For a century, steel and concrete have ruled the commercial building market. Today, a small but growing movement suggests that timber—the structural resource they replaced—may be poised for a comeback.
With new and stronger-engineered timber products, designers are beginning to put their trust back into wood for structural applications in timber buildings, some of which reach as high as 18 stories. Construction is now underway in Williamsburg on the first two wooden office buildings in decades. With an increasing number of projects around the world and efforts to recognize mass timber products in some building codes, designers and builders are starting to take note.
Cross-laminated timber (CLT), the material used in timber buildings, is typically comprised of three, five or seven layers of kiln-dried dimension lumber glued together to make one strong, lightweight panel. Because of its compact design, CLT has strong acoustic, fire, seismic, and thermal performance. With fast and easy installation and almost no onsite waste generated, CLT production continues to grow. Recent reports from Europe estimate that CLT production will double until 2020.
Barriers to CLT
Still, adoption has been relatively low in the United States. Most building codes in the U.S., including New York City’s, don’t allow wood construction over six stories. Manhattan’s first-ever planned timber skyscraper was scrapped after the developer cited a ‘downturn in the market’ alongside the height regulations on wooden towers. At 10 stories, the proposed Chelsea building would have required a change in city law. And though the 2015 International Building Code included standards for CLT, nearly all U.S. cities and states—except, most recently, Washington State— have yet to follow suit and make this adjustment to their codes.
Developers have also shied away from CLT in the United States for fear of creating a fire-prone building. Although CLT is fire-resistant and regulations throughout the United States require timber to survive two hours in a furnace at 2,000 degrees, mass timber has yet to earn the full trust of developers.
Lastly, CLT requires an immense amount of pre-construction planning, as panels come prefabricated, with openings already cut out. Developers looking to construct a CLT building will need advanced 3-D modeling programs, full coordination between manufacturers and developers, and patience.
Benefits of CLT
Light but very sturdy, CLT offers both design flexibility and a low environmental impact. Because it is quick to assemble, CLT can save costs by reducing construction time, with some case studies saving up to 12-15 weeks.
CLT is viewed as an eco-friendly alternative to steel and concrete. In addition to being composed of wood, a renewable resource, CLT is largely produced from timber grown in sustainably-managed forests throughout the world. By utilizing wood from forests that re-plant multiple trees in place of each one that is logged, over-logging and habitat destruction can be kept to a minimum.
CLT also helps improve weather-tightness earlier in the construction process, as the panels themselves are weather-tight. And mass timber has significant carbon-storage qualities. At a recent event hosted by the Center for Architecture, engineers and architects discussed how using timber can improve a building’s total embodied carbon storage by nearly 68 percent.
CLT, What’s Next?
While CLT is gaining popularity in North America, and even here in NYC, the market is still very much in its infancy. Building codes should continue to keep pace, adapting as engineering improvements continue to make larger and higher timber buildings safe. With fewer code barriers and greater market knowledge, time will tell if the benefits of mass timber construction—both environmental and economic—are sufficient to propel this growing trend to the mainstream.