This story was originally published in Dwell’s May 2009 issue.

Chicago preservation architect Thomas “Gunny” Harboe prefers not to dismantle architectural monuments. But at Mies van der Rohe’s 860–880 Lake Shore Drive apartments, built in Chicago in 1951, one of the first steps in the preservation process was to remove the broad travertine pavement between the two towers.

Taking up nearly 1,100 slabs—each weighing as much as 175 pounds, many crumbling and leaking water into the garage below—was the easy part. The challenge was to create a replacement faithful to the original in all respects but one: It would be designed to last.

Harboe’s solution, designed with colleague Rico Cedro of Krueck & Sexton Architects, used techniques that eluded Mies six decades ago: waterproofing the plaza’s concrete base, creating a slight pitch, and cutting the stone (from Tivoli, like the originals) slightly thicker and some with shallow ridges or valleys to carry away rainwater.

The interventions barely alter the original design but prove what Harboe’s experience in restoring midcentury-modern buildings had already taught him: In the less-is-more aesthetic, “sometimes less wasn’t enough,” he says.

Harboe’s passion for conservation goes back to a taste for antiques acquired from his family. He studied history and material culture at Brown University, followed by conservation at Columbia University. While working on the reconstruction of the Frank Lloyd Wright Room at the Metropolitan Museum of Art as an intern, he noticed that “architects were the ones calling the shots”—so he pursued an architecture degree at MIT. “I saw the past through objects,” he says. “And buildings were the largest objects I could find.”

His professional career began more than two decades ago with an unexpected opportunity: He was hired by the Chicago design firm chosen to restore the Rookery, one of Chicago’s noblest early skyscrapers. Harboe led the effort, won a National Trust for Historic Preservation award for it, and then directed the restoration of the equally lush Reliance Building nearby, which garnered additional accolades.

In the mid-1990s, Harboe helped establish the U.S. chapter of Docomomo, an international organization dedicated to preserving modern architecture, and in 2006, he founded Harboe Architects, specializing in high-end restorations such as Mies’s Crown Hall at the Illinois Institute of Technology in 2005 and currently Frank Lloyd Wright’s Beth Sholom Synagogue in suburban Philadelphia.

Harboe’s goal is to make his own work invisible—though sometimes there are exceptions. In the travertine pavement at the Lake Shore Drive apartments, a small section of original slabs will be darker than the rest—deliberate evidence that the present restoration is an essential piece of the building’s history.