Construction worker building home

Cross-laminated timber construction

With its positive attributes of being a strong, lightweight, and sustainable building product that speeds up the construction process, CLT is gaining worldwide popularity.



By Phyllis Modlin, Executive Claims Examiner, US Construction Casualty Claims
With thanks to Ray A. Howard, Director, Risk Solutions Services at Markel Service, Incorporated, for his technical input.

Cross-laminated timber (“CLT”) is a new and innovative wood product with widespread use in Europe before entering the American construction market. European builders have used CLT for over the last two decades. Today, it is a common construction product in Canada and is now gaining popularity in the United States. CLT is known for its strength, appearance, versatility, sustainability, and lower cost than traditional building products like steel and concrete. Studies predict that CLT usage could grow into a $4 billion market in the United States. So what exactly is CLT, and why is it viewed as one of the hot new building products of the 21st century?

The CLT manufacturing process

CLT is a wood panel product comprised of different layers of solid-sawn lumber that are kiln-dried and glued together. The layers of wood are bonded with adhesives at perpendicular angles and pressed tight to form panels that frequently range from 12-18 inches thick, 10 feet wide, and 40 feet long. These solid panels are compressed and dried, which creates improved structural rigidity. The end result is a superior wood product that does not demonstrate the inconsistencies of unmodified wood.

The typical nine-step CLT manufacturing process:

  1. Primary lumber selection
  2. Lumber grouping
  3. Lumber planning
  4. Lumber cutting
  5. Adhesive application
  6. Panel layup
  7. Assembly pressing
  8. Quality control and assurance
  9. CLT product shipping

A CLT project’s material and labor costs are lower than traditional concrete or steel. Additionally, it is lighter and requires a smaller foundation. From an engineering standpoint, CLT possesses improved strength, fire resistance, and is considered to be dimensionally stable. Industry proponents also argue that CLT’s carbon footprint is lighter than steel or concrete.

CLT production and applications

CLT is used in a variety of different applications. The panels can function as walls, floors, furniture, ceilings, and roofs. A panel’s thickness and length can be adaptable to each project. Manufacturers assemble and cut panels as specified by the project designer. At the mill, CLT panels are cut to size, including openings for doors, window openings, utility piping, and ductwork. Offsite work can help reduce the chance of construction site accidents. The other potential benefit is that installation proceeds at a faster pace.

In 2015, the International Building Code (“IBC”) recognized CLT as a code-compliant construction material. After this designation, cities across the United States began adopting the 2015 IBC. The Pacific Northwest city of Seattle was the one of the first to allow CLT in construction. The Seattle City Council was influenced by CLT’s ability to reduce carbon emissions, minimize waste in production, and withstand natural disasters making it a viable alternative to concrete and steel. The International Code Council (the body that established the IBC) is studying fire safety requirements and allowable heights for tall mass timber buildings. New provisions will appear in the 2021 IBC edition.

With IBC recognition and local building code changes in US cities, CLT’s use increased in a variety of structural systems, including the assembly of exterior walls, floors, partition walls, and roofs. Developers now use CLT to erect tall wood structures. Canada changed its building codes to allow for 12-story CLT buildings, doubling the prior limit. Large-scale CLT projects are underway throughout the United States. In 2019, the University of Arkansas completed the nation’s first large-scale CLT project—a five-story, 200,000-square-foot residence hall with 368 rooms.

Collapse at Oregon State University

Not all CLT projects in the United States have gone smoothly. In 2014, Oregon State University decided to replace the home of its forestry school with a new building. The new Peavy Hall was intended to symbolize the rebirth of the state’s timber industry by showcasing CLT. In March 2018, a 1,000-pound section of the third floor buckled and crashed onto the floor below. No one was injured, however significant property damage increased construction costs and delays resulted from the incident.

A protracted investigation followed the failure to determine the cause, extent of the damage, and a strategy to resume construction. Because CLT was so new to US construction, no acceptable protocol existed for testing. Engineers traced the panel’s failure to delaminating glue, which caused poor binding between the adhesive and the wood layers. Construction experts found 85 panels showing signs of delamination requiring replacement. The CLT manufacturer ultimately took responsibility for the bad panels. Poor quality occurred from temperature variations that inadvertently caused premature curing of the adhesive, resulting in poor binding.

Oregon State completed the project, but it highlights the risks of CLT. The collapse created months of delay, added costs for experts and engineers, and replacement panels adding millions to the cost of a project that climbed by 32% to $79 million.

