Two of the most interesting trends in the industry in recent years have been the rise of modular construction and the rise of cross-laminated timber (CLT).

Modular construction can significantly shorten construction times by allowing units to be assembled in the factory at the same time as foundations are poured and other site work is completed. And CLT—slabs of lumber glued in layers, with the grains of each layer going in different directions—offers exceptional structural strength and fire resistance, sequesters carbon, and speeds construction time. So we thought, why not combine the two?

We called up DCI Engineers, a structural engineering firm we’ve created award-winning architecture with before and asked them to collaborate with us on developing ways to incorporate CLT into modular construction. Then we approached Factory_OS, a local manufacturer of modular housing. Founded in 2017, Factory_OS relies on wood stick framing in its modules, which is great for medium-density multifamily buildings—five stories or less over one or two levels of concrete podiums—but is not an option for high-rises.

The arrival of the 2021 International Building Code (IBC) was a game changer, finally allowing mass timber buildings up to 18 stories tall. This created an opportunity to bring CLT to bear on modular high-rises. We collaborated with DCI Engineers to design CLT modules that are highly rigid. DCI developed a method that uses a post-tension system—typically used only with concrete construction—to provide structural strength.

Next, we worked with Factory_OS to apply for one of the U.S. Forest Service’s Wood Innovation Grant Awards. These grants support the development of consumer products that promote the use of wood to develop an innovative building material or a renewable energy source. Factory_OS won a grant and is building a mock-up.

The 2021 IBC currently only allows mass timber high-rises to use a gravity structural system, so for now, we are required to use concrete between some of the units to create lateral shear walls. However, as we test these systems further, DCI expects to prove that post-tensioning renders the concrete unnecessary. Eliminating the concrete would have several benefits, making the buildings faster to assemble and lightening the structural load on the foundation system.

Mechanical, electrical, and plumbing connections are among the most complex aspects of modular construction. We asked Point Energy Innovations to review our initial layouts to make sure that sizing and routing of all three systems were accurate. We dropped ceilings in bathrooms and closets so that we could fit in mechanical ducts to keep them within the module. We placed all primary mechanical/electrical/plumbing connections along the corridors, which enables the various trades to easily make the necessary cross-connections from the hallway, without having to enter each unit.

In terms of costs, we expect CLT modular construction to be very comparable with steel modular systems. However, the CLT system has the advantage of being lighter than steel or concrete, so the cost of the concrete podium and the footings and foundations is significantly less. In addition, CLT has a much lower carbon footprint than steel, because a tremendous amount of energy is required to acquire the raw material and turn it into usable pieces in steel construction.

Our next step was to design a building using the CLT modular system, to provide an example of the architectural possibilities made possible by using a system that has both a low-carbon footprint, but also affords the designer with flexibility to create a unique, memorable piece of architecture.

Given rising construction costs and the demand for housing in urban areas, we need all the innovations we can get. Layering CLT onto modular construction is one way to reap the benefits of both—building a greener, taller, stronger future.

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Interested in exploring CLT and modular construction for your next project?