Firefighters stood helpless to stem the blaze Monday at Notre Dame Cathedral, likely because of Gothic engineering techniques that waterproofed the roof, consigning it to destruction.

The roof and spire of Notre Dame were all but doomed to burn once flames began to spread, possibly begun by a renovation accident, according to Ted Henderson, who studied archaeology at Dartmouth College. Henderson said a Gothic engineering technique used to waterproof the top of the cathedral involved placing a lead sheet roof over a wooden truss structure on top of the groin vault that served as the ceiling of the cathedral’s nave.

The wooden truss became highly flammable as it dried over hundreds of years, but was completely sealed from the outside by the lead sheets, and therefore impervious to firefighters’ efforts to douse the flames with water, Henderson said.

“So the wood catches on fire and it’s not actually exposed to the outside of the building anywhere,” Henderson told The Daily Caller News Foundation.

“In fact, the reason the roof is covered with lead is so water can’t get inside. It’s specifically fireproofed. So when the firemen, you know, arrive on the scene and they’re spraying [the] cathedral with water, it’s running off the sides of the lead roof and can’t access this void where the fire is actually spreading.”

As the fire grows hotter, the lead super-heats and melts, falling down onto the stone vault of the nave, which overloads the vault and causes it to collapse inward, creating an imbalance of weight throughout the entire structure and possibly leading to a total collapse, Henderson said.

A number of Gothic cathedrals have suffered near total collapses due to fire throughout history, including the Papal Basilica of St. Paul Outside the Walls, which burned in 1823 because a workman repairing the lead roof accidentally began a fire.

Notre Dame, however, has partially survived. The spire collapsed but the bell towers are visibly intact. The organ has survived, as well as the treasures, such as the crown of thorns relic, officials say.

"Good news: all the works of art were saved. The treasure of the Cathedral is intact, the Crown of thorns, the Holy sacraments. #NOTRE_DAME" https://t.co/X0FSl7eHO7

— Stephen Miller (@redsteeze) April 15, 2019

The crown of thorns was kept in Constantinople during the Byzantium empire until Emperor Baldwin II offered it to King Louis IX of France in exchange for hefty financial support of his nearly bankrupt empire. The crown of thorns was considered at the time to be “the most valuable single object in the face of the earth,” according to Henderson.

The crown of thorns has been in near continuous royal possession since Constantine the Great, with the exception of the time between when it was stolen during the French Revolution and when it was returned.

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