How the Notre-Dame Cathedral Fire Spread

It took less than an hour for a fire to spread from the attic of the Notre-Dame Cathedral in Paris, race across the wooden rooftop and topple its 300-foot spire.

Around 6:30 p.m., Paris time, smoke began to pour out of the cathedral’s roof, near scaffolding that had been put up over the last few months to conduct renovations on the spire.

The fire started in the cathedral’s attic, said Jean-Claude Gallet, commander of the Paris firefighters.

The attic is an oddly shaped space, seldom visited, that lies above the soaring stone arches visible from the floor of old European cathedrals. Diagrams of Notre-Dame and official descriptions of the space indicate that it is crisscrossed by giant, timber trusses that add structural integrity to the cathedral.

“Once these massive timber structures start to burn, they almost never can be stopped,” said Jonathan Barnett, an international fire safety authority at Basic Expert in Australia. “We focus on their masonry walls and forget all the massive timber within.”

Each beam of the cathedral attic’s wooden frame is made from an individual tree — more than 1,300 in total — giving it the nickname “the forest.”

A dry space like the attic tends to accumulate dust and debris, making it even more flammable. “That’s where your risk really is,” said Andrew Tremlett, the Dean of Durham Cathedral in the United Kingdom, which has a similar wooden vaulted roof like Notre-Dame’s.

After taking over the attic, the blaze quickly spread across the roof and engulfed the spire, a wood-frame structure covered in lead.

Fire safety experts said houses of worship can pose special problems for firefighters. The cathedral’s wood roof, for instance, is made of a flammable material and is difficult to reach.

“These cathedrals and houses of worship are built to burn. If they weren’t houses of worship, they’d be condemned,” said Vincent Dunn, a fire consultant and former New York City fire chief.

Images taken on Tuesday showed the scale of the damage inside the cathedral.

Around five hundred firefighters were battling the blaze on Monday evening, according to France’s interior ministry.

By 11 p.m., Gen. Jean-Claude Gallet, the Paris fire chief, said that the structure, including the two magnificent towers soaring above the skyline, had been “saved and preserved as a whole,” but that two-thirds of the roof was destroyed. The cause remains unknown.

BY LARRY BUCHANAN, WEIYI CAI, JAMES GLANZ, MIKA GRÖNDAHL, EVAN GROTHJAN, ALLISON MCCANN, YULIYA PARSHINA-KOTTAS, KARTHIK PATANJALI, JUGAL K. PATEL, SCOTT REINHARD, BEDEL SAGET, ANJALI SINGHVI AND JEREMY WHITE

Elian Peltier and Aurelien Breeden contributed reporting from Paris. Christiaan Triebert contributed from New York.

An earlier version of a diagram with this article incorrectly labeled a portion of the cathedral as the nave.