If everybody knows what wood is, the material itself and the construction field are still full of mystery for most of us. Wood is a breathing, natural and beautiful element largely used for buildings, shelters, boats, tools, furniture as well as design and decoration. A large part of the elements and objects surrounding us in our daily life are made or can be made from wood.
Wood crafting and manufacture have belonged for a long time to the artistic domain, requiring knowledge, savoir-faire, and even finesse. The final wood products are commonly appreciated for their resistance, durability, history as well as natural and gorgeous aspects.
Everybody should be able to find a variety of wood that they adore, as the choice of wood is considerably huge.
Due to wood characteristics, the possibilities of uses are enormous and the variations of pattern, color, finishing… also make possibilities endless when it comes to work and transform this material.
To give you a general understanding as well as complete vision of wood, its origin, transformation, and uses, we focus in this rapport on this versatile and promising material.
For years, botanists, researchers, scientists, and specialists put all their efforts together and managed to index around 60 000 different tree species, growing worldwide. The species are listed under different varieties (pine, oak…), also defined as Genus (plural genera). The GloablTreeSearch database will be evolving in parallel with discoveries, and classification development. The database can be visited on the Botanic Gardens conservation international website, a membership organization representing botanic gardens in more than 100 countries worldwide.
Trees can live in their natural environment for thousands of years. The oldest trees in the world are aged around 5 000 years, and about three trillion trees are estimated as mature.
The trees are composed of different parts. The trunk, used for timber, contains solid woody tissue and vascular tissue.
To better understand the growing process of trees, let’s imagine a trunk from outer to inner parts.
The tree’s outer bark is a protective layer, keeping it away from moisture and drying, acting as insulation, and protecting it from insects and other exterior attacks. The second layer of bark, called inner bark or phloem, circulates the nutrients. This layer is generated fast, the part in contact with the outer bark dies and becomes a section of the outer protective barrier.
This dying phloem, jammed between the inner and outer bark is also the cork.
The cambium cell layer is situated between the bark parts and the sapwood, also described as new wood. It is an essential element in the production of fresh bark and new wood. The auxins hormones, reacting with the phloem content, stimulate the growth of the cells. The auxins circulate in the tree parts from Spring when they are produced by leaf buds on branches.
The sapwood circulates water within the tree parts. As new rings of sapwood are created, the old ones, closer to the tree center, naturally turn into heartwood, due to the impoverishment of their cells. The sapwood is lighter than the heartwood, contains a high percentage of moisture, and holds, for certain species, the tree sap.
The heartwood is the hardest part of wood contained in the tree. It is its central pillar. Its strength will remain intact, and pieces of timber coming from this part will support substantial weights.
Below the ground, the roots spread out widely to extract and cumulate the moisture and nutrients from the soil. At the top of the trees, the leaves perform photosynthesis to provide the tree with other vital nutrients. All tree species do not perform photosynthesis the same way, as this is the case for needled trees.
In the construction industry, a large choice of trees is available and many wood varieties are used for different purposes. From a historical perspective and as men started to build shelters, the wood appeared as the second building material after stones. With time, understanding, experiences, and scientific discovery, some trees have been revealed to be more adapted to building and construction than others.
The structural features and advantages of wood can only be enhanced when it comes to construction. It is not without reason, and beyond the fact that wood has been an abundant resource, that men have always selected this material for buildings.
Generally speaking, wood is a strong, resistant, light, flexible and durable material. If burnt, it provides a new energy resource, and if used in constructions, it makes the buildings benefit from a natural sound and thermal insulation. Besides, wood is a natural, ecological, and renewable resource. Wood can balance its structure with its environment, absorb moisture thanks to its hygroscopic characteristics, and store CO2.
This breathable material has been subject to many studies conducted around the world to demonstrate and enhance its positive effects, not only on the environment but also on the house climate and occupants of the dwellings.
Wood is recommended in the seismic areas, as the assembly system makes it less likely to collapse than another type of building. It can support heavy and tensile weights and is well adapted to various grounds. In addition to be a naturally good insulation material, wood also prevents static electricity or dust circulation, making the house interior more pleasant for allergic persons, for instance. It has also been proven that wood is a considerable asset for health, offering a relaxing atmosphere as well as physiological and psychological benefits.
