What does it take to do a massive spire-to-sidewalk, inside-to-outside restoration of one of Manhattan's best-known landmark churches? The crew assigned to the three-year, $177-million upgrade of St. Patrick's Cathedral has found it comes down to preparation, coordination and an enormous amount of scaffolding.

The project includes replacing marble on the facade and interior of the historic structure, which looms from Fifth to Madison avenues and from East 50th to East 51st streets and is on the list of National Historic Landmarks. It also involves repairing or replacing more than 200 windows and cleaning the pipes on one of the biggest organs in Manhattan.

But perhaps the most considerable feat for crew members is working around the clock, with some minor exceptions, while masses are conducted and tourists stream in and out of the hushed and sacred spaces.

"If you come in once a week to light a candle for St. Jude or St. Anthony, you need to be able to get to it," says Eileen McCarthy, a senior project manager with construction manager Structure Tone, which oversees a project team that can swell to 120 at peak times and typically works six days a week.

To that end, the team and church officials must closely coordinate activities, such as temporary shutdowns of the central aisle when scaffolding needs to be relocated or stopping work for special services such as funeral masses.

The project, which launched in 2012, is in the second of three phases. Phase 1 focused on the exterior on the Fifth Avenue side, while Phase 2 is targeting the middle section, or the transept, between East 50th and East 51st streets. Phase 3 will restore the rear of the building along Madison Avenue and then take aim at the altar area. Work is concurrent on several aspects of the project, however, including upgrades to the heating/cooling and IT systems.

Phase 2 is running slightly behind schedule due to weather and humidity issues and is expected to wrap up in September of 2014, Structure Tone says. But the entire project is still expected to be completed in 2015.

One of the most challenging aspects of the project has resulted in the most notable change: the cleaning and refurbishing of the white marble blocks that make up the facade of the 1879 buttress building, which was designed by James Renwick Jr., who also designed Grace Church in Manhattan.

To scrub away the pollution that has built up on the stone since its last major restoration in the 1940s, crews had to erect a lattice of scaffolds around the building's exterior. Starting from the busy sidewalks on Fifth Avenue, the scaffolding reaches to nearly the tip of the crosses on the building's dual spires, which top out at an elevation of 347 ft.

The exterior scaffolding, which was in the process of being peeled away in late November, took seven months to erect. To keep it from being top-heavy, platforms could not be inserted all at once among the metal bars, but instead were constantly shuffled around as the crews moved from place to place, says Derek Trelstad, an associate at Robert Silman Associates, the structural engineer. The scaffolding had to be safe, attractive and "gentle" to the cathedral in that it could not dig into the structure. It also could not be too obtrusive; black netting shields interior scaffolds from parishioners and visitors below.

To clean the marble, workers avoided harsh chemicals and instead used a more delicate low-pressure rotating vortex technology that blends air, water and powder. Besides turning the stone from an ashy hue to what looks like sparkly chalk, the technology helped reveal where chips had fallen out over the years and where marble had cracked, says Jeffrey Murphy, a partner at designer Murphy Burnham & Buttrick Architects.

"The cleaning was really a preservation measure," Murphy says. "The added benefit was a lighter and brighter building."

But finding stone to fill those gaps, with color closely matching what was there, was tricky, McCarthy says. The Westchester County quarry that produced the Tuckahoe marble that makes up much of the cathedral—stone that was also used in New York City's Marble Collegiate Church and Met Life Tower—was closed and filled decades ago.

Building Conservation Associates, the conservationist on the project, managed to track down 60 raw tons of the stone, however, in different sizes that were gathering dust in a Westchester cemetery. Petrillo Stone Corp., Mt. Vernon, N.Y., cut it roughly into shape, while fine-tuning was done on site.

The cathedral has presented other hurdles as well, many stemming from the fact that there are few surviving building documents, such as sketches, schematics or blueprints. This made it difficult to know, for example, whether the 19th-century wood trusses in the building's attic, which is at a height of 120 ft, could support a new trolley-and-track structure, Trelstad says. The new structure was needed to service a modern fire-protection system where none existed before.

Silman had to coordinate detailed tests on the trusses, including making sure that they did not have too many knots, which can indicate weakness, Trelstad says. The beams turned out to be solid, he adds.

Similar detective work took place to determine where to place scaffolds. Each leg can support 8,000 lb, compressed into a tiny column, like a high-heel shoe on a carpet. Silman had to make sure the sidewalks and floors could withstand that tremendous pressure, though in the case of the floor, it can be hard to tell where the strongest parts are.

