Nowadays the main building materials used in the construction industry are concrete, steel and timber. From the point of view of ecological sustainability, there are four important differences between these three materials: first, timber is the only material of the three that is renewable; second, timber needs only a small amount of energy to be extracted and recycled compared to steel and concrete (but the implementation of its potential is not as developed yet); third, timber does not produce waste by the end of its life since it can be reused many times in several products before decomposing or being used as fuel and; and fourth, timber traps huge amounts of carbon from the atmosphere – a tree can contain a ton of CO2  – and the carbon absorbed remains embedded as long as the wood is in use.
Considering the fact that 36 percent of total carbon emissions in Europe during the last decade came from the building industry, as well as 39 percent of total carbon emissions in the United States, the materiality of construction should be a priority for governments’ regulations in the future as measurements against global warming. The amount of CO2 in the atmosphere and the level of carbon emissions of the big economies across the globe are big issues that need to be solved with urgency in order to avoid larger, more frequent climate catastrophes in the future. The current regulation in several countries of the EU, which is incentivizing the use of renewable materials in buildings, is showing the direction the building industry in many other parts of the world should follow. And if these measures are adopted across the EU and beyond – if other countries start to follow this tendency as well – there will be significantly more wood in cities.
However, though the use of wood is one of the most effective mechanisms to decrease CO2 emissions in building construction, there are other considerations that should be made at different scales of the built environment. City density, for example, is directly related to carbon emissions. It is a fact that dense cities are significantly more sustainable than sprawling cities; therefore one path to more sustainable forms of living might be the planning and regulation of compact wooden cities.
But a dense city necessarily requires the construction of high-rise buildings, posing challenges to wood construction technologies, since wood has traditionally been used in small buildings where the structural demands are lower. Also, the durability of wood due to moisture decay and fire has been a problem for timber structures. Fortunately, new timber-based products are being developed which are structurally stronger and last for longer periods of time without any moisture and fire complications. These new products allow us to build high-rise buildings, turning timber into a feasible and convenient alternative to traditional high-rise building materials such as concrete and steel. Wood technology will undoubtedly keep developing along this path, making it possible to build skyscrapers in the future.
Even though material innovation and new technologies have increased the durability of timber significantly, there are still people who argue that steel and concrete are much more durable and, therefore more sustainable. However, the difficulty of reusing these materials is an issue. Nowadays, cities are very dynamic and are constantly changing, and thus the average life span of a building is not as long as it used to be in the past; today buildings die young. A study of residential buildings in the United Kingdom claims that 46 percent of demolished structures were between 11 and 32 years old at the time of their demolition. The same study shows that in Japan, the typical life span of office building is between 23 and 41 years. The data is very similar in many other countries around the world. In the current circumstances, steel and concrete buildings are constantly producing waste – demolished buildings – which means that their durability properties are a disadvantage in light of the “early” demolition of a considerable amount of the built environment. On the other hand, wood is a material that can be easily reused or recycled, or even used as fuel at the end of its use for construction purposes. This energy can be used to heat other wooden buildings or to produce wood-based products. This way, timber can easily become a carbon-neutral material.
High-rise timber buildings will need the development of new structural systems if the industry is pursuing the construction of buildings higher than twelve stories – the highest wooden buildings erected up until today. New structural systems are starting to use a variety of different wood-based products, taking advantage of the qualities and properties of each product for the diverse functions that structural systems require. A skyscraper is a very complex structure and it cannot be built using timber exclusively, therefore in the future the structural systems will probably be mixed, but they should always use as much timber as possible and decrease the amount of steel and concrete.
