No structural material is left unchanged by fire – steel, concrete, and wood all have different strengths and weakness in the event of a structural fire. However, wood is fundamentally different from the others because it not only changes, it burns.
When wood burns, it chars from the outside in at a generally predictable rate [1]. The surface of the piece is the only area where all three corners of the fire triangle are available – fuel, heat, and crucially, oxygen. Underneath the charred layer, toward the core of the component, there is a slightly cooler layer where the wood is undergoing pyrolysis, or thermal decay in the absence of oxygen. In this zone, the wood is losing some of its strength but it is not yet burning. At the core, beyond the heated zones, is unaffected wood.
The edge of the charred layer will continue to move inward toward the center of the piece, for as long as the fire is heating it, but the rate it moves inward slows over time because the char that already exists acts as a growing layer of insulation [1]. The size of a piece of wood greatly impacts how long this process takes. It’s easy to picture this effect by thinking of a campfire: a small stick of kindling burns quickly compared to the log that burns slowly all night.
Once wood is charred, it loses essentially all of its strength, so the overall load carrying capacity of a wooden structural element depends on its uncharred component [2]. In a small timber, such as regular dimension lumber, the piece will char through and lose its structural capacity relatively quickly. A larger piece of wood takes exponentially longer to char through, so the piece will retain its structure for much longer. The size of the elements is key to the fire behavior of mass timber.
Testing has shown that mass timber typically has a similar charring rate to solid wood [1] However, any gap in the material that allows oxygen further in can increase the depth of the charred layer and affect the wood structure and behavior of the fire, as can contact with metal elements or connections that conduct heat [3]. Connection performance and strength can also be impacted by the heat, even if they are not directly exposed [6, 7].
Not all mass timber behaves quite the same, either. Delamination at the adhesive layer is a particular concern, especially in dimension lumber-based composites like CLT where each lam represents a significant proportion of the total cross section. Layers may delaminate if the char fully penetrates them, depending on how well the adhesive used in the panel withstands heat, and how well the wood and adhesive have bonded. If a delamination does occur, it has the twofold effect of adding fuel to the fire and revealing unburned surface for further charring, both of which can extend the fire’s timeline.
Delamination is a well-established challenge and an active area of research. It is well established that some adhesives perform better than others in elevated temperatures, but these effects are tempered by many factors, including the structural loading in place during the heating, the time-temperature profiles, and ambient moisture content, among others [6–8].
Approaches that can help mitigate the direct effects of fire on mass timber in service can be applied at multiple scales. Mass timber materials governed by the PRG 320 are now required to utilize heat resistant adhesives, which reduces the vulnerability of the material itself [9]. The mass timber elements in buildings can be upsized to account for the loss of strength to charring, and they can be covered by non-combustible barriers and/or intumescent coatings. Spaces can be designed in ways that can improve the likelihood that fires will self-extinguish [1,2,6,10,11]. Though quite a bit of work is being done on all of these fronts, more research is needed.
The effect of fire on the materials used in a building is important to know, so that they can account for those effects in the design of the building. Researchers continue to look at how mass timber systems behave in fire, in order to provide engineers, designers, and architects with the tools to build safe structures that still have the benefits of mass timber.
The upcoming Oregon Fire Testing Facility on the OSU campus will enable researchers close to home to continue investigating many of these critical questions. There is only one other fire testing facility on the Pacific Coast, and no other university-run laboratories west of Michigan. The opening of the fire testing lab will be a huge boon to researchers throughout the western US.
The next post in the fire series will discuss how the behavior of fire and fire testing in wood materials is translated to fire design research and mitigation strategies at the structural scale.
[1] D. Barber, R. Gerard, Summary of the fire protection foundation report – fire safety challenges of tall wood buildings, Fire Sci. Rev. 4 (2015) 1–15. https://doi.org/10.1186/s40038-015-0009-3.
[2] J. Liu, E.C. Fischer, A.R. Barbosa, A. Sinha, Experimental Testing and Numerical Simulation of Timber-Concrete Composite Floors in Fire, J. Struct. Eng. 149 (2023) 04023151. https://doi.org/10.1061/JSENDH.STENG-12577.
[3] E.C. Fischer, S. Bhandari, A. Sinha, Fire Testing of Glue-Laminated Beam-to-Column Connections, J. Struct. Eng. 151 (2025) 04025020. https://doi.org/10.1061/JSENDH.STENG-13888.