Concerning its thermal properties, wood has the advantage to be resistant to high temperatures, and have a low heat conductivity compared to materials such as aluminum, glass, or steel.
The tree genera are classified under three main categories, according to their common features. The three main categories are the softwoods, the hardwoods, and the engineered woods. Following this classification, is it also easier to determine the uses of wood.
The softwood category is mainly composed of coniferous trees. The coniferous trees are mostly evergreen trees, whose needles replace the usual tree leaves, producing cones, and without flowers. The coniferous trees are also known as gymnosperm plants, literally meaning ’naked seed’.
As the softwood trees do not possess pores, the density of wood is low, due to water circulation inside the tree structure. However, all the wood belonging to this category does not present the same characteristics when it comes to density. Therefore, the right wood should be selected according to the project and its future application.
Many trees belonging to this category produce sap. The softwood trees are known to grow faster than hardwoods, making them easier to harvest and to refurnish the plantations. Another notable feature of the wood from the softwood category is the resistance of certain varieties to insects, making wood ideal for external constructions.
Many softwoods are strong and used in various different ways to obtain diverse products. From decoration and design to the construction industry, softwoods are also used to produce paper pulp, for instance. The most common uses are construction frames, building structures, doors, window frames, or furniture.
From an engineering point of view and per their uses, softwoods density and thickness are relatively easy to monitor, and allow to achieve high-quality wooden elements.
The products made out of softwoods are offering a clean and seamless finish. If not treated, the woods are generally lightly colored with clear tones.
Very often, it is common to think that the terms softwood and hardwood have something to do with the actual softness of a wood piece and product. However, this is not necessarily true and proven. The softwood is, generally speaking, easier to work with than the hardwood, especially when it comes to resistance, but some varieties of softwood are extremely hard. It is for instance the case of the Douglas Fir, “hard” wood classified in the softwood category, and the Balsa wood, providing with soft pieces of wood but belonging to the hardwood group.
The hardwood category gathers any trees, which do not produce needles or cones, but whose branches are garnished with leaves and flowers. Many trees belonging to this category are described as “deciduous”, a botanic term, and are also called in the scientific field angiosperm plants, as they produce seeds and leaves, shaped differently depending on the tree species.
Many hardwoods are easily identifiable thanks to their dark color and wood grain aspect. In opposition to softwoods, showing loose wood grain, many species show distinct patterns and close grains.
In general, hardwoods are dense and heavy, and this density can be observed with a naked eye. Products and wood elements made with hardwood are resistant and durable. This is why, hardwoods are commonly employed to create beams, walls, ceiling and floors, furniture, or used for outdoor constructions.
The great resistance of the species gathered under the hardwood category can be explained by the tree structure and the relative absence of sap, and the slow-growing process, making it more mature when harvesting.
Some exotic trees are also classified in the hardwood category, such as Bamboo and Palms, even though they are not described as deciduous trees. These varieties of trees belong to the monocotyledons class, but also have similar features to hardwood species, reasons why they are classified within the same category.
The last main category of wood, engineered wood, gets under the same distinction as the manufactured wood. Engineered wood is the wood which is not occurring naturally and whose properties are hardly found within the harvested wood. Therefore, and to meet some requirements, the wood needs to be transformed to possess specific features.
The wood belonging to this category is produced from waste wood or an association of different wood varieties and is commonly known as composite wood.
The engineered wood pieces and boards can also be manufactured through a chemical process, as well as a heating process. The engineered wood can be made in one piece and in different sizes. The wood can also be produced to match a desired pattern or veneer not existing in the natural environment.
Plywood, oriented strand boards, fiberboards, glued laminated timber, or composite boards, particleboards are all examples of engineering woods. Most often, these types of wood are used largely from residential to commercial projects, including industrial constructions.
Classification of wood and timber depends on many indicators and is (or mainly) related to trees or to timber boards and transformed wood.
