PARIS • The architect who oversaw the design of the fire safety system at Notre-Dame acknowledged that officials had misjudged how quickly a flame would ignite and spread through the cathedral, resulting in a much more devastating blaze than expected.

The system was based on the assumption that if the cathedral ever caught fire, the ancient oak timbers in the attic would burn slowly, leaving ample time to fight the flames, said architect Benjamin Mouton.

Unlike at sensitive sites in the United States, the fire alarms in Notre-Dame did not notify fire dispatchers right away.

Instead, a guard at the cathedral first had to climb a steep set of stairs to the attic, a trip Mr Mouton said would take a "fit" person six minutes.

Only after a blaze was discovered could the fire department be notified.

Even a flawless response had a built-in delay of about 20 minutes from the moment the alarm sounded until firefighters could arrive and climb to the attic with heavy equipment to begin battling a fire.

Those delays turned out to be catastrophic.

"I was stunned by the speed with which the oak in Notre-Dame burned," Mr Mouton said. "Oak that old can't burn like a match. It's absolutely incomprehensible."

Experts said two of the top officials on the project, Mr Mouton and former firefighter Regis Prunet, appeared to have miscalculated what was needed to protect such an unusual building.

Scientists consulted by The New York Times said fire dynamics indicated that, while the dense timbers may take time to burn completely, a fire would naturally race across the original timbers at Notre-Dame. It was a mistake to assume otherwise, they said.

Mr Mouton was the architect in charge of Notre-Dame between 2000 and 2013, during which he oversaw a revamp of fire safety.

The designers were determined not to alter the attic with features like sprinklers or firewalls.

Lieutenant-Colonel Prunet said sprinklers were not added as they would "drown the whole structure". They said the team had banked on prevention and detection instead.

Two guards monitored the roof structure, day and night. The cathedral was covered in smoke and heat sensors. Three times a day, someone went up to check that the system was working.

But that approach appears to have been flawed, beginning with the response to the first alarm at 6.20pm on Monday (12.20am on Tuesday Singapore time).

The guard, seeing no fire, gave the all-clear and came down.

By the time the second alarm sounded at 6.43pm and a guard climbed the stairs again, the fire was already a conflagration.

The call finally went to the fire brigade at 6.51pm.

"We could have avoided all this with a modern detection system," said Mr Guillaume Poitrinal, president of Fondation du Patrimoine, an organisation that promotes French architectural heritage.

NYTIMES