees: These trees grow outward by adding a distinct ring each year, called an annual ring. By counting these rings, you can determine the age of the timber. Experts further divide exogenous trees into the following categories.

  1. coniferrous and
  2. deciduous.

Coniferous trees have cone-shaped leaves and fruits. Their leaves stay on the tree until new ones grow. These trees yield soft wood.Deciduous trees have broad leaves. Their leaves fall in autumn, and new ones grow in spring. These trees yield strong wood, so builders commonly use them in construction.

Commercial industries classify timber as softwood or hardwood based on its properties. The table below shows the key differences between softwood and hardwood:

  1. Softwood shows distinct annual rings, whereas hardwood shows indistinct ones.
  2. The colour of soft wood is light whereas the colour of hard wood is dark.
  3. Soft woods have lesser strength in compression and shear compared to hard woods.
  4. Soft woods are light and hard woods are heavy.
  5. Softwood resists fire poorly compared to hardwood.
  6. Softwood has a resinous structure, while hardwood has a close-grained structure.
The cross-section of a exogeneous tree is as shown in the Fig. 7.The following components are visible to the naked eye:
Cross-section diagram of a tree trunk labeled with parts including outer bark, cambium layer, sapwood, heartwood, pith, medullary rays, and inner bark.
Figure 7
  1. Pith: It is the inner most part of the tree and hence the oldest part of exogeneous tree when the plant becomes old, the pith dies and becomes fibrous and dark. It varies in size and shape.
  2. Heart Wood: This is the portion surrounding pith. It is dark in colour and strong. This portion is useful for various engineering purpose. This is the dead part of wood. It consists of several annular rings.
  3. Sap Wood: Sapwood forms the layer next to heartwood. It represents recent growth and contains sap. It actively supports the tree's growth by transporting sap upward. The annual rings in sapwood show less distinct separation and appear lighter in color. People also refer to sapwood as alburnum.
  4. Cambium Layer: The cambium layer forms a thin strip of fresh sap between the sapwood and the inner bark. It holds sap that has not yet transformed into sapwood. If you remove the bark and expose the cambium layer to the atmosphere, the cells stop functioning, and the tree dies.
  5. Inner Bark: It is a inner skin of tree protecting the cambium layer. It gives protection to cambium layer.
  6. Outer Bark: It is the outer skin of the tree and consists of wood fibres. Sometimes it contains fissures and cracks.
  7. Medullary Rags: Medullary rays consist of thin radial fibers that extend from the pith to the cambium layer. They hold the annual rings together. In some trees, these rays break, while in others, they appear less prominent.

(b) Endogeneous Trees: These trees grow inwards. Fresh fibrous mass is in the inner most portion. Examples of endogenous trees are bamboo and cane. They are not useful for structural works.

(ii) Classification Based on Modulus of Elasticity: We classify timber based on modulus of elasticity by determining Young’s modulus through a bending test. Based on the results, we categorize timber as follows:

  • Group A: The modulus of elasticity (E) is 12.5 kN/mm².
  • Group B: The modulus of elasticity (E) is 9.8 kN/mm².
  • Group C: The modulus of elasticity (E) is 5.6 kN/mm².

(iii) Classification Based on Durability:Researchers at forest research establishments conduct durability tests by burying test specimens (600 × 50 × 50 mm) halfway into the ground. They monitor these specimens regularly over several years. Based on the observed performance, they classify timbers as follows:

High durability: If average life is more than 10 years.
Moderate durability: Average life between 5 to 10 years.
Low durability: Average life less than 5 years.

 

(iv) Classification Based on Grading: IS 883-1970 classifies the structural timber into three grades-select grade, grade I and grade II. The classification is based on permissible stresses, defects etc.

(v) Classification Based on Availability: Forest departments classify timbers based on the availability as

X—Most common. 1415 `m^3`
Y—Common. 355 `m^3`
Z—Less common. Less than 355 `m^3`

Properties of Timber

  1. Colour: It should be uniform.
  2. Odour: It should be pleasant when cut freshly.
  3. Soundness: A clear ringing sound when struck indicates the timber is good.
  4. Texture: Texture of good timber is fine and even.
  5. Grains: In good timber grains are close.
  6. Density: Higher the density stronger is the timber.
  7. Hardness: Harder timbers are strong and durable.
  8. Warping: Good timber do not warp under changing environmental conditions.
  9. Toughness: Timber should be capable of resisting shock loads.
  10. Abrasion: Good-quality timber resists deterioration caused by wear. Engineers and builders should consider this property when selecting timber for flooring.
  11. Strength: Timber should have high strength in bending, shear and direct compression.
  12. Modulus of Elasticity: Builders prefer timber that has a higher modulus of elasticity for construction.
  13. Fire resistance: A good timber should have high resistance to fire.
  14. Permeability: Good timber has low water permeability.
  15. Workability: Timber should be easily workable. It should not clog the saw.
  16. Durability: Good timber is one which is capable of resisting the action of fungi and insects attack.
  17. Defects: Good timber is free from defects like dead knots, shakes and cracks.

