Steel and concrete have long been the preferred tools for constructing high-rise buildings thanks to their exemplary strength and structural simplicity.

But steel and concrete aren’t without their downsides, particularly in an increasingly environmentally conscious world. Scientists estimate that more than six percent of global greenhouse gas emissions comes from the production of steel; the production of concrete adds an additional five percent. The growing realization of the carbon impact of these materials has led many in the A/E/C industry to refocus attention on one of the original building materials – wood.

While producing steel and concrete requires emitting carbon, wood acts as a carbon sponge – storing greenhouse gases for generations. With modern day sustainable forestry practices now the standard, wood is also a renewable resource.

While wood is widely used for low and mid-rise construction in the United States, long-standing building codes have stood in the way of wood being more prevalent in high-rise urban construction. Tragic urban fires around the turn of the century led to the proliferation of limits on the height of wood construction. Builders have attempted to push the envelope over the years, to the point where Type III-A buildings with sprinkler systems can go up to 85 feet above-grade, but concerns over fire safety have continued to place a ceiling on urban wood construction.

But in recent years a new product is shifting the paradigm of what builders can safely do with wood. Cross-laminated timber (also known as “mass” timber) is fundamentally simple – multiple layers of timber are stacked on top of each other at 90 degree angles and laminated together to form a building block, similar to a glulam beam or a large, thick piece of plywood. These custom-made panels can be as large as 64 feet and as thick as 16 inches. The pieces are then transported to the building site and assembled on-site.

CLT has been used internationally for more than 20 years, including in a nine-story apartment building in London and a 10-story apartment in Australia. Now the technology may be catching on in North America, as architects and engineers get more comfortable and the industry gets more information on CLT’s technical capabilities. Vancouver, B.C. architect Michael Green designed the 96-foot Wood Innovation and Design Centre at the University of Northern British Columbia – currently the tallest all-timber building in the world. Green and other wood builders think this is just the start – they’ve begun penciling out preliminary designs on 30 or 40-story all-wood buildings. Green recently gave a TED talk on the subject of CLT and high-rise wood construction:http://www.ted.com/talks/michael_green_why_we_should_build_wooden_skyscrapers?language=en

As Green outlines, CLT is potentially advantageous in a host of ways:

• It’s quick and efficient. CLT panels can be perfectly crafted off-site and quickly assembled in urban settings where assembly space is hard to come by. The Bridport House in London was assembled in 12 weeks by four skilled laborers and one supervisor.
• It can be made from trees that might not otherwise be used. While much wood construction material comes from large, mature trees, CLT can be made from underutilized tree species or small pieces of timber. Some companies are even experimenting with using pine trees that have been killed by the pine beetle infestation that has afflicted forests in the western United States and Canada. These dead forests are at risk of massive forest fires, which would release tons of carbon into the atmosphere. British Columbia alone has 44 million acres of beetle-infested trees. Repurposing this timber into building material that could supplant steel or concrete could greatly impact Canada’s total carbon emissions.

While the advent of CLT is promising, it will have to clear some hurdles before gaining popularity, including:

• Fire protection. Tests of CLT have shown that it performs very well during fires due to its tendency to char on the outside, preserving the structural integrity of the inside layers. CLT performs better than conventional wood framing and on par with heavy timber construction. But testing in more dynamic situations will be necessary to convince American jurisdictions to alter building codes and allow for high-rise wood construction.
• Cost. While CLT pencils out as a cost-effective construction method, there are currently only a few manufacturers in North America, meaning high shipping costs for some projects. CLT could help revitalize rural communities that saw huge job losses with the decline of the timber industry in recent decades, but timber producers need assurance that the market for CLT will be healthy in the coming years before investing in the necessary production capacity. Federal and state governments have stepped up with grants to kick-start the industry in recent years, including Oregon, which hopes to create the first CLT plant in the state near the city of Riddle.