New construction is no small feat, but architectural conservation requires next-level expertise. And in the case of St. Patrick’s Cathedral—a majestic Gothic-Revival church across the street from NYC’s Rockefeller Center—it took an army of engineers, designers, artisans, and historians; 30,000 interventions; and $177 million to restore the 136-year-old structure to its original luster.

Building restoration requires architects to be schizophrenic in some ways. One day, they’re detectives poring over documents, news articles, and books to figure out how to alter the design while preserving its original essence. The next day, they’re doctors diagnosing the structural weaknesses and repairs that need to take place. They’re also masters of disguise in that they often have to wedge modern elements—like heating, cooling, and fire-suppression systems—into a historic edifice. And you have to manage expectations on top of that, too. (In the case of Scotland’s Stirling Castle, which was recently featured on an episode of the podcast 99 Percent Invisible, the walls were restored to their original ochre hue, but the public hated it in spite of its historic fidelity.)

Murphy Burnham & Buttrick Architects spearheaded the master plan to restore St. Patrick’s Cathedral, and marshaled a team of experts to execute it.

“When we came to this project, the cathedral was magnificent, and it’s still magnificent to this day,” says Jeffrey Murphy, a founding partner of Murphy Burnham & Buttrick. “The net result to this restoration is you’re seeing a building that looks more like it did when it opened in 1879 than it did four years ago.”

James Renwick Jr., responsible for a number of churches along the Atlantic Seaboard, designed the landmarked structure. Builders set the first cornerstone in 1858 and completed the final touches in 1888, nearly 10 years after it first opened to the public. Over the years, dirt and soot accumulated, plaster chipped, and mortar crumbled. It was time for a refresh.

While the building was well documented, few of those records were digitized. One of the first things the restoration team did was cobble together digital drawings and plans that were input into building information modeling systems to help triage the structure better and to delegate and track repairs more efficiently.

On a one-to-ten scale of difficulty, Murphy ranks the restoration challenge at an 11. “It was built with such quality and a high level of craft that I think everybody who worked on this building has a sense of awe on what was able to be accomplished 150 years ago,” he says. “In some ways it’s daunting because the bar is so high on this building. There’s so much of everything to do: There’s woodwork, stained glass, protective glazing, bronze doors, wood doors, and six types of stone, which all require different treatments.”