Over the past decade, the design industry has been increasingly looking toward timber as a building material for the construction of tall buildings. This interest is partly due to the development of new engineered timber products and the potential economic benefits of prefabricated timber elements and composite building systems. However, a recent emphasis on the importance of green and sustainable architecture and an understanding of the potential sustainability benefits of tall timber buildings can be seen as the primary motivator for many architects, owners, governments, and other building stakeholders wanting to design with timber.


Owners, managers, designers, and some government agencies have been placing a greater importance on sustainability in building construction and operation, as buildings are a major contributor to greenhouse gas emissions in their construction and operation.1 This has led to increased interest in the use of timber in buildings, as timber can be considered an attractive material for green building construction.

The use of timber in building construction can positively contribute to sustainable building practices in many ways:

  • Timber is considered a renewable resource and the forests supplying timber can offer a natural carbon sink;
  • The resource extraction and manufacturing phases of timber products demand a very low amount of energy relative to more conventional structural materials used in construction; and
  • Innovative timber systems designed for prefabrication and disassembly allow for reuse of the material and a more resource-efficient product life cycle than typical demolition and down-cycling.

In addition to the sustainability benefits, timber has other positive attributes relative to other building material types:

  • Possibility of offsite prefabrication and minimized onsite work allowing for high-quality certified production, independent from weather and a rapid erecting progress;
  • Reduction of building weight, resulting in savings in foundation works when compared to other construction materials;
  • Ease of alteration onsite; and
  • Increased flexibility in architectural design options.


Timber products, assemblies, and methods of construction have evolved over time. Conventional experience with timber buildings is typically limited to low- and mid-rise residential and commercial buildings. These buildings generally utilize light timber frame construction and are limited in size and open area. This is different from heavy timber frame construction that is increasingly being used for mid- to high-rise residential and commercial applications.

While light and heavy timber framing are used for different applications, the primary differentiator between the construction types is the section size of the timber members used in construction. Although there is currently no universally accepted definition of "light” and "heavy” timber, timber can be considered as heavy where its minimum dimension of solid wood exceeds approximately 80 mm (3").2

In general, light timber frame construction is composed of a greater number of small-section stud members to form wall and floor assemblies, typically enclosed within cladding to form wall or floor framing elements. Light timber frame construction is typically used in low- and mid-rise residential buildings and is often used in buildings up to five- and six-stories, typically above a reinforced concrete ground floor. Framing methods include "platform” and "balloon,” or stick-framed construction.4

Heavy timber frame construction is composed of a lesser number of large-section engineered products to form the building superstructure. While this includes solid sawn lumber sections, modern timber buildings generally use engineered timber products. Relative to solid sawn lumber, engineered timber products offer greater strength and design flexibility and have enabled greater ambitions in architectural and structural design.

The use of heavy timber frame construction allows for greater design flexibility (relative to light timber frame construction) including longer unsupported spans, open-plan areas, and taller construction. The two predominant forms of heavy timber construction include post and beam construction and panelized construction.

Characteristics of light and heavy timber frames are discussed in a number of design guidance documents.2,3,4,5

Post and Beam Construction

Post and beam construction utilizes a range of different products. This includes traditional products such as solid sawn lumber, and contemporary engineered products such as glue-laminated wood, laminated veneer lumber and cross-laminated timber.

Solid sawn lumber consists of large-section timber members that are cut down to size. Given the size of the elements, solid sawn lumber is typically used as structural column and beam framing members. Glue laminated wood (glulam) is an engineered product that consists of smaller pieces of wood, nominally 2 in x 4 in (50 mm x 100 mm), which are adhered, or laminated, together. This produces a structural element that is stronger than solid sawn lumber as it is more homogenous and reduces the impact of knots and other imperfections. Similar in size and structural application as solid sawn lumber, glulam elements are commonly used as structural beams and columns.

Laminated veneer lumber (LVL) is composed of multiple layers of thin wood veneers, approximately 3 mm (1/8") thick, which are laminated parallel to each other under heat and pressure. Slicing the timber into thin veneers and laminating them together reduces the effect of imperfections in the wood, resulting in improved structural performance compared to solid sawn timber members.6

Panelized Construction

Panelized construction consists of solid timber panels as the primary structural elements. These panels are composed of an engineered product referred to as "cross laminated timber” (CLT).

Cross laminated timber is an engineered product that consists of multiple layers of stud members that are laminated perpendicular to each other to achieve strength in multiple directions. Multiple perpendicular layers are built up to create CLT panels for use as structural elements.

