Seattle Project Showcases Mass Timber’s Strengths

The use of mass timber in U.S. construction has increased rapidly in recent years, as more developers embrace the use of engineered wood products as an efficient, low-carbon and aesthetically appealing alternative to concrete and steel.

Among the 979 mass timber projects built or in design across the country, a new apartment project north of Seattle illustrates how wood can be efficiently harnessed to create a unique and sustainable mid-rise development. The Postmark, a recently completed 243-unit community in Shoreline, Wash., marks one of the first projects in the U.S. to use cross-laminated timber (CLT), a panelized structural engineered wood product, for floor and ceiling assemblies.

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Developer The Wolff Co. partnered with construction firm Katerra to realize the two-building, five-story project at 17233 15th Ave. NE, whose market-rate apartments are now being leased out. Katerra’s design team in conjunction with Via Architecture decided to construct the building using natural materials for an aesthetically appealing exterior with comfortable and relaxing interior spaces, explained Chester Weir, Katerra’s head of mass timber residential design.

“Sustainability, comfort and wellness were the core factors that informed The Postmark’s design process,” Weir noted to Multi-Housing News. “The prefabrication and speed of installation were also a consideration, as was the ability to pioneer mass timber in multifamily design and construction.”

Prefab solution

The Postmark. Image courtesy of Aaron Locke

CLT, made of multiple layers of lumber with alternating grain orientation that are pressed together with an adhesive, offers superior strength and fire resistance to individual boards. The panels can be customized for specific projects and fabricated offsite, dramatically speeding up construction and reducing onsite labor needs.

Katerra opened a 270,000-square-foot mass timber factory in Spokane, Wash.—the largest single-use CLT facility in North America—in September 2019, but the company delivered CLT from Europe for The Postmark, as construction started before the new facility become operational.

“Many containers of mass timber came, and I saw them come out of the container,” said Hans-Erik Blomgren, director of testing and certification for structural products at Katerra, who was involved in The Postmark’s construction. “All of (the panels) might need to go in a certain spot, a certain sequence, and fit, and they did. There were no reject panels.”

The industry has roughly a decade of experience with offsite fabrication of walls and exterior frames, but the challenge for Katerra’s first multifamily CLT project involved building the whole structural frame offsite. “We cut our teeth, literally, on this project,” said Blomgren. “The fundamental structure was changed on the floors.”

Built above a two-level, below-grade parking structure, The Postmark uses CLT, along with glulam beams, as the floor slabs and roof structures. This reduces structural loads and dramatically improves build efficiency compared to concrete-and-steel construction, the firm says. As the prefabricated panels arrive with all the joints precut, the crew needs only three days to install an entire 30,000-square-foot floor.

Katerra created the building prototype using virtual design and construction (VDC)/3D modeling technology, which enabled the company to have components made to its exact specifications. “I think this is fundamentally key to what makes mass timber work,” said Blomgren. “Wood, with its versatility and ease of use, gets to be put into a factory setting where after it’s pressed, it goes downstream into a five-access CNC gantry mill, that is tied to a digital model.”

Timber appeal

The Postmark. Image courtesy of Katerra

A key attribute of the project is its use of CLT, an inherently fire-resistant solution, to achieve 60-minute fire rating, allowing for enhanced structural integrity and durability in the face of natural disasters. Mass timber also helps reduce carbon dioxide emissions in contrast with more conventional materials such as cement, concrete and steel.

Built next to a microbrewery on the former site of Shoreline’s post office, The Postmark features studio, one- and two-bedroom units in a Seattle suburb with a population of about 57,000. Amenities include a freestanding clubhouse at the podium level and a gym, resident lounge and outdoor courtyard.

“In addition to the structural and aesthetic benefits, owners and developers are drawn to the mass timber building and exposed wood due to its track record of driving premium resale and rental rates,” Weir noted, adding that CLT provides “phenomenal long-term ROI potential for building developers.”

There were 534 mass timber projects in design across the U.S. as of September 2020, adding to 445 completed projects, according to a report by WoodWorks. The nonprofit found that business buildings such as offices and restaurants accounted for the most popular application of mass timber in 2019, followed by education, multifamily and public assembly buildings.

In the same year, mass timber projects totaled 4 million square feet, with CLT accounting for 50 percent of building projects but 62 percent of the square footage, indicating that CLT projects tend to be larger, according to a report by the Forest Business Network.

Katerra has another CLT multifamily project underway in Denver, a 292-unit, seven-story community called Cirrus. The 230,000-square-foot development west of Mile High Stadium is constructed use five-ply and seven-ply CLT that is structural and will remain exposed in unit interiors.