By Katrin Bennhold and James Glanz

New York Times News Service

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 they had anticipated.

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

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 Mouton said would take a “fit” person six minutes.

Only after a blaze was discovered could the fire department be notified. That means 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 hundreds of pounds of equipment to begin battling a fire.

“I was stunned by the speed with which the oak in Notre Dame burned,” 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, Mouton and a former firefighter, Lt. Col. Régis Prunet, appeared to have miscalculated what was needed to protect such an unusual building.