31 OCT 2019

"It is the darkest hour of Notre Dame," says Dany Sandron, professor of art history and medieval archeology at the André Chastel Center1  and head of the 3D mobile scanning platform (Plemo3D). 


cathedrale en feu
Notre-Dame cathedral on fire © Virginie Bodin

Nine centuries old, the great lady had resisted the Revolution and wars. Nothing, until this terrible fire, had ever threatened the heart of its structure to such a point since its construction at the end of the twelfth century.

According to the first findings of the investigation, the fire started in the attic of the cathedral. The fire then spread very quickly because of the wind, devouring one of the oldest structures of Paris made up of hundreds of oak beams. The lead roof of several hundred tons that was resting on this "forest" went up in smoke. The 93-meter-high spire above the transept’s cross collapsed in less than an hour. Under the cathedral’s sub-structure, "the temperatures could reach 2000 or 2500 degrees Celsius, well above the temperature to melt lead," explained Guillaume Legros, professor-researcher at the ∂'Alembert Institute2  and former doctoral student of José Torero3 , an international specialist in large fires.

How are fires spread?

According to Guillaume Legros, the first flames can initially be quite weak and produce a lot of smoke particles that give this reddish appearance to the flames. The latter radiate intensely, thus "preheating" the surrounding surfaces until reaching a temperature that can trigger their ignition.

The fire of Notre-Dame de Paris © Steeve Leulier

Contrary to popular belief, "for the fire to spread, the scale of the initial flames is relatively unimportant to the initial scenario of a conflagration," says the scientist. Some combustions, called "smoldering combustions" are hardly detectable because they emit no visible flame. This type of combustion, which often forms in porous media such as foams, spreads very slowly and can lead to a large-scale fire."

Clues to determine the causes of the fire

At present, all the scenarios concerning the cause of the fire are envisaged. If it is necessary to wait for the progress of the investigation to emit solid hypotheses, some elements may, according to the researchers, help to determine the origin of the fire.

"We know for example, explains Guillaume Legros, that the initial focus of a fire by an electrical short circuit is very localized and very black. Unlike a fire caused by a blowtorch, combustion is slow, inefficient and generates a lot of smoke that darkens the origin of the fireplace. On the other hand, when it comes to arson where, for example, ethanol has been deliberately dumped, the initial focus will be much more extensive and less readable."

A necessary diagnosis of the structure

In parallel with the investigation of the origin of the drama, in-depth analyzes of the structure of the building will be conducted. While the work of the firefighters inside the building has saved Notre Dame, "we must now perform a technical diagnosis that will take several weeks to identify the structural damage suffered by the cathedral," says Dany Sandron.

Indeed, if the two belfries resisted the blaze, part of the vaulted ceiling collapsed in three places and the destruction of two thirds of the roof has weakened the facades of the transept, which are no longer supported at their summit.

"A Gothic structure, such as the cathedral, holds together because of its ensemble of elements. It's a skeleton. If something is missing, it weakens the whole building, "says Sandron.

Gothic architecture is based on the opposition of forces. This is why, according to Dany Sandron, the flying buttresses were created to compensate for the lateral thrusts of the arched ceiling. However, with the disappearance of parts of the vaulted ceiling, the buttresses risk pushing the walls of the facades towards the interior.

Schema arc-boutant
The flying buttresses compensate for the lateral thrust to the outside of the crossed-arching vaults © Sorbonne University

On the other hand, says the art historian, "we are currently unable to say whether the masonry has not suffered or will not suffer because of the thousands of cubic meters of water that have been poured on Our Lady." This water, however, is a great danger for architecture. It soaks the stones, dissolves the mortar joints, which risks dissociating elements of the structure of the cathedral and threatening its equilibrium.

This diagnosis, as well as the work of the researchers, will be valuable to guide the choices of the future reconstruction of this jewel of Gothic architecture.

Read also:

"The Voice of Notre Dame de Paris"

"Recreate the acoustics of Notre Dame de Paris"


1 Shared research structure: Sorbonne University, CNRS, Ministry of Culture

2 Co-supervised by Sorbonne University, CNRS

3 University College London

Credits of the photo on the homepage: Steeve Leulier