[4] K. Mahr, A. Sinha, A.R. Barbosa, Elevated Temperature Effects on Performance of a Cross-Laminated Timber Floor-to-Wall Bracket Connections, J. Struct. Eng. 146 (2020) 04020173. https://doi.org/10.1061/(ASCE)ST.1943-541X.0002737.
[5] K. Mahr, A. Sinha, A.R. Barbosa, Experimental Investigation and Modeling of Thermal Effects on a Typical Cross-Laminated Timber Bracket Shear Connection, J. Mater. Civ. Eng. 32 (2020) 04020111. https://doi.org/10.1061/(ASCE)MT.1943-5533.0003122.
[6] L. Muszyński, R. Gupta, S. hyun Hong, N. Osborn, B. Pickett, Fire resistance of unprotected cross-laminated timber (CLT) floor assemblies produced in the USA, Fire Saf. J. 107 (2019) 126–136. https://doi.org/10.1016/j.firesaf.2018.12.008.
[7] M. Cheung, A. Zaben, C. Maluk, Experimental method for studying the effects of elevated temperature in adhesives used in laminated Mass Timber, Fire Saf. J. 146 (2024) 104058. https://doi.org/10.1016/j.firesaf.2023.104058.
[8] A. Čolić, F. Wiesner, M. Spearpoint, D. Hopkin, L. Bisby, Influence of Adhesive Type on Heat Induced Delamination in Tension Loaded Shear Lap Cross‐Laminated Timber Samples Exposed to Radiant Heat Flux, Fire Mater. (2026) fam.70041. https://doi.org/10.1002/fam.70041.
[9] Standard for Performance-Rated Cross-Laminated Timber, (2025). https://www.apawood.org/guides-tools-training/technical-document-library/standards/ansiapa-prg-320-2025-standard-for-performance-rated-cross-laminated-timber/.
[10] J. Liu, E.C. Fischer, Review of large-scale CLT compartment fire tests, Constr. Build. Mater. 318 (2022) 126099. https://doi.org/10.1016/j.conbuildmat.2021.126099.
[11] F. Wiesner, H. Xu, D. Lange, V. Gupta, I. Pope, J.L. Torero, J.P. Hidalgo, Large-scale compartment fires to develop a self-extinction design framework for mass timber-Part 2: Results, analysis and design implications, Fire Saf. J. 152 (2025) 104346. https://doi.org/10.1016/j.firesaf.2025.104346.
Degenkolb's structural engineering practice reflects more than eight decades of commitment to technical expertise, exceptional client service through close collaboration, and life-long learning. Degenkolb pioneers innovative structural system designs to support the needs of our clients, including mass timber projects as engineer-of-record, peer reviewer, and in construction engineering.
DLR Group is an integrated design firm delivering architecture, engineering, interiors, planning, and building optimization for new construction, renovation, and adaptive reuse. Our promise is to elevate the human experience through design. This promise inspires sustainable design for a diverse group of public and private sector clients; local communities; and our planet. DLR Group is 100 percent employee-owned and fully supports the initiatives and goals of the 2030 Challenge, and is an initial signatory to the China Accord and the AIA 2030 Commitment.
FFA has recently created 20 projects utilizing mass timber. For our team this is a design and research opportunity, leveraging post-occupancy evaluation, performance tracking, and material sourcing studies to continuously refine its approach. The dedication to using high-tech wood products for small and large projects helps us connect to our region through a range of attributes from aesthetics to economic solutions. This endeavor brings together multiple experts from across the research, design, and construction industry together creating better solutions for the community we serve.
Forell | Elsesser (F|E) is an award-winning structural engineering firm known for innovative design solutions that are practical and scalable. The firm brings deep expertise in mass timber design, with particular strength in vibration mitigation and performance-based seismic engineering for complex buildings. An active collaborator with academic partners and industry stakeholders, F|E contributes targeted research insights that help close knowledge gaps and support broader adoption of mass timber solutions.
Holmes is an international engineering firm specializing in advanced structural and fire engineering across the Pacific Rim. With deep expertise spanning research, design, and construction, Holmes has contributed to industry-advancing initiatives including collaborative research with academia, development of industry design guides and participation on multiple code advancement committees. This expertise extents particularly to mass timber, where Holmes has been involved in some of the largest and most innovative mass timber buildings in North America, New Zealand, Australia and beyond.
