Resilience: Wood for the Modern Age
Recent events have seen a flurry of promotion around a value-added softwood lumber product called cross-laminated timber, or CLT. CLT is essentially plywood made out of 2x3s. In 5- to 9-layer thicknesses, it is strong like hollow core precast concrete but more versatile. As a bio-based product it is much lower in embodied energy and impacts on the environment. As a prefabricated product it is manufactured to exacting tolerances and is an ideal substrate for prefabricated building assemblies.
It’s hard to overemphasize how big this moment is in terms of Canadian construction products. A value added softwood lumber product! The expansion of the CLT market pushes back against Canada’s reputation for sticking to the role of raw material supplier rather than developing value added products for sale. The presence of CLT is a thing to be celebrated.
While significantly lighter than concrete, it is still heavy enough to require cranes for installation, which reinforces the role of advanced mechanization in construction. This in turn opens up the construction field to a better gender balance where brains are prioritized over brawn.
Another advantage of prefabrication is speed of installation and reduction in the number of persons needed for installation crews. In a 2017 presentation on the Brock Commons residence at UBC, architect Russell Acton expressed his surprise at how quiet the construction site was compared to business-as-usual. The relationship between the construction site and the adjacent neighbours is not yet something commonly considered in Canadian construction. As Canadian cities begin a cycle of substantial Infill construction, fast, accurate, and quiet building assembly will be increasingly recognized as an advantage.
CLT is promoted under the banner of sustainability. But often claims made about CLT and carbon storage suggest more than is possible. Trees absorb carbon from the atmosphere and store it in the growing wood. The “storage” stops increasing when the tree is cut and turned into CLT. Long-term carbon storage is the product of being protected by other building elements so the CLT does not rot (water damage) or burn (exposure to building fires). The embodied impacts (energy consumption in harvesting and manufacturing, and releases to land, air, and water) of CLT are smaller than steel and concrete, but no different over time.
There is one thing however, that is discordant with the current promotion of CLT as a part of a “sustainability” equation. That is the assertion that a good use of CLT is in the construction of tall buildings. This really has more to do with attracting investment through the Daniel Burnham “make no little plans” strategy than with sustainability.
Furthermore, the meaning of the word “tall”1 in the promotion of CLT is stretched, leading to the description of low and mid-rise projects as “tall” buildings. “Tall” no longer means tall, it means “bigger than we’ve seen previously.” If any promotional campaign has to distort the use of common words to make its claims, critical review is necessitated.
In social and planning terms, tall wood is no different than tall concrete or tall steel. Tall buildings are isolating, and commonly sterilize the ground plane around them by removing the potential for vibrant street life. Tall buildings are not particularly resilient, requiring more energy for operations than can be gathered from the sun falling on the site, or even the site plus the building exterior. City mains water pressure cannot deliver water any higher than the 5th floor, so tall buildings need an infrastructure of pumps to ensure critical services are delivered.
With all of the advantages of CLT, what is needed now is pricing that will move the product. That needs volume in production and local manufacturers serving local markets. This is another place where the argument for tall buildings is counterproductive. Given the time in planning, approvals and financing required for tall buildings, one could argue that many smaller buildings would generate more CLT sales faster than tall towers. A larger number of smaller buildings would be more effective in the expansion of the market than fewer tall ones. The best target for the use of CLT may be low-rise, 6- or 7-storey mixed-use buildings that could be delivered on old commercial streets in many Canadian downtowns.
This is where the new wood products could really shine. Quiet, fast, minimally disruptive, and lightweight for smaller foundations. Low carbon CLT mass timber buildings are ideal for intensification on the avenues. But, no. Promoters of the technology have adopted the “bigger is better” argument, which cannot be supported on the grounds of sustainability. Are those of us in the sustainable building community supposed to cheer for wood high rises?! Cheer for the same building type that sterilizes the urban ground plane? Cheer for the buildings that need electricity to deliver potable water to the majority of their residents rather than relying on mains pressure? No thanks.
One of the most remarkable and hopeful statements in the short tenure of Jennifer Keesmat as chief planner of the City of Toronto2 was that the 5 million new people Toronto expects to receive by 2050 could all be housed in 6-storey mixed use buildings on the avenues. This is an important observation in the discussion of tall buildings regardless of construction material. Discussions of sustainable building start with demographics. If there is no demographic evidence that “tall” is needed, then why incur the negative aspects of that approach?
CLT is a terrific technology but it is missing its real calling. Many more CLT buildings will be built if small markets are developed. The success of Brock Commons says clearly, look for base hits rather than the home runs of wood skyscrapers. Sustainability is ubiquitous, small, and local. Why should the most sustainable structural technology go anywhere else?
NOTES:
According to Wikipedia, “The Emporis Standards Committee defines a high-rise building as ‘a multi-story structure between 35–100 meters tall, or a building of unknown height from 12–39 floors.’” But this seems a low bar for tallness by today’s standards, given the current passion for ever-taller buildings.
Even better news, perhaps, Ms. Keesmat has announced her candidacy for mayor of Toronto in the upcoming election.