For several decades there has been a debate in the building industry as to whether wood-frame or steel-frame construction is more sustainable — wood being a renewable material while steel has recycled content, often incorporating 70-80 percent old automobiles. Perhaps the debate is finally being decided due to a panel technology called cross-laminated timber, or CLT for short. Developed in Europe in the 1990s, it has only recently been gaining popularity here.
A CLT panel usually consists of three, five, seven, or nine layers of kiln-dried boards stacked in alternating directions, bonded with structural adhesives, and pressed to form a solid, straight, rectangular panel. Surprisingly, CLT has good fire-resistant properties: It is hard to ignite, and once lit resists fire spread. Because the layers are oriented perpendicular to each other, the CLT panels are exceptionally strong, stiff, stable, relatively lightweight, and able to handle load transfer on all sides. They can be used for walls, floors, and roofs in a single building system or used interchangeably with other wood products.
Most commonly, CLT panels are 40-60 feet long, but they can be as long as 100 feet. They are up to 18 feet wide and any thickness up to 20 inches. These panels are widely used in Europe, Australia, Canada, and Japan. The possibility of large panels is revolutionizing how 10-, 20-, and 30-story buildings are being built. Currently an 18-story, 400-student residence (174 feet high) at the University of British Columbia is the largest CLT structure, but a 24-story tower is under construction in Vienna, and a 35-story building in Paris is in the works. The most ambitious proposal to date is London’s CLT-framed, 80-story Oakwood Tower.
Not only are CLT panels frequently made using small-diameter trees, but they also can use less desirable wood from pest-damaged trees, or even trees killed by wildfires, without compromising the panel’s overall integrity. These small, less-than-perfect inputs to panel manufacturing are leading to better utilization of forest resources. Pulling out small- and medium-sized timber, as well as dead trees, contributes to healthier forests.
Processing these culled trees into CLT panels, which then get incorporated into buildings, sequesters carbon from the atmosphere. Not only does CLT emit less carbon dioxide during the manufacturing phase, but the finished buildings then help sequester carbon for longer periods. Scientists estimate that buildings made with these materials result in a 25-30 percent reduction in global warming potential compared to those made with traditional materials — concrete, masonry, and steel.
Because they lend themselves to design versatility, fast installation, reduced waste, and good thermal and seismic performance, CLT can reduce construction costs by up to 50 percent. Perhaps the biggest advantage, however, is sequestering carbon while creating healthier and more resilient forests.