Last month, painters suspended from mountain climbing gear finished painting the dome of St. Francis de Sales Catholic Church.

Out of breath at the top of the scaffolding, the project's lead architect, Annabelle Trenner, said the dome gives you a sense of the scale of the Byzantine church.

Sixty feet across, the dome caps a roofscape of platforms and arches. Raphael Guastavino of Spain engineered the building in the early 20th century.

It's an incredible building that presents an enormous challenge. The entire renovation of the century-old building in West Philadelphia could take 15 years. However, completing its centerpiece is a major milestone.

Trenner calls the complex renovation a "hundred-year restoration." Workers should not have to return to the roof for another hundred years.

Guastavino originally covered the dome with a pattern in his signature ceramic tiles, but Philadelphia winters proved less kind than Spanish ones. In the 1950s, it was covered over again with subway tiles, which were failing again when the archdiocese brought in Trenner from Historic Building Architects.

After stripping the 1950s tiles, the team took a new approach. After taking cores out of the dome to find the original colors of Guastavino's tiles, they reapplied his pattern in more weather-resistant paint.

Besides the damage to the tilework, water pooling on the rooftop had also begun working its way inside the walls. Trenner identified 37 different leaks, using infrared during a rainstorm.

"Literally, there was so much water coming down it washed off the wall," said Trenner.

People brought umbrellas to mass.  

Trenner had plans to climb the vaulted walls to pinpoint the last leak.

Work on the interior has really just begun. Netting lines the side walls. Scaffolding climbs over the altar, protecting the priest from falling bricks. Trenner has ordered more scaffolding placed outside as a temporary restraint.

Grants from the Partners for Sacred Places and money raised year by year by the archdiocese have paid for the work. The whole project will cost about $26 million.

People who have come to sit in the pews over many, many years have fond memories of the building and, Trenner believes, recognize its incredible value.

"Philadelphia has all these lovely beautiful secret buildings that people don't know—they're sort of like a hidden treasure," Trenner mused. "More people need to enjoy them and figure out how complicated they are and how beautiful they are."

Trenner will know St. Francis de Sales intimately by the time she's finished here—renovations are expected to take 15 years.

"It's a huge undertaking for the church, but it's such a glorious space," said Trenner. "I mean, it's breathtaking. I think ... whatever faith you come from, this is just a spiritually nurturing place. It's just wonderful."

"I think they recognize that," she continued. "It's a wonderful, wonderful project. I'm lucky to be working on it, I think."