That's because the floor is a concrete slab topped with marble and terrazzo, which is thick and layered and, as a result, hard to examine. Similarly, the area underneath can basically only be accessed through heating ducts, Trelstad says. "We had to scramble a little bit," he adds. This included using radar and metal detectors to figure out where the fortified rebar-lined sections were.

Quiet Time

Work must be done without major interruption to church life, which for St. Patrick's includes 2,400 masses, 150 weddings and about 5 million visitors a year. Aside from Sundays, when crews are off, no work can occur from 7 to 7:30 a.m. daily to allow Cardinal Timothy M. Dolan and other priests to televise a mass without the sound of hammering in the background. Work must also stop from noon to 12:30 p.m. for services taking place on the high altar.

Visitors to the cathedral may notice a brightness that wasn't there before. Workers have methodically cleaned, repaired and reglazed the cathedral's 73 stained-glass windows, which in many cases had oxidized and become cloudy, obscuring their intricate violet, red and orange patterns. The 26 leaded-glass clear windows that dot the lower sections of the towers have also been scrubbed, while 160 windows in the top parts of the towers have been replaced.

As of November, a third of the pews in the cathedral—enough to fill five 53-ft-long tractor trailers—had been removed and replaced by temporary stackable black plastic chairs. The pews are being restored in Middletown, N.Y., by the Keck Group; the remaining pews are scheduled to undergo a similar process starting in early 2014.

The huge pipe organ that dominates the cathedral's western wall, high above parishioners and one of three organs in the building, was also dismantled, piece by piece, in part to protect it during the restoration but also to refurbish its 8,000 pipes. The two smaller organs continue to make music but will also be refurbished after the western one is reassembled this winter.

Some items have been removed and are not likely to return. These include the chiller that sat for several years outside the East 51st Street side of the cathedral. The cathedral expects either to tap into the heating and cooling system used by Rockefeller Center across the street or to install its own geothermal plant. Paint on the bronze doors has also been removed, by hand and sometimes using dental tools. Crews then treated underlying corrosion, added a patina and waxed the doors.

Among the more modern touches is a new fiberoptic-based system for ringing brass bells. Cooler burning LED bulbs will replace the existing LEDs and will be installed along with new flat-screen TVs. But unlike before, TV conduits will be tucked out of sight to maintain the building's beauty, workers say.

Only a handful of items were left untouched. The verdigris roof, including one atop the Lady Chapel on the Madison Avenue side, as well as coping and gutters across the complex won't be restored to their original copper color but will be left in a weathered state. There will be only minor work done to the chapel itself.

Step Training

While erecting sky-high scaffolds may be complicated, moving around on them requires special preparation too. As required by the Dept. of Buildings, every worker was required to take a four-hour course in scaffold safety. Most priests took the course as well, so they would be safe while exploring the building.

Other training included optional lessons in somewhat arcane architectural terms. Some team members took it upon themselves to learn the difference between a narthex (the vestibule inside the front entry) and a cathedra (the archbishop's throne). They say this has helped them communicate effectively with church officials, architects and conservators.

"From my seat in the bleachers, this is one of the most complex jobs I've done," said Ron Pennella, a senior project manager with Structure Tone, on a recent tour as he bounded up a south tower spiral staircase. Pennella has also worked on restorations of Marble Collegiate, on West 29th Street; the Federal Reserve Bank of New York, on Liberty Street; and even Ground Zero, as part of a fraud-investigation team.

Not everything about the project has run smoothly, however. The project is about three months behind due in part to Superstorm Sandy last year, Pennella says. Humidity has also been a problem, he adds. Throughout last summer, humidity became trapped in the cathedral's eaves, making it difficult for a super-thick plaster used for ceiling vaults, capitals and rosettes to dry. Until it did, the plaster could not be painted, which finally happened in October, Pennella says.

Though the lack of blueprints might have thrown contractors a few curves, it has also allowed for thrills of discovery, Pennella says. St. Patrick's, which was begun in 1859 but stalled during the Civil War, is like a palimpsest of construction techniques ranging from hand-cut marble to machine-cut trusses. In contrast, the trusses on the older Marble Collegiate church were cut by hand, "and one can tell as much because their edges are slightly rougher," he says. He adds that uncovering such details inspires "awe of the workmanship done from years and years ago."