On Monday, a fire started in the attic of 850-year-old Notre Dame Cathedral. The fire quickly spread across a large part of the roof, eventually causing the spire to collapse, and badly damaging about two-thirds of the roof. The flames devoured the roof's wooden frame, which is more than 100 metres in length and nicknamed "the forest." Thankfully, 400 firefighters prevented the fire from causing any more damage.

In the days since, about €880 million ($994 million) has been raised for what is guaranteed to be a massive rebuilding project. Paris Mayor Anne Hidalgo and French President Emmanuel Macron have said that the project must be finished before the city hosts the Olympic and Paralympic Games in 2024.

But the project could run into trouble if they want to rebuild the cathedral with original techniques. Bertrand de Feydeau, vice president of the preservation group Foundation du Patrimoine, told France Info on Tuesday that France simply doesn't have trees tall enough to rebuild the roof as soaring.

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The cathedral spans 427 by 157 feet, while the roof is 115-feet high. And according to Notre-Dame's website, 1,300 trees were felled for its roof between 1160 and 1170. It's believed that most of those trees were between 300 and 400 years old at the time, and incredibly tall as a result.

De Feydeau told CNN that he doesn't believe France has enough old trees to restore the roof as it was.

“The roof was made of beechwood beams over 800 years ago. There are no longer trees of that size in France,” he said.

Asked if there were enough trees of that size across the entirety of Europe, he said, “I don’t know.”

"We will have to implement new technologies that will leave the appearance of the cathedral as we love it," De Feydeau said.

Foundation Fransylva, a federation representing private timber growers in France, has already asked members to offer up their trees.

"Loggers want Notre-Dame's 'forêt-charpente' re-constructed with French oak trees, in keeping with the same traditions and good quality of the first builders," they said.

Groupama insurance company has offered to pay for some 1,300 100-year-old oaks from the forests of Normandy, in keeping with the original construction.

But with a lack of trees tall enough for the project, some modern techniques will likely have to be used.

Jean-Michel Wilmotte, the architect of the Russian Orthodox Cathedral in Paris, and who restored the Collège des Bernardins, believes steel beams could be the answer.

Wilmotte suggest to the BBC that a lighter structure of steel beams and titanium panels in place of the oak beams and lead sheeting that were lost could help preserve the structure into the future.

Denis Dessus, president of France's National Council of the Order of Architects, believes that while the structure could be rebuilt exactly as it was due to thorough documentation, a lighter, more flexible structure, is likely to be used to "allow this cathedral to live for another 10 centuries."