Seasoning of Timber

Seasoning is the process of reducing moisture content in a freshly cut tree to a suitable level. This process increases the durability of timber. We classify the various methods of seasoning into:
  • Natural seasoning
  • Artificial seasoning

(i) Natural Seasoning: Natural Seasoning includes air seasoning and water seasoning. In air seasoning, workers stack timber balks on a platform about 300 mm high inside a shed, ensuring proper air circulation around each balk, as shown in Fig. 8. Over time, natural processes reduce the moisture content. Well-seasoned timber contains only 15% moisture. This method works slowly but provides good seasoning results.

Workers carry out water seasoning on riverbanks. They place the thicker end of the timber pointing upstream. After 2 to 4 weeks, they remove the timber. During this time, the process washes out a large amount of sap from the timber. Then, workers stack the timber in a shed with free air circulation for further drying.

 

Diagram showing an elevation and section of timber storage. Timber members are stacked with spacers on a platform supported by pillars, roof above.
Figure 8

(ii) Artificial Seasoning: Workers season timber artificially in a chamber where they regulate heat, control humidity, and maintain proper air circulation. This process completes seasoning in just 4 to 5 days. People use different methods for artificial seasoning, including:

  1. Boiling: Workers immerse timber in water and boil the water for 3 to 4 hours in the boiling method. Then they dry the timber slowly. Instead of boiling water, they may circulate hot steam around the timber. This seasoning method works fast but costs more.
  2. Kiln Seasoning: Kiln seasoning uses an airtight chamber called a kiln. Workers place timber inside the kiln and force fully saturated air at 35°C to 38°C into it. The heat gradually penetrates the timber. Then they slowly reduce the humidity and increase the temperature, maintaining these conditions until the timber reaches the desired moisture content. Kilns can be stationary or progressive. In a progressive kiln, carriages carrying timber move gradually from one end to the other. Workers supply hot air from the discharging end, so the temperature increases gradually from the charging end to the discharging end. This method suits large-scale seasoning.
  3. Chemical Seasoning: Chemical seasoning involves immersing the timber in a solution of a suitable salt. After soaking, workers dry the timber in a kiln. This preliminary chemical treatment helps the timber season uniformly, both inside and out.
  4. Electrical Seasoning: Electrical seasoning involves passing a high-frequency alternating electric current through the timber. When the timber contains more moisture, it offers low resistance to the current. As the moisture content decreases, the resistance increases. By measuring this resistance, workers can determine the right time to stop the seasoning process.
However, this process costs a lot. Nevertheless, some plywood industries have tried this technique. In fact, despite its advantages, manufacturers do not use it for seasoning timber on a large scale. For instance, due to high expenses and time requirements, the method is less popular. Moreover, alternative techniques are often preferred. Consequently, large-scale adoption remains limited. Therefore, industries consider cost-effectiveness. In addition, efficiency plays a role. Meanwhile, quality and speed are important factors. Besides that, environmental concerns also influence decisions. As a result, manufacturers weigh pros and cons carefully. Furthermore, ongoing research seeks improvements. Ultimately, adoption depends on multiple variables. Hence, widespread use is yet to be achieved. Thus, the industry continues exploring options.

Defects in Timber

We can group various defects that may occur in timber into these three categories:

  1. Due to natural forces
  2. Due to defective seasoning and conversions
  3. Due to attack by fungi and insects

(i) Defects due to Natural Forces:

The following defects are caused by natural forces:

(a) Knots: As a tree grows, many branches fall off, and the tree covers the stumps of these branches in its trunk. When workers saw the timber, these branch stumps appear as knots. Knots appear as dark, hard pieces where the grain distorts. Figure 9 shows some types of knots. If the knot stays firmly attached to the surrounding wood, we call it a live knot. If it loosens or falls out, we call it a dead knot.

 

Four black-and-white illustrations compare wood knots: top left, a clean live knot; top right, irregular decayed; bottom row shows cross-sections, live and decayed.
Figure 9

(b) Shakes: The shakes are cracks in the timber which appear due to excessive heat, frost or twisting due to wind during the growth of a tree. Depending upon the shape and the positions shakes can be classified as star shake, cup shake, ring shakes and heart shakes [Ref. Fig 10].

