'The dilemma is the authenticity': What it will take to rebuild Notre-Dame

This won't be your typical renovation project. It's not often thatthe reconstructionof a building whose origin dates back centurieshas to be contemplated.

But while officials at the site of Paris's historic Notre-Dame Cathedral still are assessing the damage caused by Monday's devastatingfire, French President Emmanuel Macron is already touting a quick rebirth.

"We have so much to reconstruct. So yes, we will rebuild the Cathedral of Notre-Dame, more beautifully," Macron said in a televised national address. "I would like it to be achieved in five years from now. We can do it. And we will mobilize."

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International expertise and money is already pouring in. And within France, some of the country's richest citizens and companies have pledged close to 500 million euros($753 million Cdn) towardthe cathedral's restoration.

Before doing any work, experts say the true extent of the damage must still be fully evaluated.

The next 24 hours will be key.

UNESCO director-general Audrey Azoulay said work must begin immediately to protect the remaining structure, warning that parts of the cathedral remain "extremely fragile." Notre-Dame is part of a UNESCO heritage site and the agency has offered its expertise.

Caroline Bruzelius, an art historian and professor at Duke University who spent years working inside the cathedral, said parts of the structure could still be at risk of collapse due to fire damage. She said her main concern is the concrete vaulting that supports the cathedral and whether it can be stabilized after the heat damage it endured.

Bruzelius is not sure how long the renovations will take but said that while it may be safer and more practical to use modern materials, she questioned whether that's what the people of France will want.

Caroline Bruzelius talks more about repairs:

Notre-Dame Cathedral could still be at risk after fire, art historian says

5 years ago

Duration 1:15

Annabelle Radcliffe-Trenner, a preservation architect at Historic Building Architects in Trenton, N.J., who specializes in religious buildings, says there will be an initial stage of "peeling back" the fire damage to figure outwhat remains.

"The next step is really to diagnose the condition of everything and shore and brace everything that needs to be stabilized and protect it from weather," she said.

It will all start with the church's sprawling ceiling structure, where architects have identified three main holes.

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Officials say most of the wooden roof beams have been burned, and parts of the concrete vaulting that hold up the roof have collapsed. The damage from the extensive amount of water used to extinguish the fire is also being assessed.

"We would have to assess the condition of what is remaining there, especially of the vaulted ceilings," saidmaster stonemason Frank Helmholz, whois currently working on a pyramid restoration project in Egypt.He has worked on numerous projects across Europe, including the restoration of the Basilica of Saint-Denis in northern Paris.

"There are parts of the roof that have a hole in them, which, of course, destabilizes the surrounding parts. And so we would have to assess how fragile and how vulnerable this may be," he said.

Re-rafting and rebuilding parts of the ancient roof will bea meticulous and lengthy process, Helmholz said.

All damaged and loose pieces must initially be removedand then shoring put in place to ensure no additional pieces fall. Then, he said, a temporary roof will likely be put in place, before each individual arch is rebuilt.

"An arch, structurally,only works and is stable and strong when it is complete," Helmholz said. "When it is not complete, when a part is missing, then it folds together and collapses."

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It's time-consuming work:But only after the stone arches are repaired can a new wood roof be laid on top, he said.

What materials officials will actually have available to restore the cathedral is another story. Some of the damaged original wood and stone could be salvageable but much of it likely won't be.

And according to Helmholz,there are a number of issues to consider with ancient stone.

"Prolonged intense heat will destabilize the stone. At first, that just turns it somewhat pinkish, which is not a problem," he said. "But if it is hot enough and the exposure is long enough, then the stone becomes brittle. And it just doesn't have the same properties as it [once] had."

Helmholz also said that some of the structure's wood, swollen by water, is likely pushing against stones in the roof.

Officials have acknowledged that much of the wood that formed Notre-Dame's roof was destroyed and that replacing its 800 wooden beams will be next to impossible.

"We don't, at the moment, have trees on our territory of the size that were cut in the 13th century," said Bertrand de Feydeau, vice-president of preservation group Fondation du Patrimoine.

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Salvaged wood from older structures is often used in such projects, saidRadcliffe-Trenner.But this ambitious project will require a mix of old and new technology, she said.

"There's a multitude of ethical conservation preservation questions to be asked and to be thought through very carefully," she said.

"It takes a lot of time to do that in a way that is sensitive to the historic fabric, but as well, viable in terms of buildability. So the dilemma is the authenticity and how much of the original fabric you preserve and how much you change."

It's likely a combination of building materials will be usedwith an emphasis on fire safety.

"For example, we used fibreglass beaded board [on a recent project]because it was less vulnerable to fire damage in the future," Radcliffe-Trenner said. "I mean,nobody, when you walk, would know. That was a compromise we made on material and a decision we made because it would protect the building in the future."

Extent of Notre-Dame fire damage revealed

5 years ago

Duration 0:44

Helmholz saidpeople often think using different or more modern materials, in conjunction with what exists, undermines a restoration. But in the case of Notre-Dame, it may not be possible to get the same stones from the same source.

"Initiallyyou would see the difference, but these things blend together pretty quickly, especially given the pollution and the exposure to the weather on the stones," he said.

Both Helmholz and Radcliffe-Trenner acknowledge the challenges of restoring Notre-Dame to its former glory, but insist it will happen. Although, they say, Macron's five-year timeline may be a little ambitious.

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"Is it possible? Yes, I do believe it's possible. I think 10 years is possibly not even enough, given the prominence of the building," Helmholz said. "Maybe there will be some effort to work in many [sections] at the same time, and yet I don't think it can be rushed. Some things just have to be built step-by-step."

"I mean, you can restore anything," saidRadcliffe-Trenner. "It takes a lot of money and it takes a lot of very skillful, talented people who have expertise in this area and there are plenty in France and in Europe."

Europe has a long history of restoring or rebuilding churches that were damaged during the First and Second World Wars, Helmholznoted. That work has provided key insights into how ancient architecture was conceived and how it can be most effectively restored.

"The structures, the shapes, the mouldings all these things are very well known and documented. So we wouldn't have to guess about how to build it or how to shape them," he said."And so we wouldn't be building it in the wrong way; we know what needs to be done."

And if asked, Helmholzsays it would a "great privilege" to lend hisexpertise.