CLT panels can be vertically oriented as load-bearing walls and shear walls, or horizontally as load-bearing floors or roofs. The use of CLT panels for structural wall and floor elements is typically used for mid- and high-rise residential construction as load-bearing walls and floors. Walls typically consist of three- to five-layer panels, whereas floors consist of five or more layers for greater stability. CLT panels can be designed to create internal and external partitions within the structure that makes their use practical for housing units in residential buildings. One of the primary benefits of CLT panels is the use of offsite prefabrication. Holes and notches in panels can be pre-cut prior to arrival to site. This minimizes work onsite, reduces construction time and costs, and increases the accuracy of structural components and quality of workmanship.


The recent emergence of tall timber buildings presents several primary challenges for the design, approval, and construction of these new and innovative structures.

Perception of Fire Risk

One of the primary challenges for tall timber buildings is a concern that they might present greater risks compared to high-rise, non-combustible structures. This could be motivated by historical precedent for catastrophic fire cases, recent fire incidents in timber structures, or even a general misunderstanding of the fire performance of heavy timber as a building material.

Identifying the fire performance of light and heavy timber members is an important distinction. Experience with timber in fire could be limited to small section members, characterized by kindling and light timber framing. Research has shown that heavy timber elements exhibit different fire performance compared to light timber.7 The section size of heavy timber members achieves an inherent fire resistance that protects the element due to the formation of a charring layer. This results in improved fire performance for heavy timber members relative to light timber.

Gaps in Knowledge

Over the past 15 years, research and testing has been performed to better understand the fire performance of timber structures. However, opportunities for further research remain. Providing greater understanding of these gaps in knowledge is intended to better characterize the fire performance of timber structures and clarify fire risks. Research and testing could lead to performance benchmarks and design tools that would allow a designer to characterize fire performance, engineer fire protection strategies, and demonstrate safe design.8

Contribution of Exposed Timber to Room Fires
Previous fire testing has shown that exposed timber has the potential to contribute to the fuel load in compartment fires.9 Fire tests have shown that delamination can occur in exposed CLT panels.10 This delamination may result in an increased burning rate for a limited period of time and also can result in an increased char rate for the exposed solid wood as it is instantly exposed to the compartment temperatures.

A better understanding of the contribution of exposed timber to room fires has the potential to better account for the contribution, identify if and when delamination might occur, and engineer strategies to meet the specific risks. Further, research also can evaluate the potential for self-extinguishment in exposed timber applications.

Connections Between Timber Components and Timber Composite Assemblies
As new timber technologies are developed to meet architectural ambitions and structural demands, it is important to consider the fire performance of these new structural assemblies and, in particular, the connections that transfer load between structural elements.

A high level of confidence in structural fire performance will be required to demonstrate that safety is achieved. This could require advanced modeling analysis or fire testing to demonstrate safe design. Design solutions must be balanced by structural-efficiency, cost-effectiveness, aesthetics and, importantly, fire performance.

Penetrations for Services
Understanding penetration behavior is critical to demonstrate that compartmentation is achieved for fire safety in timber buildings. The combustible nature of fire-rated structural elements presents unique challenges compared to noncombustible structures. Charring behavior must be considered for appropriate fire-stopping solutions to be developed to meet the fire performance requirements.

Two possibilities to solve this challenge include the development of fire test standards that account for combustible bases or substrates, and proprietary products for walls and floors in timber buildings.

Gaining Approval

While there is a growing global precedent for tall timber buildings, the building regulatory process in the U.S. has yet to approve the design for a high-rise timber building. The current prescriptive guidance in the International Building Code11 restricts combustible construction to approximately five to six stories, well below the practical limit of approximately eight-plus stories for heavy timber buildings.

In the current regulatory environment, approval for a high-rise timber structure would require the proposal of an alternative solution to the building code. This requires a designer to provide technical justification that the alternative solution meets the intended level of safety required by the prescriptive code requirement. This is a high burden of proof that relies on a comprehensive understanding of timber fire performance, an engineered fire protection strategy, and possibly fire testing, to justify safe design.

A significant amount of ongoing research is aimed at filling the gaps in knowledge and leading to a better understanding of the fire safety challenges in tall timber buildings. A particularly valuable tool is fire testing to clarify fire performance issues and validate analytical tools for fire engineering design and alternative solutions.


Europe is one of the first continents to embrace the use of new timber technologies in mid- and high-rise buildings. Sweden, Germany, and the United Kingdom, among others, have constructed tall timber buildings of seven-stories or more. Most recently, Australia has completed the tallest modern timber building in the world.

