Mass timber is part of an advancement in sustainable construction that uses engineered wood products as the primary load-bearing building structure in a building. This class of materials includes Glue Laminated Timber (Glulam), Cross Laminated Timber (CLT), Laminated Veneer Lumber (LVL), Nail-Laminated Timber (NLT) and Dowel-Laminated Timber (DLT) . 

While some products such as Glulam have been in use for over one hundred years, the invention of others such as CLT enables larger-scale construction in timber, and the substitution of more energy-intensive materials. 

Unlike lumber, which is typically used in traditional stick-frame construction of houses and low-rise buildings, mass timber is used to construct everything from multifamily housing to civic buildings, office and industrial buildings, and more.

Mass timber stands out as the only low-embodied carbon structural material that can be used to build larger structures. What is embodied carbon? According to the Carbon Leadership Forum, embodied carbon represents the greenhouse gas emissions arising from the manufacturing, transportation, installation, maintenance and disposal of building materials. 

Mass timber production produces significantly less carbon emissions compared to the energy-intensive processes involved in producing concrete and steel, and timber is also a renewable resource. 

When looking at the building holistically, the embodied carbon of a mass timber building is further reduced as a result of a smaller foundation and smaller lateral force resisting system (due to mass timber’s lighter weight, relative to concrete), and a reduction in finish materials since the timber structure can often be exposed to view. 

 It is estimated that one cubic metre of mass timber sequesters one metric ton of carbon dioxide.  By contrast, it is estimated that every tonne of manufactured cement produces about 0.47 tonnes of carbon dioxide.  Likewise, 0.6 tonnes of carbon dioxide is emitted for every tonne of iron manufactured. By supplanting concrete and steel in mid-rise developments with mass timber products, the carbon footprint of construction can be mitigated.

For more information, check out our Carbon resources and read Comparison of Embodied Carbon Footprint of a Mass Timber Building Structure with a Steel Equivalent.

If we are to continue constructing new buildings, our most sustainable solution is to build with renewable materials that do not require tremendous amounts of energy to produce. The buildings sector as a whole is responsible for 40% of global greenhouse gas emissions. While much of this total results from the energy required to operate our building stock, approximately 15% of global GHG emissions come from the manufacture of concrete and steel - primary materials used in construction. Mass timber can be used as a substitute for concrete and steel in commercial construction, reducing the carbon footprint of the built environment while promoting sustainable forest management.

Additionally, as wood is 50% carbon by dry weight, the wood in a mass timber structure stores carbon within itself for as long as that building remains standing and the wood remains intact. By serving as carbon storage, large-scale mass timber buildings are actually helping to transform cities into carbon sinks, while we continually replant our forests.

Read More:

• ‍Mass Timber Building Life Cycle Assessment Methodology for the U.S. Regional Case Studies

A rise in mass timber construction will be positive for the forest products sector, as mass timber products represent a new, high-value market for wood products. An increased demand for mass timber will spur investment in the mass timber supply chain, and economic development for rural communities where the economy is reliant on forest products manufacturing.

Timber structures have the durability to stand for hundreds of years, with historic precedents across the globe, from Japan to Scandinavia. Furthermore, deconstructing mass timber buildings at the end of their lives and reusing materials is one way to ensure that carbon remains stored and not emitted. Mass timber products can be upcycled as load-bearing columns, beams, and panels, repurposed as furniture, or downcycled into particle board or wood chips. Detailing for durability will help mass timber buildings stand for centuries, and designing for disassembly will allow future generations to make the greatest use of materials as they are recovered.

Read More: 

• ‍Durability of Mass Timber Structures: A Review of the Biological Risks

By code, all mass timber structures must meet or exceed existing fire and seismic codes in the jurisdictions in which they are built. Mass timber structures achieve fire resistance through their mass, similar to a log in a campfire. When exposed to a fire, mass timber elements will char on the outside to create a layer of insulation that protects the interior from damage and loss of structural integrity.  As such, mass timber can withstand intense heat, and buildings can be designed to have fire resistance ratings up to two hours. 

Fire testing has been conducted on cross-laminated timber buildings by the International Code Council (ICC), the Fire Protection Research Foundation, and the U.S. Forest Service, which also conducted successful blast testing. Features like early warning systems, redundant sprinkler systems, and gypsum wallboard can help mitigate fire risk.

