Doubt has been cast on a pledge by French President Emmanuel Macron that the restoration of Notre Dame will be completed within five years.
Mr Macron said the renovations to restore the cathedral’s iconic 19th century spire, vaulting and two-thirds of its roof would be completed in time for the Paris 2024 Olympics.
“We will rebuild the cathedral to be even more beautiful and I want it to be finished within five years,” he said.
But experts have said that the ambitious timeline appears insufficient for such a massive operation.
Even French Prime Minister Edouard Philippe – while supporting the government timeline – acknowledged that it would be difficult.
“This is obviously an immense challenge, a historic responsibility,” Mr Philippe said.
Prominent French conservation architect Pierluigi Pericolo told Inrocks magazine it could take triple that time.
“No less than 15 years … it’s a colossal task,” Mr Pericolo said.
Mr Pericolo worked on the restoration of the 19th century Saint-Donatien basilica, which was badly damaged by fire in 2015 in the French city of Nantes. He said it could take between “two to five years” just to check the stability of the massive cathedral that dominates the Paris skyline.
“It’s a fundamental step, and very complex, because it’s difficult to send workers into a monument whose vaulted ceilings are swollen with water,” Mr Pericolo told France-Info.
“The end of the fire doesn’t mean the edifice is totally saved. The stone can deteriorate when it is exposed to high temperatures and change its mineral composition and fracture inside.”
The comments came as Notre Dame’s rector said he would close the once-functioning cathedral for “five to six years” acknowledging that “a segment” of the near 900-year-old edifice may be gravely weakened.
Firefighters said Notre Dame’s rose windows are in good shape after the fire, but that their support structures are at risk.
Spokesman Gabriel Plus told reporters that the rose windows are “in good condition” but that “there is a risk for the gables that are no longer supported by the frame”.
He said firefighters took down statues inside the gables above the rose windows to protect them, and took care not to spray water too hard on the delicate stained glass.
He said firefighters and experts are still closely monitoring the building to determine how much damage the structure suffered and what needs to be dismantled to avoid collapse.
A Paris fire official said the towers of Notre Dame would have fallen if firefighters had not deployed massive equipment and acted swiftly.
Philippe Demay denied there was any delay and said firefighters acted as fast as they could.
Mr Demay told reporters that the operation was extremely difficult and that the towers could have collapsed “if we hadn’t put heavy equipment in place”.
The questions over the timeline came as nearly a billion euro poured in from ordinary worshippers and high-powered magnates around the world.
Contributions came from near and far, rich and poor – from Apple and magnates who own L’Oreal, Chanel and Dior, to Catholic parishioners and others from small towns and cities around France and the world.
Experts have put this in the threshold of realism – estimating the restoration would cost into to the hundreds of millions, although they acknowledge it is too early to be certain.
Presidential cultural heritage envoy Stephane Bern told broadcaster France-Info on Wednesday that 880 million euro (£762 million) has been raised in just a day-and-a-half since the fire.
The French government was gathering donations and setting up a special office to deal with big-ticket offers.
Some criticism has already surfaced among those in France who say the money could be better spent elsewhere, on smaller struggling churches or workers.
Mr Philippe said a competition will be held to see if the spire should be rebuilt.
“The international competition will allow for the question to be asked, should the spire be rebuilt?” he said. “Should we rebuild the spire envisaged and built by Viollet-le-Duc under the same conditions … (or) give Notre Dame a new spire adapted to the technologies and the challenges of our times?”