Introduction: New Possibilities

In the 2021 update to the International Building Code new possibilities opened with the expansion of Mass Timber under Type IV construction. Under the existing Type IV, three sub-types were added, IV-A, IV-B and IV-C, and the existing base Type IV was now labeled IV-HT. This defined and clarified the capabilities of mass timber, particularly in terms of fire resistance, the key jurisdiction concern over these types of structures. With the new code, Type IV-A has the most stringent fire resistance and protection requirements but allows buildings up to 270’ or 18 stories. Next, Type IV-B allows buildings up to 180’ and 12 stories. The third subtype, Type IV-C and the renamed Type IV-HT both permit buildings up to 85’, but where IV-HT only allows up to 6 stories, in a residential use IV-C can achieve 8 stories [1].  This introduced a departure from previous heavy timber and stick framed construction – and presented an opportunity.

The successful completion of Heartwood demonstrated the viability of using Type IV-C Mass Timber in the Seattle Market. It was Seattle’s first tall mass timber structure and the first 8-Story Type IV-C building in North America. Designed by local architectural firm AtelierJones and constructed by Swinerton, the building used Timberab’s product to deliver missing middle housing in the heart of Seattle [2]. The building was awarded a 4 Star Built Green certification and is notable for its sequestered carbon, bringing the impact to the building’s Global Warming Potential below net zero. This chapter will investigate what Seattle can do to enable similar structures to proliferate across the city, with Heartwood as a case study.

Why Mass Timber? Rational for Studying the Case

This research was admittedly prompted by a personal connection to the site, the neighborhood, and sustainability as a whole. Having grown up in Seattle, environmental stewardship was a topic of discussion from a young age both in school and at home. My grandfather, Harry Leavitt, was an avid environmentalist and founding member of the Mercer Island Recycling Center in the 1970s while teaching at Mercer Island High School. Since 2017 I have lived in what was once his house, only a block from Heartwood. As such, I was able to see the fascinating nature of the construction daily. It was something more akin to building a model from a kit, than erecting a steel and concrete high-rise.

In previous research around the environmental impact of alternate concrete materials I dove into the impact the construction industry has the environment globally. As noted by Bennet, Visintin, and Xie in their work on Recycled Aggregate Concrete (RAC) “the construction industry consumes an estimated 50% of all natural resources and 40% of all energy produced, while simultaneously generating approximately 50% of all global waste streams” [3]. This results in Construction being responsible for 40% of global emissions. Steel manufacturing in general is responsible for 10% of global admissions and 8% of total emissions – 15-20% of construction emissions – results from the production of cement, most of which is used for concrete [4]. While steel and concrete are within Heartwood, during construction they were noticeably minimal – The steel augments the core and the foundation is minimized.

Further research by Duan, Huang and Zhang revealed that to comply with the Paris Climate Accords “all the new buildings and 20% of the current building stock are expected to be zero-carbon by 2030” to achieve carbon neutrality by 2050. While this goal seems lofty, albeit necessary, they did find in their study that a percentage increase in wood usage corresponds to an equal or slightly greater reduction in carbon emissions from building materials [5]. This aligns with Heartwood’s success in carbon sequestering, and provides a hopeful outlook for mass timber’s role in mitigating the environmental impact construction. However, to do so requires using a newer construction method with limited adoption.

With no significant personal background in mass timber, mass timber in the Northwest still seems an obvious choice. Seattle was built around the timber and maritime industries, and timber remains prominent in the state – in 2021 revenue from the forest products industry exceeded $36 billion [6]. This includes Weyerhaeuser, the company that produces TJIs – the brand name in engineered wood joists. With historic ties to sustainability, timber, and engineered wood materials, Seattle seems like a perfect opportunity for mass timber to take hold. This led to the formation of my research question.

What enabled the Heartwood Project to be successful? What can Seattle do to enable the widespread use of Mass Timber in Mid Rise Construction?

Seattle at the Forefront: An overview of the Heartwood Case

The Heartwood project was innovative in many ways. The 66,580 square foot apartment building on 14th and E Union delivered 126 units in an expansion to a nonprofit Community Roots Housing’s footprint on the block. In addition to being Seattle’s first tall mass timber structure and the first 8-Story Type IV-C building in North America, Heartwood is also one of the first affordable housing developments nationwide to use Cross Laminated Timber [7]. The lot had previously been the parking lot for Community Roots Housing’ Helen V Apartments, a 38 unit, 3 story brick low-rise built in 1901 [8]. Renters for Heartwood fall within 60-100% of the median income for the area – providing the “missing middle” housing that is a rising issue in Seattle.