For those related to trees and as previously mentioned in this article, two main categories are helping to identify trees and features of the wood. Exogenous and endogenous trees have different ways of growing. If exogenous trees will grow wider extensively by adding each year one ring at their trunks, the rings on endogenous trees are hardly visible, due to the fact that trees are growing inward and that their grain pattern is denser. The age of the exogenous trees can be determined by counting the rings.
The softwoods are also known to have a lower strength in compression and shear than the hardwoods and lower fire resistance.
The wood can be classified according to its availability. The availability is determined thanks to the volume occupied by species (density of the species in m^3 per year.) The most available species are classified as most common and the rarest as less common.
The lifespan of trees also helps to classify timber. To evaluate the durability and tree growth average, tests are conducted by observing the growing conditions over the years. The classification is a division of three groups.
Timbers living more than 10 years belong to the high durability class, between 5 to 10 years, they go into the moderate durability group, and are part of the last category called low durability if they go mature within 5 years.
Other gradings are used to differentiate the timbers, their features, but also similarities and determine uses.
The timbers are classified into groups according to their elasticity (thanks to bending tests), permissible stresses, defects, as well as their resistance to tension, compression, shear. Therefore, the strength and stiffness of wood will be a great indicator to determine the best purpose of the timber.
Another example is the classification of wood depending on its density. Light density woods will belong to the first class, whereas the densest will be placed in the last groups. The grading systems and class names can vary from a country or institution to another.
This is why, regular tests are conducted on the timber, to prove that the wood chosen for application is really adapted. Pieces of wood are carefully selected from the harvest, in order to use proper wood in any situation (construction, heavyweight to support, decoration, outdoor structures…).
As some of the construction and architectural needs and challenges can not be fulfilled by the natural features of the wood, growing in nature, engineering wood also occupies an important role in the construction field.
Several steps need to be completed to transform the tree, standing in its natural environment into a beautiful piece of wood, which will be used for houses, furniture, or other applications.
The first step of the supply chain is the harvest. Not all forests can be harvested the same way, at the same pace, and for similar purposes. All the locations are not naturally garnished with fauna and flora the same way, and the different species of trees do not need all the same amount of time to develop, grow, and mature. This is why harvesting has a lot to do with forest management. Forestry harvests are also very much directed by ecology, species, and environment protection.
Many actors play a role during the harvesting step including the landowner or investor, the wood dealer, establishing prices, and acting like an intermediary, the lodger, who will harvest, and the sawmill. Eventually, other actors can intervene in the process, as professional forestry consultants.
To select the trees to harvest and value the trees, the professionals will be looking at different indicators, for example:
The species, and the needed type of wood to produce the final wooden element. As all the wood types can not be used for the same purpose, the selection of the genera and species will have a great impact on the quality of the final product. Also, the species have a lot to do with the value of trees.
The size and age: the sizes and age mainly depend on the variety selected. However, width trees will provide a larger amount of wood and tall ones will offer long solid pieces of lumber, essential for certain work and final results.
The quality and location: the quality of the final wooden product can be depending on the quality of the forest and its environment. Also, as trees and logs are heavy to transport and require proper equipment, the harvest and usually not done too far from the sawmill.
Before being transported to the sawmills, trees can be stored on the spot, allowing meanwhile, a small amount of water to evaporate. The water contained in trees is also affecting their weight.
When the wood arrives to be transformed, the first cutting will help to shape the wood, clear it or not from clark, and roughly cut the first boards out from the piece of log. The following cuts serve to shape the wood pieces at good dimensions and make them usable for final products.
Different patterns can be chosen to cut a log into several lumbers, slats, or boards, with various dimensions and for diverse applications. The patterns help to optimize the log and generate the less waste possible from the piece. Different sawing machine types will perform various cutting patterns. The cutting direction will also play a role in the final visual of the piece of wood, giving it its authenticity and uniqueness.
Throughout all the process of wood transformation, the wood will be drying, naturally, while it is stored, or with high temperature and thanks to a kiln dryer.
This process is meant to reduce and remove the quantity of water and moisture held in the tree.
