Mass Timber Buildings

Introducing PMX 15: Our model for a mid-rise timber building

We’ve spent the past year refining our approach to factory-made mass timber buildings. This two-part blog series unpacks key findings for a 15-story model based in Seattle.

By Mark Bauernhuber, Lily Huang, and Kristin Slavin

A rendering of the PMX 15 mass timber building model.

A rendering of PMX 15: Sidewalk Labs’ proto-model for a 15-story mass timber building. (Sidewalk Labs)

Mass timber is one of the most promising innovations in urban development, with the potential to help cities dramatically reduce their carbon footprint. It’s also catching on fast. Several notable mass timber projects have recently been approved or entered construction, including the 25-story Ascent tower in Milwaukee.

For the past few years, Sidewalk Labs has been researching and developing a factory-based approach to fabricating mass timber buildings. By manufacturing a core “kit of parts” offsite, we think it’s possible to produce sustainable, high-quality mass timber buildings with greater speed and reliability, at a cost that meets market requirements. As part of that work, we developed Proto-Model X — basically, a proof-of-concept for how factory-made mass timber buildings could work. We call it PMX for short.

In early 2020, we released a PMX design for a 35-story mass timber building — a height that has yet to be achieved in practice. This work taught us some important lessons. We demonstrated that it’s feasible to reach this height with an all-timber structural system, further reducing the building’s carbon footprint. We also learned how to design a kit of parts that’s efficient to manufacture without compromising on design quality.

Next, we wanted to test our ability to design a safe wooden structure in a seismic region. We decided to do that with a mid-rise building 15 stories tall, for a couple reasons. Many cities have a greater market demand for mid-rise buildings, and U.S. states and cities are also starting to adopt the 2021 International Building Code, which grants regulatory approval for timber structures up to 18 stories tall.

Another key difference with PMX 15 is that we took the model all the way to construction documents, or the 100 percent design stage. That means we created all the supporting technical documentation needed for submission to local authorities for a construction permit. Going this far taught us even more about our kit of parts and its constructability, costs, and carbon impact.

Building on our PMX 35 explorations, this new two-part blog series will focus on some of the key lessons from PMX 15:

  • PMX 15, Part 1: Designing for a high seismic zone while meeting latest building code
  • PMX 15, Part 2: Improving the factory kit of parts to achieve low-carbon, high-quality design

As with PMX 35, we worked on PMX 15 in collaboration with a world-class team of architects, engineers, and environmental designers (their firms are listed below). Together, we’re pushing forward every day in the quest for more sustainable cities. If you’re a developer or timber professional who wants to help us advance mass timber building design or offsite manufacturing, send us a note through our website or email

The PMX team wishes to thank the exceptional consultants who helped make this work possible. The core consultant team includes Gensler, Aspect Structural Engineers, Interface Engineering, and Vortex Fire. Other consultants include Aercoustics Engineering, RDH Building Science, CadMakers, and Atelier Ten.