Business of Farming: Cross-Laminated Timber

BY DR. ADAM MAGGARD

What it is and its relevance to Alabama

Originally published in the Backyard to Back 40 Newsletter Volume 3, June 23, 2020

Wood is one of most well-known construction materials for consumers, but it is often perceived as boards such as two by fours and as plywood, when it is much more than that. Engineered wood products and mass timber systems, like cross-laminated timber (CLT), are increasingly used for mid- and high-rise construction in the United States. Several high-tech wood products firms operate within Alabama, producing such engineered wood products, including the first producer of CLT products in the Southeast with a facility located in Dothan. CLT is a prefabricated, engineered solid wood panel made of layers of dimensional lumber that is stacked crosswise at right angles, glued and pressed into place (Figure 1). The panels are then transported to the construction site where they are precisely connected using joints and structural adhesives. CLT originated in Europe in the 1990s. Since its origination, CLT is now used around the world, including several locations in the United States, including the Southeast.

The Southeast is a growing hot spot for mass timber production because of its large supply of southern yellow pine timber and historical production rates and efficiency of growing southern yellow pine. Alabama has a total land area of over 32 million acres, of which approximately 71% is considered productive timberland. Alabama ranks third in overall timberland in the contiguous Unites States, leads the nation in loblolly pine growing stock (~544 million tons), and it is estimated that roughly half of the standing timber in the state is of sawlog size (sawtimber) or larger. Sawtimber class timber is a source of raw material for producing CLT.

The use of mass timber has many benefits, but also provides a new market and opportunities in Alabama for sawtimber. Forest products are a critical part of our State’s economy, and CLT manufacturing provides an opportunity to strengthen that industry. Extensive research and outreach education on CLT are ongoing as it is a relatively new material to architects, building contractors, forest products industry, and consumers in North America and the Southeast. Some advantages of CLT as found through research are listed below:

• Environmental advantages – In addition to energy efficiency, as a wood product it has a smaller carbon footprint than steel and concrete because wood products continue to store the carbon absorbed by the trees once they are manufactured. CLT is a more sustainable material than steel or concrete. CLT produces a net reduction in atmospheric carbon dioxide and minimal waste production.

• Design flexibility – CLT can be used in walls, roofs and floors. It is also relatively easy to increase its thickness to meet many specifications.

• Energy efficiency – CLT panels are thick and tightly jointed when constructed resulting in less potential for passing airflow and require less added insulation.

• Disaster resiliency –

- Fire protection: Due to the thickness and cross-section design, CLT provides resistance to fire due to slow charring panels. The char provides added protection from further deterioration acting as a natural insulator protecting inner layers from flames.

- Seismic and wind performance: Continuing research has shown that CLT has excellent dimensional stability and rigidity; research on seismic testing, such as earthquake-like shaking of multistory construction, has found that CLT performs extremely well against collapse. Similarly, research shows that CLT performs extremely well against damaging winds; both instances in part are due to CLT’s structural flexibility that reduces the chances of break.

• Acoustic performance – Research shows that the mass of CLT panels and their airtight construction contributes to acoustic performance providing adequate noise control limiting airborne and impact sound transmissions.

• Construction efficiency – CLT is lighter than concrete and faster to construct because of prefabrication and preciseness of panels.

• Aesthetics – Prefabrication of CLT acts as the inner skeleton of a building; therefore, specific external visual requirements are not an issue (Figure 2A).

The use of mass-timber products such as CLT is the future of sustainable building. Alabama currently serves an important role in the production of CLT in the Southeast. Figure 1. CLT design concept of using dimensional lumber to create multilayer wooden panels. Figure credit: USDA Forest Service, Forest Products Laboratory.

Figure 2. (A) The first hotel in the United States constructed using CLT and shelled for a specific appearance. Redstone Arsenal, Huntsville, Alabama. (B) Photo of a CLT display at the hotel in Redstone Arsenal demonstrating CLT design and construction.