Nowadays the most widely used wood-based products available on the market are Glued Laminated Timber (Gluelam), Cross-Laminated Timber (CLT), Laminated Veneer Lumber (LVL), Laminated Strand Lumber (LSL), and Parallel Strand Lumber (PSL). Gluelam is produced by gluing together individual planned timber laminations to form continuous timber members, creating a homogenous composite material without limitations of width and length. The individual pieces are joined with finger joints, so there are no potential weak points. Because of this, it is possible to standardize the quality of timber and develop timber structures with engineering precision, therefore providing an ecological alternative to steel and concrete. CLT is a massive panel of several individual plies glued together at 90 degrees to each other. The deformation seen in solid wood due to variations in moisture conditions is practically nonexistent in CLT, and this stability can result in very precise tolerances for prefabrication construction applications, making it possible to build with the same precision as steel and concrete. LVL is produced using thin layers of softwood veneer glued together and usually oriented in the same direction. It can be very strong in the longitudinal direction parallel to wood fibers, and as large dimensions for floors, roofs and walls or as a columns and beams. LSL is similar to LVL but instead of layering thin veneers it is made from layering flakes of wood pressed together with adhesive. PSL is manufactured from strands or strips oriented in the same direction and combined with adhesive to form large format billets. It is used in applications where high bending and/or compression stress is needed, such as long spam beams.
All of the wood-based products available in the market are frequently used for different parts of buildings, fulfilling particular functions according to the specific characteristics and properties of each product. But all of these products require huge amounts of timber, and the concerns of many people regarding deforestation in service of the construction industry are more than justified. The demand for wood in a scenario where timber is the main material of construction in cities could be catastrophic for forests and the environment – if the extraction is not well managed. The practice of logging and re-logging an area, each time taking the best of what has grown there with no provision for the future could be a disaster for the environment. When the eventual regrowth of the forest is based on stunted and malformed trees, the new forest is of lower quality. This is currently a problem in many countries, but forest management in the EU is demonstrating that it is possible to produce more forest than what is being harvested. Therefore, better management is imperative in order to be able to maintain and even increase the area of our forests while still using them intensively for construction. Silvicultural and genetic improvements have been increasing productivity and will even more in the future. Today, an integrated modern operation can convert more than 80 percent of a tree into useful products, with most of the rest converted into fuel. In order to increase productivity, reliance on smaller and younger trees is necessary. Logging trees when they are young essentially means producing smaller pieces in bigger numbers than larger ones. Considering the today's advanced timber technology this should not be a problem since very stable and strong products can be made from small, lower quality pieces. Using young trees as a material for wood-based products is also more sustainable since trees absorb CO2 faster in their first years, so more carbon will be embedded in timber if we cut young trees and rapidly regrow new ones. If forests are well managed and the technology continues developing, the demand for timber can be covered by the forestry industry without problems.
In conclusion, the challenges of global warming and emissions of CO2 should be solved partially through the densification of cities using timber as the primary material of construction. In order to achieve this, structural systems and timber-based products must continue to develop, and the forestry industry should be prepared to respond to a higher demand for wood in the future, which can be achieved by increasing the productivity and efficiency of the extraction of this renewable resource. Compact wooden cities seem to be viable and effective way of creating a sustainable built environment in which many people live. However, the adoption of wooden construction in cities needs to happen faster than it is taking place at present. This is possible, I believe, only through increasing construction regulations which promote the use of wood as a building material and the development of new and innovative wood technologies, in order to catch up with accelerating global warming.
- Mayo, J. (2015). Solid Wood: Case Studies in Mass Timber Architecture, Technology and Design. New York: Routledge, p. 9.
- European Commission for Research and Innovation (2016). Challenges Ahead. Retrieved from: http://ec.europa.eu/research/industrial_technologies/eeb-challenges-ahead_en.html
- US Green Building Council (2016). Buildings and Climate Change. Retrieved from: http://www.eesi.org/files/climate.pdf
- O’Connor, J., & Dangerfield, J. (2004, June). The environmental benefits of wood construction. In proceedings, 8th World conference on timber engineering (Vol. 1, pp. 171-176).
- Jeska, S. & Pascha, K. S. (2015). Emergent Timber Technologies: Materials Structures Engineering Projects. Basel: Birkhäuser Verlag GmBH, p. 52.
- Mayo, J. (2015). Solid Wood: Case Studies in Mass Timber Architecture, Technology and Design. New York: Routledge, p. 17.
- Ibid., p. 15.
- Ibid., p. 15.
- Hoadley, R. B. (2000). Understanding wood: a craftsman's guide to wood technology. Taunton press, p. 255.
- Ibid., p. 256.