Wood Construction: Evolution towards a more Sustainable Building Model

The decarbonization of cities is currently one of the main challenges that governments worldwide must address. In this context, reducing carbon emissions from buildings during their construction and throughout their life and subsequent maintenance is a key factor in achieving this goal.

The industrialization of construction, digitization of the sector, and the use of natural and sustainable materials such as wood construction are three fundamental pillars of this evolution towards a new model that will lead to more sustainable and healthy cities for their inhabitants.

Wood as a Renewable and Sustainable Material

Choosing wood construction means opting for a natural material that can be reused infinitely until its complete degradation. Wood is the only renewable construction material that grows naturally and removes CO2 from the atmosphere by capturing a significant amount of embedded carbon inside. Additionally, during its transformation, it requires minimal energy compared to alternative materials such as steel or concrete.

In the quest to capture carbon and prevent it from entering the atmosphere, various techniques have been developed to transform it into stone or use it for fracking by injecting it into the ground. However, nature has already provided a material that performs the dual function of absorbing CO2 and transforming it into oxygen, capturing carbon within its mass: wood.

This material, present since the dawn of civilization, provides a tool to actively contribute to solving a global problem that even with advanced scientific techniques is not yet solvable. Around 50% of the weight of wood is carbon extracted from the atmosphere in the photosynthesis process where carbon dioxide, water, and energy convert into oxygen and glucose. Additionally, a stabilized forest (adult, no longer growing) absorbs very little CO2 because when trees decompose or burn in fires, they release part or all of the absorbed CO2.

The only way to turn a forest into a permanent CO2 absorption machine is by actively managing the forest, sequestering the embedded carbon in buildings through wood construction. The use of what we call “technical wood” for the mass construction of buildings enables us to retain the embedded carbon, sequestering it throughout the material’s entire lifespan.

Buildings constructed with wood thus become carbon stores, and the construction industry would shift from being a source of emissions to a sink for emissions—a radical change that the planet urgently needs.

Technical Wood in Construction

Throughout the evolution of civilization, wood has always been present, initially as fuel, and as progress occurred, it became a fundamental element in city construction. Faced with the need to find a material capable of meeting the global housing demand without further impacting the planet, scientists turned their attention to a material invented in Austria in the early 20th century: technical wood.

The natural structure of wood makes it weakened by knots, faults, and cracks that occur naturally. Therefore, the precise cutting and joints (finger joint) represent a fundamental quality leap, providing constant strength throughout the entire piece.

Technical Wood or Laminated Wood
Types and Characteristics for Construction Use

Laminated wood results from bonding layers of wood with durable and moisture-resistant structural adhesives for construction use. Currently, there are different types of laminated wood:

Glulam o Glued Laminated Timber

This method of treating wood refers to the manufacturing of Glulam, a product composed of linear elements formed by sections of solid wood. This process allows obtaining lengths and thicknesses that would be challenging to find in the natural world. The procedure begins with the repair of cracks and knots, followed by solid connections that not only enhance the structural behavior but also enable the production of long boards. The process concludes with the joining of these boards using adhesives and pressing, allowing the creation of sections significantly larger than traditional lengths, thus expanding the limits of structural use in wood.

Glulam, as mentioned, is composed of linear elements with sections of solid wood approximately 20-40 mm thick. These elements have been repaired of knots and imperfections and joined using a finger-joint assembly. This type of joint maximizes the mechanical properties of wood, especially in pillars, where fibers work under compression, or in beams, where fibers work under bending (tension and compression). One dimension of Glulam is variable, while the other is a multiple of 20 or 40 millimeters, depending on the thickness of the board. The common modulation is typically 20 millimeters for both sides of the section. This process and product are essential for achieving specific lengths and dimensions that exceed the limitations of natural wood in its raw state.

CLT: Cross Laminated Timber

The third leap in quality occurs by overcoming the limitation of anisotropic behavior with a simple gesture: crossing glued boards at 90º, creating a new material called Cross Laminated Timber or CLT. The origin of CLT dates back to 1947. This young material, developed in its modern conception only about 30 years ago, matches the structural behavior of steel and concrete.

CLT consists of surface elements, boards created by an odd combination of layers – 3, 5, 7… each of them 20, 30, or 40 mm thick, which can be combined, resulting in thicknesses ranging from 60 mm to 90, 120, 150… and even reaching up to 500 mm. The dimensions of the board are related to transportation capabilities, so the length is usually limited to 12 or 14 m, and the width range, although dependent on each manufacturer, typically includes modulations of 2100, 2400, 3000, 3300 mm, and can go up to 3500 mm in board width.