Much on the new construction in the surrounding area and across Seattle follows the five over one or five over two podium style. This maximizes the allowable stories for wood construction under Type III-A. Prior to the 2021 update, the 2018 IBC version of Type IV allowed mass timber up to 85’ but to a maximum of 6 stories in a residential building. Heartwood’s eight stories are another unique feature, since, as discussed earlier, the new Type IV-C construction allowed an additional story not typically seen in Midrise construction.

Findings

Following my literature review and background research, this data was collected through a semi scripted in person interview with Professor Tyler Sprague, PhD, a University of Washington professor, published author on Seattle’s structural history, and keynote speaker at the International Mass Timber Conference. I was also able to correspond over email with Ian Maples, the Technical Manager and Heartwood Subject Matter Expert at AtelierJones. Ian offered a first person view of the Heartwood project and was able to recommend a follow up study on mass timber that the award-winning team at AtelierJones had participated in. I am truly grateful for all their irreplaceable knowledge, experience and time, as it was critical to this research.

In my interview with Professor Sprague, he referenced mass timber as a third option, with traditional frames of steel and concrete being the other two. Before academia he was a structural consultant for MKA, where his world revolved around concrete and steel. During his PhD, his work focused on history, which is notably rarer on the engineering side. This led to research on Christiansen and thin-shell concrete. This use of concrete simultaneously blends structure and design. Recently his research has shifted to researching mass timber. But he drew comparisons between the two – both are somewhat contradictory – light concrete and heavy timber. Currently he is an Associate Professor in the Department of Architecture where he teaches an architecture studio focusing on mass timber. In reference to the class, he mentioned looking at contemporary examples, Founders Hall, Dunn Family’s project Northlake Commons, but also historic examples like national parks, lodges, log cabins. A model of Mount Rainier’s Paradise Lodge behind his desk alluded to this.

\While mass timber in its current form is novel, in some ways the third option is a reintroduction of a historic material which can be used in conjunction. Most mass timber buildings are still a fusion of steel and timber, Heartwood included. And while concrete use is minimized due to reduction in building weight, the foundation remains concrete.

Theme One: The Owner

A theme I was interested in was the owner and developer’s part in mass timber. To understand what Seattle as a jurisdiction could do to enable mass timber, I wanted to understand the viewpoint and experience of the project owner. Tyler referenced that projects considering using mass timber often ran concurrent designs, one for traditional methods and one for mass timber. This was credited towards owners focusing on cost to ensure the project is viable. But while mass timber design may be advanced in tandem with a traditional material design, this doesn’t allow the designer to focus on the design and maximize it, and in turn adds to the feasibility cost. However, in the Heartwood case mass timber was considered from the onset. AtelierJones principal Susan Jones was awarded a USDA grant that allowed the developer and owner to produce a very robust feasibility study. This was typical to Ian’s experience, he has mostly dealt with parties already committed to using mass timber – AtelierJones works almost exclusively in mass timber – so every project he had worked on since joining the firm had looked to use mass timber. To him “the next step is finding owners and developers willing and interested in pursuing that further.”

Theme Two: Jurisdiction

In my interview with Professor Sprague, he explained that the city, and in general the authority having jurisdiction, is most concerned about fire safety, so the code reflects this. The current code mandates a certain ratio of encapsulation of the wood while some of it can be left exposed. I was unfamiliar with encapsulation and followed up on this, so Tyler explained that the encapsulation is typically rated drywall. In Heartwood’s case, the project was well received and supported by the city. Ian noted that Seattle was an early and pioneering adopter of code allowances for mass timber. Susan Jones’s house was designed and built between 2010-2015 using CLT, predating the updated IBC. He noted by name his counterparts at SDCI and their contribution to Heartwood’s success saying “I would include in that success the willingness and engagement of SDCI personnel… who worked closely with us to review and approve the project. It could not have succeeded without SDCI’s own interest in making it work.” Concerning what Seattle could do beyond the support they gave Heartwood, Ian noted that that to enable mass timber Seattle could offer general monetary incentives for building with Mass Timber.  This could be due to local and regional industry benefits and climate impact benefits. With Heartwood a USDA grant allowed an in-depth feasibility study, similar grants could help interested parties pursue mass timber projects.

