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

Only after a blaze was discovered could the fire department be notified and deployed. 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 hoses and equipment to begin battling a fire.