Repairing and rebuilding the Notre Dame Cathedral, one of the most famous buildings in the world, will take time - maybe decades.
Architecture experts said the repairs could require a delicate balance of restoring the majestic building's unique look with fortifying the structure for the future.
"This is going to be a slow process and one that's going to take a lot of time," said John J. Casbarian, dean emeritus at Rice University's School of Architecture, who oversees the school's program in Paris.
Most of the rebuilding will focus on reconstructing the roof, originally built with wood, Casbarian said, and he expects it to take at least 10 to 20 years.
Luckily, the church has an extremely accurate documentation system for everything inside, he said. "It's just a matter of figuring how to manufacture and replace every piece that was destroyed."
Over the years, historians and archaeologists have made exhaustive plans and images, including minutely detailed, 3D laser-scanned re-creations of the interior.
Stephen Murray, a professor emeritus of medieval art history at Columbia University who worked on scanning the roof, said he hopes the new roof isn't a replica but rather a high-tech, modern structure - one better prepared to handle hazards.
French President Emmanuel Macron said Tuesday he wants to see the 12th-century cathedral rebuilt within five years. Casbarian expects it to take more time because of the large area that was destroyed, which will require a lot of scaffolding, and the level of craft work it will require to reconstruct each lost piece.
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"A lot of construction is covered up so that you don't actually see it. In buildings like this, you see everything," he said.
Many French Gothic cathedrals, like Notre Dame, were built thin, slender and fragile, which means restoration has been ongoing throughout their history, said Jean-François Bédard, a professor at Syracuse University's School of Architecture.
"It's not like this monument is fixed in time and suddenly destroyed. It's an ongoing process," said Bédard, who expects the rebuild to take at least 20 years.
An earlier restoration
After the French Revolution left the cathedral in a state of disrepair in the 19th century, restorers Eugène-Emmanuel Viollet-le-Duc and Jean-Baptiste Lassus comprehensively rehabilitated the structure, Bédard said.
It was Victor Hugo's 1831 novel, "The Hunchback of Notre Dame," that largely sparked interest in the renovations, according to Casbarian.
Viollet-le-Duc did much of the work restoring the building's roof, and he designed the spire that collapsed Monday.
That restoration work dramatically changed the character of the cathedral, Bédard and Casbarian said, and much of the cathedral remained in its 1800s condition.
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French officials haven't said whether they will rebuild the roof with wood, as it had been since Viollet-le-Duc's work, or a more fireproof material such as stone.
Though the building's design kept it from being leveled by the blaze, Murray and other architects said some sort of modification with fire prevention in mind is needed to prevent another disaster.
"I don't anticipate that they will want to do a replica of the old wooden roof," Murray said. "How are you going to do that in wood in the 21st century? A wooden roof in any case is not a good idea."
Why is the structure intact?
As a world watched the fire drama play out Monday night, there was a global ripple of joy when fire officials announced that the main towers were safe.
Erik Inglis, an art historian from Oberlin College, credits two factors: "First, the firefighters, who put a stop to the fire on the roof and attic.
"Second, the stone vaults of the church which, in combination with the walls and flying buttresses, proved stable. They are built in a sort of cellular system, in which each unit helps to support its neighbors while being supported by them," he said.
Because the buttresses are outside the building, they probably did not take on as much heat and were able to support the structure, Casbarian said. "Of course, it will never be the same, but the foundations I presume are as solid as ever," he said.
"It is also possible that the church may have benefited from a built-in redundancy, as the techniques of Gothic architecture were still being developed, and the architects may not have pushed the building to its limits," Inglis said.
The stone vault "acted as a kind of fire door between the highly flammable roof and the highly flammable interior" - just as the cathedral's medieval builders intended, said Tom Nickson, a senior lecturer in medieval art and architecture at London's Courtauld Institute.
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Between the wooden roof and the stone vaulted ceiling lies a layer of mortar that protected the rest of the building from the flames, a form of built-in fire-proofing, Murray said. The spire collapse is probably what caused much of the damage beyond the burnt roof, he said.
"Gothic architecture is built precisely with this type of disaster in mind," Murray said. "The renovation of a Gothic cathedral is simply a matter of life."
A massive fundraising campaign was underway Tuesday to rebuild Notre Dame. At least $700 million has been pledged.
France owns the cathedral, which has been at the center of a years-long conflict between the nation and the Paris archdiocese over who should finance badly needed restoration work to collapsed balustrades, crumbling gargoyles and cracked facades, Reuters reported.
It was too early to estimate the cost of the damage, according to the heritage charity Fondation du Patrimoine, but it is likely to run into the hundreds of millions of dollars.
Statues, relics, artifacts saved
All was not lost at Notre Dame. The religious statues that sat atop the cathedral had been removed as part of a $6.8 million renovation of the towering spire that fell to the ground in Monday's blaze. Some of the sacred artifacts housed at the cathedral are safe, too.
Notre Dame's heritage director, Laurent Prades, told The Associated Press that the only piece of architecture damaged inside the building is the high altar, which was installed in 1989.
"All the 18th-century steles, the pietas, frescoes, chapels and the big organ are fine," he said. The cathedral's famous three large stained-glass rose windows were damaged by the heat but not destroyed, Prades told the news agency.
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Copper statues usually set atop the cathedral were removed just last week. Workers sent the statues representing the 12 apostles and four evangelists to southwestern France as part of the planned restoration project.
French authorities said treasured relics such as the Crown of Thorns, which many believe was worn by Jesus Christ, were safe from the flames.
Bédard stressed that saving the religious relics - the reason why many cathedrals are built in the first place - was essential.
"You can always rebuild, but you can never replace the relics," he said. "The cathedral cannot be divorced from the religious aspect."
Contributing: The Associated Press
This article originally appeared on USA TODAY: Why Notre Dame didn't completely crumble in the blaze. And why it could take decades to repair.