Theme Three: Education

A key point across my conversations with both Tyler and Ian was the need for more education on the product. AtelierJones has specialized in mass timber in recent years, as such it was hard to gage the popular sentiment as “most of [their] conversations with AHJs and Owner/Developers are all with people talking to us because they are already interested in Mass Timber”. When asked what the industry needs to do to meet Seattle’s needs as a stakeholder, Ian responded that regardless of jurisdiction, education is the first step to increasing adoption of mass timber. And in discussing the existing obstacles to mass timber in Seattle He also noted that its “General construction cost, exacerbated by the additional cost expectation of material I suppose as well as general average unfamiliarity with the material perhaps”. This resonated later with a passage from Mass Timber Tipping Point where a key highlight of their work was that “Despite perceived advantages in carbon reduction, construction speed, and aesthetic appeal, mass timber faces persistent misconceptions about cost, performance, and practicality that lead designers to eliminate it as an option early in the design process,” [9]. Across multiple sources, the need for education was prevalent.

Further Research

This case study evaluated the question of what Seattle can do to enable the widespread use of mass timber, through the lens of a small case study around the Heartwood project. Much of the findings show a jurisdiction that is already encouraging and open to mass timber. Due to the limited scope of the study further research could include the following areas:

• Comparison of similar jurisdictions adoption of mass timber
• Potential effects of zoning changes on the feasibility of mass timber
• Availability of education on mass timber
• Cost analysis of mass timber
• Production capacity of manufacturers and it’s effect on price

These are a limited selection of many possible areas for future research. These topics are put forward in this chapter because they were identified as gaps in knowledge that could not reasonably be included in the limited scope. Evaluating these topics could help provide a more informed and detailed picture of how Seattle can continue to accommodate builders and developers interested in mass timber.

Conclusion

Based on the experience of Heartwood’s project team, owners and developers interested in mass timber have a supportive jurisdiction in the case of Seattle. The parties at SDCI were instrumental in helping Heartwood succeed. To enable the widespread use of mass timber in midrise construction will take projects put forward by parties interested in using mass timber. This case study identified two areas Seattle can help enable this.  If Seattle wants to enable the use of mass timber it can begin by promoting education on the subject across the industry. It should address both the product and method of construction. Education could highlight the capacity of the material to get more out of the existing zoning under Type IV-C. The City also could provide financial incentives to promote using locally or regionally produced products as well as green construction methods, mass timber can be both. This could offset the feasibility and material costs that hinder or are perceived to hinder current projects. Seattle is ready for mass timber and while there are steps they can take to enable it, ultimately it is up to the developers and owners to pursue it.

References

[1]         S. Breneman, M. Timmers, and D. Richardson, “Tall Wood Buildings in the 2021 IBC,” Woodworks.

[2]         “Heartwood,” Swinerton. [Online]. Available: https://swinerton.com/project/heartwood/

[3]         B. Bennett, P. Visintin, and T. Xie, “Global warming potential of recycled aggregate concrete with supplementary cementitious materials,” J. Build. Eng., vol. 52, p. 104394, Jul. 2022, doi: 10.1016/j.jobe.2022.104394.

[4]         M. Hemmati, T. Messadi, H. Gu, J. Seddelmeyer, and M. Hemmati, “Comparison of Embodied Carbon Footprint of a Mass Timber Building Structure with a Steel Equivalent,” Buildings, vol. 14, no. 5, p. 1276, May 2024, doi: 10.3390/buildings14051276.

[5]         Z. Duan, Q. Huang, and Q. Zhang, “Life cycle assessment of mass timber construction: A review,” Build. Environ., vol. 221, p. 109320, Aug. 2022, doi: 10.1016/j.buildenv.2022.109320.

[6]         Mason, Bruce & Girard, Inc., “Forest products industry Economic Impacts in Washington State – 2021,” Washington Forest Protection Association, 2021. [Online]. Available: https://data.workingforests.org/doc/WFPA_Industry_Econ_Impacts_2021.pdf

[7]         “Heartwood,” Building. [Online]. Available: https://communityrootshousing.org/building/heartwood/

[8]         “Helen V,” Building. [Online]. Available: https://communityrootshousing.org/building/helen-v/

[9]         S. Francisco, V. Martinez, E. Spiritos, and A. Waugh, “Mass Timber Tipping Point,” Tipping Point – Call Action, Mar. 2025.

About the author

name: Mikhail Kamila

institution: University of Washington

Local Seattleite and once and future UW graduate. I received my B.A. in Architectural Design in 2021, and after some time in residential construction returned to UW. I am currently pursuing a M.S. in Construction Management and Graduate Certificate in Real Estate.