In addition to accommodating an underserved market, Heartwood is notable for its novel structural system. It has a mass-timber frame that is, in large part, exposed on the interior, conforming to the requirements for what is known as Type IV-C construction under the recently adopted tall-wood provisions of the 2021 version of the International Building Code (IBC)—the model code used by most jurisdictions in the U.S. According to Susan Jones, atelierjones’s founder, Heart-wood is the first in the nation to take advantage of the new IV-C classification, which pertains to buildings up to eight stories and 85 feet tall. The new standards also include a Type IV-B for buildings up to 12 stories and a Type IV-A for those up to 18 stories. With each increase in height, the code mandates greater levels of fire protection requiring, for instance, that more of the timber be encapsulated in noncombustible materials.

It is not surprising that Jones would opt for mass timber for Heartwood’s structural system. Since founding her practice in 2003, she has developed a deep expertise in the material, using it on projects that include her own house, a church, and prototype housing for residents of Greenville, California, who lost their homes in the 2021 Dixie Fire. She has long advocated for mass timber’s mainstream adoption, serving on the International Code Council committee that developed the IBC’s tall-wood requirements. Her interest in the material is primarily environmental—a means for mitigating construction’s carbon footprint.

Mass timber, however, was a less obvious choice for Community Roots Housing, which did not set out to build with the material or even work with atelierjones. As Jones recounts, in 2019 her firm had been awarded a $250,000 Wood Innovations Grant from the U.S. Forest Service to investigate the feasibility of Type IV-C construction—but for an office project, rather than a residential one, with another client. At the 11th hour, however, site control of the project fell through. To take advantage of the funds, Jones needed a new project. After reaching out to dozens of developers—through friend and colleague A-P Hurd, founder of real-estate consultancy, SkipStone Development—she connected with Community Roots, which was working with SkipStone to build workforce housing. The nonprofit agreed to partner with atelierjones. “It was one of the most joyous yeses I have ever received,” says the architect.

People who walk by Heartwood, at the corner of East Union and 14th streets, will probably have no idea that there is something unusual going on beneath its understated fiber-cement-panel rainscreen skin. But on close inspection, one can detect a level of attention that is atypical in a mid-market multifamily project. For instance, the punched windows (all high-performance and triple-glazed) are slightly recessed and jog back and forth across the facade to create a subtle play of shadow and a syncopated rhythm. The first-floor apartments, entered directly from the exterior, are set back, creating almost the feel of townhouses. And if those passersby venture down 14th Street, they can catch a glimpse of an intimate courtyard—complete with barbecue area—shared with an existing three-story affordable apartment building owned and managed by the same client.

Behind Heartwood’s modest exterior is its mass-timber superstructure, whose wood elements were all sourced from within the surrounding Cascadia region: glulam columns and beams, five-ply cross-laminated timber (CLT) floor slabs, and mass plywood panel stairs enclosed, for seismic resistance, within steel buckling-restrained-brace-frame (BRBF) cores. The L-shaped assembly sits on top of a straightforward concrete slab and grade-beam foundation.

Working with a new code meant that atelierjones had to establish interpretations of its requirements, especially relating to fire protection. It developed details for the building’s myriad conditions, including where interior partitions or enclosed soffits meet exposed timber. The most elegant of these details is an all-wood mortise-and-tenon joint connecting columns and beams developed in collaboration with the structural consultant DCI Engineers and the company Timberlab, which performed virtual design coordination for Heartwood, as well as mass-timber fabrication and erection. The detail includes intumescent tape applied to the surfaces within the joint. If exposed to heat, the tape will expand, filling gaps and preventing fire infiltration. According to Timberlab, the absence of bolts, hangers, or other fasteners helped speed erection. On average, the beams for each level were installed in half a day, versus two to three days for steel connections.

The design and construction strategy yields compact apartments, averaging 400 square feet, with standard features, including drywall partitions, plastic-laminate cabinets, and vinyl flooring. But the timber frame and CLT ceilings provide warmth and natural appeal.

Beyond the biophilic benefits, Heartwood demonstrates a way of building with the potential for a significantly smaller environmental footprint compared with more conventional materials. According to a life-cycle analysis conducted by the University of Washington, the global-warming potential of Heartwood’s superstructure is 38 percent lower than a concrete building of the same size. If the carbon stored in the timber is included, the result is a 108 percent reduction, making Heartwood’s superstructure carbon negative.

The project is readily replicable and scalable; atelierjones has even assembled a booklet that shows how Heartwood’s guiding principles could be adapted to buildings of different sizes, heights, or site conditions. If the project proves popular with tenants, we could see hundreds of similar mass-timber structures around the country, helping tackle two of today’s most pressing problems—the earth’s warming and the housing shortage.