The main structure of Notre-Dame cathedral (巴黎圣母院) — including the two bell towers — has been saved after firefighters worked through the night to put out the big fire.

Now that it has been put out, experts have been able to explain the difficulties fire crews faced as they fought to save such a large, historic building.

It’s not yet known exactly how the Notre-Dame fire began, but it appears to have started near the top, eventually spreading across the rest of the wooden roof.

Gregg Favre, former commander at St Louis Fire Department in the US, pointed out how flammable the building is. He explained on Twitter: “Let’s pause to remember how fires actually burn. It involves key elements like fuel, oxygen, heat and a chemical chain reaction.” Unfortunately, none of these elements are easy to remove in this case.

“To start with, removing the fuel is a no go. Churches have no shortage of things to burn. Although a huge oak beam(梁) is generally difficult to get burning, as smaller timbers (木材) fuel the fire and the temperature rises, the timbers will eventually ignite (点燃) in a phenomenon known as a flashover.”

“The heat that a fire this size is putting off is terrific. Little options for interrupting that. The chemical chain is off to the races.”

“That leaves the oxygen. Unsurprisingly, even if the roof had not burnt off, churches are nearly impossible to control ventilation (通风) in. Their design is to be open and airy. Great for Sunday worship, terrible for managing fire spread.”

As the fire burned, US President Donald Trump suggested that flying water tankers (罐车) could be brought in to fight the flames from above.

Professor Rein, the head of Imperial College London’s fire-studying Hazelab said“ Trump wasn’t the first person to come up with the suggestion, however, it was not a good idea because it would have damaged the structure of the cathedral, collapsing the walls”.

The force of water coming from an air 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 this”.

Professor Rein says that “fire engineering” will need to be considered when reconstructing the cathedral. Fire engineering is now in place in modern landmarks such as the Shard in London.

“We keep seeing these buildings go up in flames like this. However, it’s relatively easy to prevent. Notre-Dame should install sprinklers (消防喷淋).” he said.

Much as the Notre-Dame fire is a tragedy, information from studying it will be priceless in protecting other buildings.