Many people, upon seeing a wooden skyscraper, believe that its construction required the felling of large and iconic trees. However, the reality is that these buildings are made with CLT produced from strips of pine or spruce wood grown for this purpose.

LVL: Laminated veneer lumber

Replace the solid wood in Glulam with thin wood veneers similar to those used in plywood boards, orienting the wood fiber in the longitudinal direction of the components. LVL structures enhance the mechanical, resistant, and dimensional stability properties of solid wood and laminated wood. Additionally, they optimize the utilization of wood extracted from trees, although they involve the use of more adhesives. Currently, LVL does not have a large distribution in Spain, but it is expected to gradually gain more market share.

LSL: Laminated Strand lumber

In this case, the material used in its manufacturing is wood chips, similar to those used in OSB boards. Pressed and glued wood chips form wood profiles suitable for use as beams, columns, and floor slabs (up to 600 mm wide), but not yet in board format. Its dimensional stability and resistance to screw pull-out and fire allow it to compete with solid wood in balloon frame structures. The use of wood chips optimizes the wood obtained from logging, making better use of forest resources. Although the type of adhesives used could potentially impact the environmental load of the product, efforts are being made in this field to comply with the environmental regulations of each country.

Challenges of Technical Wood for Construction

Until relatively recently, wood has been a highly controversial material for use in construction. As we discuss in our blog article “The Century of Wood in Construction,” the current challenge is to dismantle preconceived and highly mistaken ideas about its behavior as a structural material, despite its successful use in many structures that have endured for centuries, often unnoticed as being constructed of wood. To the attributes of structural wood, we must add the leap in quality achieved through technical wood.

Some characteristics of wood that debunk preconceived ideas:

The use of wood as fuel is an anthropological concept, which is why we associate fire with wood. Indeed, like many materials, wood burns, but the problem is not that it burns, but that it burns in an uncontrolled manner. Contrary to this, wood burns in a very predictable manner, at a rate of 0.7mm/minute, creating a protective layer that prevents oxygen from entering, coupled with the low thermal conductivity of wood, ensuring that the interior wood retains its structural capabilities (Enrique Nuere Forum Madera Construcción 2022). Solid wood is a material with a good reaction to fire, a behavior that has significantly improved with technical wood. Simply by slightly oversizing structures or encapsulating wood elements with fire-resistant plates (gypsum board), the requirements of building codes can be met.

Wood is deeply rooted in the construction tradition of all peoples, which has led to the perception that it is not a modern material. However, by paying special attention to technical wood, whether Glulam or CLT, it is easy to perceive how advanced and innovative its development is. Building with wood is currently at the forefront, representing a conscious effort by an increasing number of people and professionals to combat climate change and meet the agreements reached at each COP. Many researchers are currently working on superwoods, which are structurally more resistant, improving performance for facades, thermally treated woods, and even transparent woods, replacing lignins with a transparent material.

On the other hand, there are non-academic forums that claim wood has low resistance compared to other materials, but in reality, it is the only proven material that has withstood the test of centuries. When reinforced concrete was the star material of the 20th century and steel of the 19th century, it was thought in both cases that they were materials to last a lifetime. The passage of time has proven the uncertainty of this belief, as these materials can degrade due to a change in their chemical composition or the oxidation of their reinforcement. Tests show that technical wood surpasses concrete in compression strength, weighs five times less, and a wood beam weighs half as much as a steel one, including its deflection, while resisting the same load.

This statement deviates from reality, as wood not exposed to the elements requires no more maintenance than materials such as brick or concrete. In the case of exposed wood, design plays a fundamental role. Just like with all natural materials, wood is subject to changes in appearance caused by the passage of time. A strategy used to reduce the impact of the change in appearance is to emulate its final appearance, giving the wood a pre-aging treatment until it acquires a greenish-gray color that corresponds to its appearance after one or two years. Additionally, there are low-maintenance alternatives such as acetylated wood, thermally treated wood, furfurylated wood, and even the option of a charred finish following the Japanese tradition of shou-sugi-ban.

It is undeniable that to have wood, trees must be cut down, but as they say in European Bauhaus, “cut tree save forest.” The origin of the wood is closely controlled, coming from responsibly managed and certified plantations. This activity, called silviculture, selects trees that will perform best according to statistical patterns, removing those that may limit their growth and become fuel. Additionally, enough trees are planted to counterbalance the felling so that forest areas in countries with a tradition of forestry are growing year after year, including Spain. Building with wood increases forested areas; forests are protected against possible fires, with the resulting effects on desertification processes, increased humidity, biodiversity settlement, and CO2 absorption. Wood is the only structural and construction material that is cultivated and grows throughout the year. It is renewable and circular, and thanks to the expansion of its use, the value of forests will increase, leading to an increase in forested areas on our planet.

