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The main structure of Notre Dame Cathedral – including the two bell towers – was saved after firefighters spent the night fighting against the huge fire.
Now that it has been extinguished, the experts have been able to highlight the difficulties that firefighter teams have been facing in the fight for the safeguarding of such a vast and historic building.
A complex and flammable roof
Guillermo Rein, professor of fire science at Imperial College London, told the BBC that "what the Paris firefighters did was close to a miracle".
"It's completely different from most formations because all the material that burns is the roof," he said.
The roof of the cathedral is a complex carpentry piece with very thin and very thick columns that are all flammable, he explained. The thin parts ignite first, then thicker wood, which burns for a long time.
Image copyrightAFP
Image captionThis image taken Monday night shows flames taking hold of the roof
Gregg Favre, former commander of the St Louis Fire Department in the United States, also pointed out how flammable the building was.
He explained on Twitter: "Let's stop to remember how the fires burn.You need oxygen, fuel, heat and a chemical chain reaction.Remove the fuel is not an option The churches do not miss anything to burn.
"The heat from a fire of this size is huge, there are few options to stop it, the chemical chain is off the mark, and the horse left the team in the first five minutes.
"This leaves oxygen.Unfortunately, even if the roof had not burned, the churches are almost impossible to control the ventilation.They must be open and airy design.Ideal for Sunday worship, terrible for the management of the spread of fire. "
The firefighters of Paris, firefighters of the city, posted a video showing how they fought the fire.
Pbad the Twitter message of @PompiersParis
Paris firefighters✔@PompiersParis
#intervention Back in pictures on fire at #Our Lady from Paris who mobilized nearly 400 firefighters.
End of post @PompiersParis Twitter
Bob Parkin, a former British firefighter turned security consultant, told the BBC that in this case the only solution was to fight the fire from the inside.
"They will have to go inside the building or use airplanes, ladders and hydraulic platforms," he said.
"But you put water on the outside of the building and the only way to sort it out is to attack it from the inside."
Do you throw water from above?
When the fire burned, US President Donald Trump suggested that flying tanks could be brought in to fight the flames from above.
Professor Rein stated that he was not the first to propose this proposal, but that it was "not a good idea, as it would have damaged the structure of the cathedral by collapsing the walls. ".
The strength of water coming from a tanker is strong and no one has been trained to use one in a city. He added that Monday's fire was "not the time to test that".
Image copyrightPA
Image captionFirefighters study the damage caused by the fire the day after its burst
How can they make Our Lady safe?
Professor Rein said the "fire engineering" will have to be taken into account during the reconstruction of the cathedral. Fire engineering is now in place at modern sites such as the Shard in London.
He said: "The roof was not protected."
Mr Parkin told the BBC that in the future, Notre Dame should install sprinklers.
"We continue to see that these buildings are burning and it is relatively easy to prevent them," he said.
"In this building, you have invaluable artifacts and it is so easy to prevent by installing sprinkler systems, people are still building without them and that continues."
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