Across the street from the University of Toronto's football stadium, a new 14-story building is taking shape, but this isn’t just any ordinary construction project. Workers are assembling massive wood beams, columns, and panels, giving the half-finished tower the appearance of a giant piece of flat-pack furniture in the process of being assembled. This structure represents a growing trend in architecture and construction—mass timber, a groundbreaking technology that replaces traditional steel and concrete with engineered wood elements.
The University of Toronto building is part of a global shift toward using mass timber, a method that employs large, manufactured wood elements to create strong, sustainable structures. In its simplest form, mass timber consists of layered wood that is glued, nailed, or doweled together under high pressure. These layers form beams and panels that can extend up to 50 meters in length, creating a renewable alternative to concrete and steel, which together are responsible for nearly 15% of global carbon dioxide emissions.
Mass Timber’s Global Emergence
Mass timber is steadily gaining momentum in the construction industry, with its distinctive wooden structures beginning to reshape city skylines around the world. The 25-story Ascent skyscraper in Milwaukee, completed in 2022, is currently the tallest mass timber building, and by that year, 84 buildings of eight stories or more had either been built or were under construction globally, with another 55 proposed. Most of these developments are concentrated in Europe, with North America, Australia, and Asia slowly catching up.
In the U.S. alone, more than 1,700 smaller mass timber structures had been completed by 2023, signaling widespread interest in this new form of construction. In fact, mass timber's growth is driven not just by its aesthetic appeal but by its potential to significantly reduce the environmental impact of the construction industry.
Aesthetics and Structural Strength
One of the reasons mass timber is so appealing is its warmth and natural beauty. “People get sick and tired of steel and concrete,” explains Ted Kesik, a building scientist at the University of Toronto’s Mass Timber Institute. Timber, with its natural variations and soothing appearance, creates an environment that feels both unique and welcoming.
Beyond its visual appeal, mass timber offers surprising strength. Although wood may seem like an unconventional choice for large buildings, engineered wood composites such as glue-laminated timber (glulam) and cross-laminated timber (CLT) can be stronger than steel by weight. Glulam beams and CLT panels have revolutionized the industry, allowing wood to replace traditional materials in structural elements, floors, and walls.
Despite its strength, however, wood requires more space to support taller structures. For skyscrapers like Milwaukee’s Ascent, architects often combine mass timber with steel and concrete to maintain the necessary strength without compromising interior space.
Overcoming Fire and Moisture Concerns
One of the biggest hurdles mass timber construction had to overcome was fire safety. Historically, building codes limited the height of wooden structures due to concerns about their ability to withstand fires. However, engineers have proven that mass timber can resist fire for extended periods, primarily because it forms a protective char layer when exposed to flames. This layer insulates the inner structure, slowing the spread of fire and preventing collapse.
In 2021, the International Code Council updated its International Building Code to allow mass timber buildings up to 18 stories, signaling a major milestone for the industry. With growing confidence in mass timber’s fire resistance, more and more local governments are adjusting their building codes to accommodate taller wood structures.
While fire is no longer the primary concern for mass timber, moisture presents a different challenge. Wet wood is prone to deterioration from fungi and insects, so keeping moisture under control during construction and throughout the building’s lifespan is critical. Builders use advanced moisture management systems, including designing ventilation to minimize dampness and treating the wood with protective chemicals when necessary.
Addressing Climate Concerns
Mass timber is often hailed as a greener alternative to conventional materials, but experts caution that its environmental benefits depend on how wood is sourced and managed. A 2020 study estimated that the 18-story Brock Commons in British Columbia avoided 2,432 metric tons of CO2 emissions compared to a similar steel-and-concrete building. These savings came from both reduced emissions during production and the carbon locked within the wood itself.
However, critics argue that much of the climate benefit relies on assumptions that new trees will be planted to replace the ones harvested for mass timber and that those trees will absorb the same amount of CO2. There are also concerns about deforestation and the impact of increasing demand for timber on biodiversity and land use.
Moreover, not all parts of harvested trees end up in buildings. Branches, roots, and wood waste can decompose or be burned, releasing CO2 back into the atmosphere. Additionally, if a mass timber building is eventually demolished and the wood is sent to a landfill, it may release methane—a potent greenhouse gas—as it decays.
Integrated Design: A New Construction Model
In addition to environmental benefits, mass timber is part of a broader shift in construction toward integrated design. In traditional construction, an architect designs a building, and various firms are later hired to handle different aspects of its construction. With integrated design, however, all elements of the building—from its foundation to its ventilation systems—are planned in advance with the collaboration of all stakeholders.
This approach allows for much faster on-site assembly, with some mass timber buildings being completed up to 40% faster than conventional structures. In many ways, mass timber buildings resemble large-scale, pre-manufactured products, where components are made off-site and then shipped to be assembled on location.
As Kesik describes it, building a mass timber structure is “just like an oversized piece of Ikea furniture. Everything sort of goes together.”
The Future of Mass Timber
While the debate over mass timber’s full environmental impact continues, its potential as a sustainable building material is hard to ignore. As the world grapples with the pressing need to reduce carbon emissions, mass timber offers a promising path forward, with its combination of renewable resources, modern engineering, and unique aesthetics.
With growing regulatory approval, technological advancements, and a focus on sustainable design, mass timber could soon become a common feature in cities worldwide, reshaping our skylines and pushing the boundaries of green architecture.
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