Kalesnikoff Mass Timber is a vertically integrated manufacturer specializing in mass timber (CLT, GLT, Glulam), prefabricated wall systems and fully volumetric modular construction. Through in-house R&D and advanced manufacturing, we optimize structural systems and detailing to improve constructability, performance, and code compliance across a wide range of building types.
KPFF is a nationally recognized structural and civil engineering firm with over 1,500 employees. We have contributed to the U.S. mass timber movement since its infancy in the Pacific Northwest and have helped expand its reach across the entire United States. We are especially passionate about translating mass timber research into practice including point-supported cross-laminated timber (CLT), post-tensioned timber rocking walls, mass timber floor vibration, CLT diaphragms, timber braced frames, fire and acoustic testing, large-scale shake-table testing, and many other research and development topics.
LEVER is a leading architectural practice based in Portland, Oregon and Los Angeles recognized for design excellence that combines purpose, craft and material innovation LEVER has pioneered mass timber firsts for over a decade, with projects that are catalysts for connecting sustainable urban development to rural economic growth regionally and nationally. The company’s office, Albina Yard, was the first building in the United States to use domestically sourced CLT and their Framework project was the first wood high-rise design in the United States to receive approval for construction. LEVER has designed over 20 mass timber projects focusing on leveraging the specific qualities of a place—climate, access to materials, community, and the resources of the specific sites of their work.
Mithun is a 75-year-old design firm with offices in Seattle, San Francisco and Los Angeles. The firm’s interdisciplinary team has been working with mass timber since 2017 on diverse projects spanning commercial office, cultural, industrial and education markets. As part of the Mithun R+D program, the firm has advanced mass timber knowledge and application through self-funded research like the Building Better Schools mass timber kit of parts system that has informed multiple K-12 projects at Mithun and been widely referenced by other institutions and firms in their efforts to design economical mass timber schools. Mithun also actively participates in industry-led research such as its successful execution of two NFPA 285 fire tests on CLT walls through the USDA Wood Innovations Grant program, acoustic testing in multifamily CLT floor assemblies, and a current research project with WSU and NRRI to develop a thermally modified strand-based CLT panel for commercial use.
Rothoblaas delivers practical, high-performance solutions for mass timber and wood construction, from structural fasteners and connection systems to membranes and safety products. Supported by internal testing, technical documentation, and engineering resources, Rothoblaas helps architects, engineers, and builders navigate U.S. code requirements with strong technical support and R&D driven guidance. Through education and collaboration, we help move mass timber projects from design to jobsite with confidence.
Degenkolb's structural engineering practice reflects more than eight decades of commitment to technical expertise, exceptional client service through close collaboration, and life-long learning. Degenkolb pioneers innovative structural system designs to support the needs of our clients, including mass timber projects as engineer-of-record, peer reviewer, and in construction engineering.
StructureCraft is an industry leader in the design, engineering, and fabrication of advanced mass timber and hybrid timber systems, with a strong focus on innovation and constructability. The company has extensive experience delivering complex timber structures globally. With deep expertise in digital fabrication and in-house R&D, StructureCraft collaborates closely with architects and researchers to push the boundaries of sustainable, low-carbon construction, bringing unique technical strength and practical insight to the REACTS group.
WSP is one of the world’s leading professional services firms, uniting engineering, advisory, and science-based expertise to shape communities. Within this global network, our structural engineering teams are recognized for innovative approaches to complex building systems. We apply advanced design strategies for mass timber and hybrid structures, combining sustainability with technical precision. Through ongoing research and development, we create resilient, future-ready solutions that redefine what’s possible in the built environment.
Freres Engineered Wood is a premier wood products manufacturing company dedicated to bringing innovative, high-quality and environmentally-sound wood products to market. Founded in 1922, Freres is one of the oldest family-owned and operated businesses in Oregon, and values the personal relationships it has developed with customers and partners over the years.
Hilti stands for innovation and direct customer relationships. Based in Schaan, Liechtenstein, Hilti has about 34,000 employees around the world in more than 120 countries. Hilti provides the construction industry with advanced hardware, software, and services to make customers work more productive, safer, and more sustainable. Hilti also invests approximately 6 percent of sales each year in research and development to develop new cutting-edge solution for the customers. From product development to manufacturing, logistics, sales, and services, Hilti covers the entire value-added chain. Overall, Hilti aims to be the best partner of the customers in making construction better.