As experience and familiarity improves and a greater number of buildings are completed, the benefits of timber construction are becoming increasingly apparent, not only due to the sustainability benefits, but also due to the speed and ease of construction.

The structure of an eight-story building in Bad Aibling, Germany, took only three weeks to complete. The U.K.’s nine-story Stadthaus apartment building took 23 weeks less than a comparable standard concrete project. London’s Stadthaus also provides an interesting example of innovation facilitated by carbon reduction legislation. In June 2013, the Forté Building, a 10-story CLT residential complex in Melbourne, Australia, became the tallest occupied timber building in the world.

The development of tall timber buildings has recently expanded to North America. A six-story office building called the Bullitt Center is under construction in Seattle, Washington.


Timber is becoming an increasingly desirable construction material as many architects and designers understand the potential sustainability and construction benefits of timber buildings. Traditional schemes for timber structures as low-rise (two-stories or less) and mid-rise (three- to five-stories) are now being extended with schemes for new high-rise buildings (six-stories or greater).

Innovative technologies, such as the emergence of CLT and other engineered timber products, create the potential for timber buildings to be designed to taller heights and maintain the stability and safety of conventional construction materials, all while reducing the environmental impact.

A 2010 partnership between Arup consulting engineers and Rhomburg, an Austrian architecture firm, undertook a research study for a 20-story office building called Life Cycle Tower (LCT). The study aimed to design and detail a heavy timber commercial office building to demonstrate that high-rise buildings can be constructed in timber without compromising safety. The research project was realized with the construction of an eight-story office building called LCT One in Dornbirn, Austria in 2012.

One of the most well-known cases for tall timber construction was published in 2012.12 "The Case for Tall Wood” challenges the use of steel and concrete as essential materials in tall building construction. The study demonstrates the environmental benefits of timber buildings, while highlighting the design challenges and identifying how these can be achieved through science, engineering, and design. "The Case for Tall Wood” promotes the use of a mass timber structural solution that can be competitive with concrete construction for buildings up to 30 stories in height.

Skidmore, Owings and Merrill (SOM) published a study, "Timber Tower Research Project.”13 The study demonstrates the feasibility of a new structural mass timber system that can be designed to be competitive with reinforced concrete construction in buildings from 10 to 30 stories in height, while reducing the embodied carbon footprint by approximately 60 to 75%. The system proposes the use of performance-based design.

David Barber and Robert Gerard are with Arup.


  1. Larsen, L., Rajkovich, N., Leighton, C., McCoy, K., Calhoun, K., Mallen, E., Bush, K., Enriquez, J., Pyke, C., McMahon, S., and Kwok, A. "Green Building and Climate Resilience: Understanding Impacts and Preparing for Changing Conditions,” U.S. Green Building Council, Washington DC, 2011.
  2. Buchanan, A., Structural Design for Fire Safety. John Wiley and Sons, West Sussex, UK, 2001.
  3. White, R., "Analytical Methods for Determining Fire Resistance of Timber Members,” SFPE Handbook of Fire Protection Engineering, National Fire Protection Association, Quincy, MA, 2008.
  4. Details For Conventional Wood Frame Construction. American Forest & Paper Association, Washington DC, 2001.
  5. Ostman, B., "Fire Safety in Timber Buildings – Technical Guideline for Europe,” SP Report 2010:19. Stockholm, Sweden, 2010.
  6. Gerard, R., Fire Resistance of Connections in Pre-Stressed Heavy Timber Structures. MS Thesis, University of Canterbury, Christchurch, New Zealand, 2010.
  7. Calculating the Fire Resistance of Exposed Wood Members. American Wood Council, Washington DC, 2003.
  8. Gerard, R., Barber, D., & Wolski, A. "Fire Safety Challenges of Tall Wood Buildings.” Fire Protection Research Foundation, Quincy, MA, 2013.
  9. Frangi, A., Bochicchio, G., Ceccotti, A., & Lauriola, M. "Natural Full-Scale Fire Test on a 3 Storey XLam Timber Building,” Engineered Wood Products Association, Madison, WI, 2008.
  10. McGregor, C., "Contribution of Cross Laminated Timber Panels to Room Fires,” Master’s Thesis, Carlton University, Ottawa, Ontario, Canada, 2013.
  11. International Building Code, International Code Council, Washington, DC, 2012.
  12. Green, M., "The Case For Tall Wood Buildings.” mgb Architecture + Design, Vancouver, Canada, 2012.
  13. "Timber Tower Research Project,” Skidmore, Owings & Merrill, Chicago, IL, 2013.