Mass timber’s light weight lends to its strength in seismic events.  One-quarter of the weight of comparable concrete structures, mass timber structures require smaller foundations, which reduce the seismic forces on a building. Mass timber’s significant strength for its weight enables outstanding performance in earthquakes. In May 2023, a 10-story building made of cross-laminated timber (CLT) underwent testing on the world’s largest outdoor shake table at the University of California San Diego. In an interview with UCSD, Shiling Pei, associate professor at Colorado School of Mines and lead investigator on the NHERI Tallwood Project, said, “It performed exactly as we expected—the building remained damage free after two major design-level earthquakes back to back.”

Learn more:

• Tests Highlight Seismic Resilience of Tall Mass Timber - WoodWorks | Wood Products Council
• Testing Mass Timber’s Seismic Resilience | Passive House Accelerator

Mass timber products are made with softwood (from coniferous trees) species such as Douglas Fir, White spruce, Engelmann spruce, Lodgepole pine, and Alpine fir trees (Western SPF, as a category). 

Wood sourced for mass timber production in Washington is coming from sustainably managed lands. In eastern Washington, some of these lands are overstocked and in need of treatment to improve their resilience. In this case, mass timber can promote healthy forest management by incentivizing the removal of small-diameter trees that present a wildfire hazard.

In other instances, wood for mass timber production is sourced from Tribal lands, bringing economic benefit to Indigenous communities who have long managed forests for both ecological and human needs. 

Mass timber brings efficiency to construction.  Timber panels are prefabricated and assembled on-site, reducing construction timelines by 25%.  However, the economic viability of the material is predicated on the holistic cost of a mass timber building compared to a concrete or steel building, rather than comparing the cost of structure alone. Holistic cost assessment is important, as mass timber buildings have smaller foundations and lateral systems, and require fewer finishes than their equivalent concrete and steel buildings, yielding cost savings that may offset a cost premium associated with the mass timber material itself. A reduced schedule yields additional cost savings, reducing the soft costs of construction and helping developers to begin generating rents sooner.

Owners and developers also find that mass timber buildings are of higher value to them, due to the beauty and warmth of their structures, the opportunity for mass timber to meet company or campus carbon reduction goals, and the potential for market differentiation.  There is also anecdotal evidence that mass timber buildings yield higher rents, attract longer-term tenants, and help attract and retain employees (commercial office) and students (higher education). 

While we may not yet have reached the limit for tall mass timber buildings, the 2021 International Building Code (IBC) includes provisions for tall timber buildings up to 9, 12, and 18 stories through three new construction types: Type IV-C, Type IV-B, and Type IV-A (respectively). 

Type IV-A buildings can be constructed up to 270ft in height, with 18 stories of mass timber (allowing for a substantial concrete podium below). Type IV-B buildings can be constructed up to 180ft in height, with 12 stories of mass timber (also allowing for a concrete podium below). Type IV-C buildings can be constructed up to 85ft in height, with 8-9 stories of mass timber.  

The Heartwood project in the Capitol Hill neighborhood of Seattle was the first Type IV-C building permitted and constructed in the United States, and opened in 2023.

Read More: 

• Status of Building Code Allowances for Tall Mass Timber in the IBC - WoodWorks | Wood Products Council
• Heartwood Apartments Case Study

The Federal Government has shown an interest in supporting mass timber construction, understanding of the potential for these products to serve as a natural climate change solution, drive domestic manufacturing, and support economic development in rural communities.The following actions have been taken by various government agencies and officials to support continued growth of the mass timber industry.

In 2022, the U.S. Forest Service convened more than 130 design professionals, researchers, manufacturers, industry leaders, and government employees for the 3rd Mass Timber Research Needs Assessment. The meeting resulted in a list of well over 100 research needs, outlined in a detailed report. The assessment emphasized the importance of “comprehensive, significant, and strategic investments” in mass timber research and development to help support the growing industry and the sustainability and performance of its construction.

Research needs assessment for the mass timber industry: Proceedings of the 3rd North American mass timber research needs workshop

The U.S. Department of Agriculture’s Wood Innovations Program creates and expands markets for wood products. Wood Innovations Grants worth more than $14 million have supported at least 60 projects that studied or supported mass timber cost-effectiveness, seismic resilience, fire performance, mold and decay resistance, and more. One grant helped the American Wood Council perform fire safety tests on taller mass timber buildings, leading ICC to allow expanded use of exposed timber ceilings in buildings as high as 12 stories.

The Farm Bill provides further opportunities for mass timber development. Provisions of the Timber Innovation Act of 2017 (S.538/H.R.1380) were included in the 2018 Farm Bill, setting the stage for increased investment in mass timber. That 2018 Farm Bill complimented the Wood Innovations provisions with a sister set of provisions for what is now known as the Community Wood grant program.  Today, that Farm Bill provision is the basis for the Wood Innovations Program.