In an effort to make Oregon the leader in American CLT construction, in August 2018, the Oregon Building Codes Structure Board adopted groundbreaking language allowing wood framed buildings as high as 18 stories, three times the then current limit. Nonetheless, in the summer of 2018, the CLT industry faced another setback when developers backed out of the Framework project, a 12-story wooden tower in Portland, due to cost concerns.

Potential advantages of CLT

CLT manufacturers are promoting timber as the essential building material of the 21st century as a comparable alternative for structures that previously could only be constructed with steel and concrete. It is now considered an acceptable and even desirable building product. CLT’s potential advantages include its favorable structural properties, its ability to be prefabricated and manufactured off-site, and its lighter weight as compared to concrete and steel.

Because CLT is lighter and requires a smaller foundation, material costs are lower than traditional construction methods. Prefabrication can help reduce labor costs because fewer workers are needed to install panels. Since CLT can be installed faster than steel, costs can be further reduced through quicker sequencing of subcontractor trades. From a timing, cost, and safety standpoint, CLT manufacturers claim that mass timber buildings are 25% faster to construct than concrete, require up to 90% less construction material delivery traffic, and 75% fewer workers onsite.

CLT proponents claim such buildings store carbon while steel and concrete actually emit greenhouse gases. The construction and operation of buildings accounts for 40% of the world’s energy consumption and approximately one-third of greenhouse gas emissions. Concrete emits a huge amount of carbon but trees are known to absorb it throughout their lifetime. Studies suggest that “1” cubic meter of wood can store more than a ton of carbon dioxide. If those trees are then converted into mass timber, that carbon is locked in rather than returned to the atmosphere when the tree dies.

Fire resistance

Critics frequently point to the risk associated with CLT’s fire resistance. Supporters of CLT contend that it’s not only safe but that it’s actually preferable to steel because wood burns in a more predictable way. In fire testing, steel connectors are common points of failure. When fire retardant is added to CLT, it yields a high fire rating. Studies also show that a seven-inch-thick CLT floor possesses a fire resistance of two hours. Steel is known for being prone to sudden collapse, and at certain temperatures it can lose its load-bearing capacity.

To manage the risk of a large fire, some insurance carriers in the London market have started issuing Fire Combustibility and Fire Retardant endorsements. These endorsements restrict coverage for damage or costs arising out of, or connected to, combustibility or fire safety requirements or the fire retardant characteristics of buildings and may specifically refer to cladding and external or internal wall systems, including composite product or material.

As CLT construction becomes more prevalent in the United States, endorsements will likely develop to manage any risks associated with fires.

Moisture resistance

It is common knowledge that most construction defect claims center on water intrusion and moisture resistance. The adhesives used in CLT production are intended to not only increase the panels’ structural integrity but provide moisture resistance. Critics argue that wood products can hide moisture for years, resulting in property damage and a potential inability to withstand vertical and lateral loads. However, in the 20 years of CLT’s use, moisture resistance on large projects has not surfaced as a common concern. This is an issue to watch in the future as CLT buildings become more common.

Worldwide CLT high-rise construction

The world’s tallest timber building is in the small Norwegian town of Brumunddal. Mjøstårnet opened in 2019. The 18-story structure contains apartments, office space, and the appropriately named “Wood Hotel.”

Industry analysts view Mjøstårnet as further evidence that CLT provides a sustainable alternative to concrete and steel. Sometimes called “plyscrapers,” tall wooden buildings are quickly becoming more common throughout Europe. From an aesthetic standpoint, the developer of Mjøstårnet promoted the psychological benefits of wood describing the exposed wood columns, with their organic appearance and differing grain patterns, as possessing a certain character that uniform concrete simply can’t achieve.

Internationally, an impressive number of timber high-rises are set to open or break ground in 2020. HoHo Vienna, a mixed-use development five feet shorter than Mjøstårnet, was recently completed in Austria.

While Europe traditionally has been the CLT leader, Canada and the United States are catching up quickly. Vancouver is already home to a 174-foot-tall wooden student residence and will now add a “hybrid” condo complex comprised of a steel and concrete core with a timber frame slated to open this year. Sidewalk Labs in Toronto is another example of the clear direction of the growth of CLT use in North America. The Sidewalk Labs project will transform a riverside neighborhood in Toronto with a projected dozen timber buildings between 10 and 35 stories tall.

