timber office building—but at only six-stories and 58-feet high. Built with laminated-veneer-lumber columns and beams, and a core and floor slabs of CLT, the building could have been several times taller using the same structural diagram, maintains Waugh. But, instead, the 48,000-square-foot co­working space is at the same scale as the adjacent buildings. “It is the appropriate height,” he says.

Black & White’s embodied carbon is 410 kilograms of CO₂ equivalents per square meter—based on a comprehensive analysis that includes the enclosure and its mechanical systems—which represents a 33 percent reduction compared to a building constructed of more conventional materials, according to the architects. That’s without subtracting the nearly 1,000 metric tons of biogenic carbon stored in the timber structure. The two totals are kept separate, in part, says Waugh, because combining them would incentivize the use of more timber than is structurally necessary. To deploy the material as efficiently as possible, his firm worked with its engineering consultants, Eckersley O’Callaghan, refining and optimizing the framing system. The strategy also helped control costs, keeping the structure to $390 per square foot, which Waugh says is roughly equivalent to concrete.

Despite the arguments of those like Waugh who favor a low-rise approach, tall buildings will continue to be built, especially in rapidly growing cities. In Vancouver, British Columbia, for instance, the City Council has recently approved the Broadway Plan, a rezoning initiative whose chief aim is to increase the supply of housing. Henriquez Partners Architects is designing Prototype (M5), a residential tower in this part of the city for the developer Westbank. As a result of the new zoning, the firm expects to be able to expand from the scheme’s current height of 21 stories to 25 stories, with about 20 percent of its approximately 210 apartments slated to rent at below-market rates. The building will have a concrete core, seven-ply CLT floor slabs, and steel for its vertical structure. “Timber columns would get too large at the base,” says Shawn Lapointe, Henriquez Partners principal. The hybrid strategy, combining low-carbon concrete, mass timber, and steel, will cut the structural system’s embodied carbon in half, compared to a more conventional frame.

Prototype (M5)’s high-performance facade, which invokes traditional basket-weaving techniques, will also incorporate wood. Its interlocking panels include a sheet of wood veneer laminated between glass sheets in order to protect it from the elements. The panels’ backup structure will be CLT, which the firm plans to expose on the apartment interiors, says Lapointe.

Some firms are working on concepts for using timber in buildings that aren’t just tall, but supertall (a supertall tower is one that is higher than 300 meters, or 984 feet). The Canadian practice DIALOG has created a theoretical scheme for a mixed-use supertall tower—105 stories high—with a patent-pending CLT-composite timber floor plate system to go along with it. Designed for Toronto, the tower has a so-called double-tube structural system with a concrete core and an external steel diagrid that widens at the base.

To reduce embodied carbon, DIALOG’s design uses mass timber, but only as part of the floor plate. The firm focused on this element because of studies that show floor systems are responsible for as much as 73 percent of a tall building’s environmental impact. A pure timber solution would be impractical, they say, since wood is less efficient structurally at greater heights, and the long column-to-column spans considered desirable in commercial buildings would be unachievable.

DIALOG’s concept, which it has dubbed the hybrid timber floor system, or HTFS, is a prefabricated composite panel, 40 feet by 10 feet, made of seven-ply CLT and post-tensioned concrete. According to the firm’s calculations, using this element to create a supertall tower’s floor slabs would result in a modest 9 percent reduction in embodied carbon compared to a baseline design for a 105-story traditional steel-and-concrete structure. If biogenic carbon is taken into account, however, the reduction would be 46 percent. “If the wood is harvested ecologically, the strategy offers an effective way to tie up carbon for as long as the building exists,” says Craig Applegath, founding partner in DIALOG’s Toronto office.

The contractor EllisDon, DIALOG’s partner, is currently testing the HTFS system at its modular-fabrication facility in Ontario. With the help of a grant from the Canadian government, sample panels are being assessed for properties that include the integrity of the bond between concrete and timber, loading capacity, the potential for creep (permanent deformation), and susceptibility to vibration. Lastly, the samples will be burned in an oven to determine charring rates and their resistance to fire.

With the aim of achieving net zero performance, DIALOG’s supertall combines a range of efficiency and energy-generation strategies, including a double skin, building-integrated photovoltaics, and thermal-energy storage, as well as some still exotic technologies, such as an algae bioreactor. The bioreactor would transform the CO₂ produced by the tower’s gas-fired combined cooling, heating, and power trigeneration plant into algae, which could then be used as feedstock for products such as fertilizers, plastics, or pharmaceuticals. A similar system is currently being piloted at a Toronto power station, says Charles Marshall, a DIALOG sustainability engineering partner. “It is not so out there as to be pie in the sky,” he says.

With its range of accompanying technologies, DIALOG’s HTFS acknowledges that addressing the climate crisis will require a multipronged approach. Given the problem’s urgency, “we can’t afford to be purist about any one solution,” says Marshall. But mass timber, in not-so-tall, tall, and very tall buildings, could be a big part of the answer.

Supplemental Materials:

Craig Applegath, Cameron Veres, Thomas Wu. ““The Zero-Carbon Hybrid Future of Tall Timber.” CTBUH Research Paper, 2021.