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.