Ascent, Milwaukee, Wisconsin

In the United States, work on what will be the world’s tallest mass timber building is scheduled to begin later this year. The Ascent project in Milwaukee, Wisconsin will be 25 stories with 260 apartments. The original plans for Ascent were changed to add two floors so that Ascent exceeds the height of Norway’s Mjøstårnet.

Ascent plans a concrete base parking structure and engineered wood stacked all the way to the top of the structure. It will feature manufactured timber columns and beams along with wooden floors and ceilings. The developers of Ascent claim that its use of timber represents the equivalent of taking 2,100 cars off the road. While the US predominantly uses pine and Douglas-fir for CLT, Ascent will use spruce for a lighter and cleaner look.

The supply chain

Public and private investment is fueling the CLT industry’s growth. In 2019, the Canadian government invested almost $5 million in an Ontario CLT plant. With growing demand for CLT, a large CLT manufacturer is opening a US plant in Conway, Arkansas, in 2021. One of its first major projects will be construction of a home office campus out of mass timber with 10 mid-rise office buildings. The Arkansas CLT manufacturing plant will strengthen the CLT supply chain, which has historically come from the Pacific Northwest.

A North Carolina-based CLT manufacturer plans to invest $31 million into building a 300,000-square-foot CLT manufacturing plant in Lincoln, Maine. This CLT plant will service the high demand for building materials in the Northeast. Manufacturing costs are expected to decrease as more large-scale projects are built with CLT. Economies of scale will result in a reduction of construction costs.

A hybrid approach

The traditional pairing of concrete and steel has been used along with CLT on projects of varying sizes. The University of British Columbia’s Brock Commons Residential Hall is an example of CLT and concrete used together with a concrete core, CLT flooring and walls, and glulam columns. This combination can provide structural and seismic strength, scheduling speed, lighter foundation loads, and stability in wind events. Another project using an integration of CLT, steel, and concrete is the John W. Oliver Design Building at UMass Amherst. Buildings of the future may feature CLT for floors, shear walls, and roofs; steel used for long-span beams or beams under extreme loads; and concrete used for foundations, parking structures, and cores.

Lessons learned

Peavy Hall’s collapse at Oregon State tells us that manufacturing risks and quality control with CLT production require serious consideration. Project owners should assemble a team of experienced design and construction professionals to both manufacture CLT and plan for its proper application. This should include, among other things, a QC/QA program to prevent installation of any material with a manufacturing defect, scheduled visits to the manufacturing facility, quality control regarding use of adhesives, and the use of mock-ups and testing to ensure no manufacturing defects prior to installation. Thorough documentation of the manufacturing process will go a long way in the event any manufacturing defect claims are asserted.

Proactive CLT project management and due diligence should include the following:

  1. Product selection. Confirm the source of the CLT materials. Wood has varying physical properties, including grain, texture, moisture content, weight, density, and decay resistance. The manufacturer must ensure that each batch of CLT will be consistent with the project’s specifications.
  2. On-site QA/QC. Confirm the CLT manufacturer’s quality control standards, batch inspection practices, and testing procedures.
  3. Job-site QA/QC. Ensure job-site superintendents possess prior CLT experience and consider an outside consultant if the project is complex.
  4. Product warranties. Purchasers should review the CLT manufacturer’s warranties to ensure the risks are properly allocated and the warranty extends to the end user in the event of a product defect.
  5. Transportation planning. An experienced professional should be responsible for the delivery of the CLT to the project.6. Minimize design changes. Since CLT is prefabricated, design changes will impact production and the project schedule. To avoid schedule delays, owners and contractors should plan adequate upfront design time. Each revision in the field requires more design time before production takes place.

What’s next?

With its positive attributes of being a strong, lightweight, and sustainable building product that speeds up the construction process, CLT is gaining worldwide popularity. Timber has become an essential building material of the 21st century. All indications are that a CLT revolution is underway and changing the way buildings are constructed. As more projects are successfully completed without significant claims or delays, CLT is on its way to be the building product of the future.

The CLT industry argues that the failure at the Peavy Hall project at Oregon State University is a localized issue that will not happen again. With CLT projects like Ascent in Milwaukee, Sidewalk Labs in Toronto, and many others planned, use of CLT is expanding and production is on the rise. More and more, we will see new construction projects planned using CLT either alone or paired with traditional concrete and steel.