Diagrams of wood shakes: Cup shakes with curved lines, Heart shakes intersecting rings, Ring shakes within rings, and Star shakes with radial splits.
Figure 10

(c) Wind Cracks: These are the cracks on the outside of a log due to the shrinkage of the exterior surface. They appear as shown in Fig 11.

Circular diagram with radial segments resembling petals. An arrow points to a segment labeled "wind cracks," indicating a potential flaw. Black and white line art.
Figure 11

(d) Upsets: Figure 12 shows a typical upset in a timber. This type of defect is due to excessive compression in the tree when it was young. Upset is an injury by crushing. This is also known as rupture.

Rectangular piece of wood with wavy grain lines, featuring a tongue-and-groove joint on the left and right edges, evoking a natural, rustic feel.
Figure 12

(ii) Defects due to Defective Seasoning and Conversion: If seasoning is not uniform,the converted timber may warp and twist in various directions.Sometimes honey combining and even cracks appear. This type of defects are more susceptible in case of kiln seasoning.

In the process of converting timber to commercial sizes and shapes the following types of defects are likely to airse: chip marks, torn grain etc.

(iii) Defects due to Fungi and Insects Attack: Fungi are minute microscopic plant organism.They grow in wood if moisture content is more than 20°C and exposed to air. Due to fungi attack rotting of wood, takes place. Wood becomes weak and stains appear on it.

Beetles, marine borers and termites (white ants) are the insects which eat wood and weaken the timber. Some woods like teak have chemicals in their compositions and resist such attacks. Other woods are to be protected by chemical treatment.

Preservation of Timber

Preservation of timber means protecting timber from fungi and insects attack so that its life is increased.Timber is to be seasoned well before application of preservatives.The following are the widely used preservatives:
  1. Tar: Hot coal tar is applied to timber with brush. The coating of tar protects the timber from the attack of fungi and insects. It is a cheapest way of protecting timber. Main disadvantage of this method of preservation is that appearance is not good after tar is applied it is not possible to apply other attractive paints. Hence tarring is made only for the unimportant structures like fence poles.
  2. Paints: Two to three coats of oil paints are applied on clean surface of wood. The paint protects the timber from moisture. The paint is to be applied from time to time. Paint improves the appearance of the timber. Solignum paint is a special paint which protects the timber from the attack of termites.
  3. Chemical salt: These are the preservatives made by dissolving salts in water. The salts used are copper sulphate, masonry chloride, zinc chloride and sodium fluoride. After treating the timber with these chemical salt paints and varnishes can be applied to get good appearance.
  4. Creosote: Creosote oil is obtained by distillation of coal tar. The seasoned timber is kept in an air tight chamber and air is exhausted. Then creosote oil is pumped into the chamber at a pressure of 0.8 to 1.0 N/`mm^2` at a temperature of 50°C. After 1 to 2 hours timber is taken out of the chamber.
  5. ASCO: This preservative is developed by the Forest Research Institute, Dehradun. It consists of 1 part by weight of hydrated arsenic pentoxide (`A_s2O_5`,`2H_2O`), 3 parts by weight of copper sulphate (`CuSO_4,5H_2O`) and 4 parts by weight of potassium dichromate (`K_2Cr_2O_7`) or sodium dichromate (`Na_2Cr_2O_7,2H_2O`). This preservative is available in powder form. By mixing six parts of this powder with 100 parts of water, the solution is prepared. The solution is then sprayed over the surface of timber.This treatment prevents attack from termites. The surface may be painted to get desired appearance.

Uses of Timber

Timber is used for the following works:

  • For heavy construction works like columns, trusses, piles.
  • For light construction works like doors, windows, flooring and roofing.
  • For other permanent works like for railway sleepers, fencing poles, electric poles and gates.
  • For temporary works in construction like scaffolding, centering, shoring and strutting, packing
    of materials.
  • For decorative works like showcases and furnitures.
  • For body works of buses, lorries, trains and boats
  • For industrial uses like pulps (used in making papers), card boards, wall papers
  • For making sports goods and musical instruments

Conclusion

Timber, with its various kinds, precise testing procedures, distinct characteristics, and wide uses, continues to represent the collaboration between nature and architecture. In the realm of sustainable construction and design, timber serves as a guiding light, emphasizing the lasting beauty and utility nature's offering can enhance to the constructed setting.

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