In the late decades of the 19th century, it was discovered that moisture above 20% favored the formation of rot fungi, so it is a condition that wood remains at low humidity. Like other materials, wood has specific design and installation standards: avoiding contact with permanent moisture, ventilating to promote drying, not allowing water to pool in flat areas, not exposing joint areas, etc., are some of the recommendations for the correct use of this material. The choice of species and the design of suitable construction details constitute the foundation that every good designer and builder must master.

Like other materials, wood needs to meet the acoustic requirements of the building code, in combination with other components and through suitable joint design. Simplified acoustic calculation options, developed before the increased use of this material, excluded wood, but this does not mean that it has poor performance at all. Wooden construction systems provide comfort similar to heavy solutions in low-frequency bands, being better in medium and high frequencies. Just like with other solutions, optimal joint design and a suitable combination of coatings and acoustic elements are necessary, no more than with other materials.

Advantages of Construction with Technical Wood

To explain the differences in carbon capture between a reinforced concrete structure and a wood structure, Michael Green, a pioneer in the field, in his TED talk “Why not build skyscrapers out of wood?” states that “To build a 20-story building of cement and concrete, 1,200 tons of carbon dioxide are produced in the cement manufacturing process. If we do it in wood, with this solution, we capture about 3,100 tons, that is, a net difference of 4,300 tons. Equivalent to taking about 900 cars off the road in a year.”

Wood is a construction material that is not extracted but cultivated, a product of natural origin, recyclable, renewable, and whose production process requires low energy consumption.

It is malleable and can be machined with a high degree of precision. Wood allows a substantial change in the execution of works and enables the digitization and application of manufacturing models from other industries to construction components.

Its natural appearance produces a wide variation of tones and textures and is associated with relaxing environments and a connection to nature, enhancing comfort in homes and promoting higher performance in office and study spaces. Humans have an innate positive response to nature, known as biophilia.

Wood has been used in construction since the beginning of time and continues to be used, often without us realizing it. Most urban centers in Spanish cities are built with wooden structures. The center of Madrid, known as “Madrid de los Austrias,” is entirely constructed with wooden structures and brick masonry. In other parts of the world, there are examples of constructions that, with proper maintenance, have reached 900 years, such as temples in China and Japan.

With excellent conditions for thermal and acoustic insulation, it reduces the energy needed for the conditioning of a wooden building. According to the American Wood Council, softwood reaches the thermal insulation capacity of fiberglass, is 10 times more insulating than concrete, and 400 times more insulating than steel. Therefore, wood plays a key role in Passivhaus systems, a construction standard with very strict requirements to reduce the energy consumption of a house by 90%.

It boasts a proven efficiency that surpasses concrete in terms of safety standards, especially due to its ductility. It is highly unlikely that a wooden structure will collapse. The greater elastic deformation capacity inherent in the material, the system based on articulated joints, and the relationship between the influence of seismic forces and the mass of the structure are its main virtues. Both wood and concrete or other materials can crack and break under very powerful solicitations such as an earthquake, but in the case of wood joints (plates and screws), they can be designed to fail in a ductile manner when reaching the limit of their capacity, and they can even be replaced after the event to be put back into service.

Much of the load-bearing capacity of reinforced concrete is used to support its own weight. In the case of technical wood, its weight is about five times less than concrete, with a similar load-bearing capacity. This means that a wooden structure transmits a lower axial load to its foundations than a concrete structure. For example, in Dalston Lane, they used 2,000 tons of wood. If the same construction had been done in concrete, 12,000 tons would have been used. This substantial weight difference between the two solutions allowed the developer to use the permitted building capacity, which was highly limited due to very unstable soil.

There is a growing awareness and a great need to transform cities, buildings, and our homes. Current policies in European countries, with the UK at the forefront, are mobilizing and raising awareness worldwide about how to take the lead in low-carbon construction, specifically in wood.

Despite preconceived and highly erroneous ideas about the behavior of wood as a structural material, wood has evolved into a new-generation material that we call technical wood, which more than meets the functional requirements of structural stability, fire resistance, durability, and those related to the circularity of materials, the energy required for their production, and their carbon absorption percentage in these times.

The growing number of tall buildings built with technical wood demonstrates the possibility of transforming an industry that has been very polluting until now into one that combines respect for the environment, the required profitability in projects, and constructive beauty to achieve a cleaner, healthier, and fairer world.