Wood drying, also called wood seasoning, is an essential process as the removal of water will allow easy working. The water can cause distortions, cracks and modify the wood shape. This is even more essential given the fact that wood is a breathing material, and even after being harvested, it is naturally trying to reach a balance with its environment. A high rate of moisture can also cause the gluing process to fail.
Eventually, and once again depending on final uses, the wood can be protected from insects and fungus, treated, painted, oiled, or varnished.
Even though thousands of three varieties can be found in the forests around the world, the main genera are used for construction work. Many variations of trees and species can be found under one genus. For instance, white oak and red oak are two species categorized under the oak genus. This also explains the difference in wood density within one genus. The density of timber is expressed in kg per cube meters and calculates the actual wood material in a unit volume of wood.
PINE – Softwood
Pine is a common variety of wood used for many applications. Pine is durable, economical due to its availability, sustainability, and fast-growing process, making it a perfect renewable resource. Usually used as an indoor element, pine can also be suitable for outdoor applications. Pine is also affordable and offers various finishing.
CEDAR – Softwood
Cedar is considered ideal for an outdoor project, as the smell of the wood is also a natural insect repulsive. The wood is resistant to weather changes. Cedar wood is recognizable thanks to its color and grain and offers high thermal insulation.
DOUGLAS FIR – Softwood
Known for its structural strength, Douglas fir is commonly used as beams and posts. Mainly found in North America, it presents various color hues from yellow to reddish-brown.
SPRUCE – Softwood
Spruce is used for construction projects due to its excellent strength properties and is therefore ideal for wooden houses. Coming from an evergreen softwood tree, it has a very light color, presents finely grains, and is known to have excellent acoustic characteristics. In the past, spruce was also used for aircraft and boats but is no longer used for these applications.
REDWOOD – Softwood
Popular for outdoor building projects, redwood is also known for its color, wood pattern, and weather resistance.
ASH – Softwood
Ash is strong, heavy, durable, and affordable when speaking about the cost. The ash is light-colored, from beige to light brown. The wood rings are visible and wood presents regular grains.
OAK – Hardwood
Oak wood is a hard, strong, and heavy material, with close grains. It is resistant against insects and naturally colored with light brown hues, depending on the Oak varieties. Oak has many uses and can serve for furniture, flooring, paneling, or decking.
MAPLE – Hardwood
Maple trees are primarily growing in North American forests. Various species of maple are existing and one big difference is the softness of the trees. On one hand, hard maple is coming from the sugar maple tree, whereas soft one comes from Red Maple trees.
Softwood is making it easier to work with, but hard maple is an excellent choice for flooring and quality furniture, for instance.
CHERRY – Hardwood
This variety of trees provide rich-colored wood, smooth and fine grain. The wood is durable and flexible. It can be used for paneling, furniture, and flooring. It is sometimes used for house and roof framing.
WALNUT – Hardwood
Walnut wood is recognizable for its rich and dark brown colors, offering a unique finishing. The wood is strong, hard, and durable. Despite its dark color, the wood also has fine grains and can be easily stained and protected. It is ideal for flooring but is also used for other purposes, such as furniture or music instruments.
Birch and hemlock are also wood categorized as softwoods. Elm, willow, mahogany, and teak are part of the hardwood category.
WHAT ARE THE DIFFERENCES ?
Solid vs glued laminated
A solid log is a log cut in one unique piece of wood. Therefore one trunk is used to produce one or more solid logs (depending on their sizes).
The glued laminated logs, on the contrary, are the result of wooden strips assembly. The glulam logs and beams can be produced in many different sizes, from the assembly of a different number of strips, and manufactured in various shapes. The advantages of glulam logs are that the logs won’t be cracking through time, and the length can exceed the lengths of the solid logs.
Lumber vs timber
Lumber and timber are usually used to describe the same elements and a conflation is not rare. But is a slight difference between the two words, which depends on the stage of the wood transformation and the sizes of the wood pieces.
The wood pieces under 12 cm wide and thick, and provided by the first cuts in the tree trunk, are more likely to be called lumbers, whereas wood pieces exceeding 12 cm wide and thick and produced close to obtaining the final product, could be described as timber.
However, the difference animates debates and is not so clear, even among the specialists of the field.