Since then, work on the Gothic Episcopal church has been in full swing and apparently on schedule.

"We are on time and on budget," Philippe Jost, the man in charge of the reconstruction efforts, told a French Senate committee at the end of March. Jost took over after his predecessor, former general Jean-Louis Georgelin, died during a hike in August 2023.

The cathedral is officially scheduled to reopen on December 8, 2024.

Although it won't be ready in time for the summer Olympics in Paris, as was originally hoped, visitors to the French capital can once again see the imposing spire of Notre Dame following the recent removal of the surrounding scaffolding. The lead roof is also currently being installed.

Fire prevention measures, such as a sprinkler system and compartmentalized sections, are also part of the restoration efforts. Ongoing work includes finishing the electrical and heating systems and restoring the internal furnishings.

Short circuit or cigarette caused the fire?

Exactly five years have passed since the fire. The historic building was partially destroyed in the process. The Paris fire department fought for four hours before managing to confine the fire to the wooden roof structure. The western façade with the main towers, the walls of the nave, the buttresses and a large part of the vaulted ceiling, as well as the side aisles and choir stalls remained stable. The heat, smoke, soot and extinguishing water affected the church's furnishings, but here too there was no significant damage.

It is not yet known whether the fire was caused by a short circuit or a construction worker's cigarette.

The extent of the destruction was not as great as first feared. "Thank God not all the vaults collapsed," said German cathedral expert Barbara Schock-Werner in an interview with DW at the time. In the end, only three vaults fell.

There was a hole in the choir. The Gothic Madonna, however, remained intact, although the crossing tower crashed right next to it. "That's the miracle of Notre Dame," said Schock-Werner.

Window restoration in Cologne

The images of the burning cathedral went around the world. They aroused worldwide consternation and a wave of desire to help.

French donors alone pledged 850 million euros (915 million dollars). But the money and experience also came from Germany. Schock-Werner took over the coordination of German aid.

The Cologne Cathedral construction office restored four stained glass windows that had been severely damaged by flames and heat. The four stained glass windows with abstract shapes are the work of French glass painter Jacques Le Chevallier (1896-1987), done in the 1960s.

In the glass workshop at the Kölner Dombauhütte, they were first freed from the toxic lead dust in a decontamination chamber. The restorers then cleaned the glass, glued the cracks in the glass, soldered the fractures in the lead mesh, renewed the lead on the edges and resealed the outer sides of the window panels. The restored "Colony" windows were reinstalled in 2023.

Sensational discovery after the fire

As dramatic as the fire was, a discovery made by French researchers at the site of the fire was equally sensational: iron staples held the stones of the structure together. Dating and metallurgical analysis revealed that these iron reinforcements date back to the first phase of the church's construction in the 12th century. This may make Notre Dame the oldest building in the world with this type of iron reinforcement.

But more importantly, the mystery of why the nave was able to reach this height was solved.

When construction began in 1163, Notre Dame, with a nave over 32 meters high, quickly became the tallest building of the time - thanks to a combination of architectural refinements. The five-nave plan, the cross-ribbed vault with thin struts and the open buttress arches on the outside of the nave, which transferred the load of the structure from the walls, made the enormous height possible.

The later cathedrals received iron reinforcements in addition to the stone and wooden structures. This gave them stability.

Reconstruction in the old style

By a stroke of luck, the highlights of the cathedral - the statues of the 12 apostles and the four evangelists that architect Eugene Viollet-le-Duc grouped around the ridge tower he designed in the 19th century - survived the fire unscathed, because they had been removed from the roof shortly before to be restored.

To rebuild the medieval roof frame, 2,000 oak trees were felled. To turn the trunks into beams, the craftsmen used special axes with the façade of the cathedral engraved on the blade. These can be seen in a special exhibition at the Paris Museum of Architecture. The exhibition also describes the painstaking work that went into reinstalling stones and wood in their original places to make the reconstruction as faithful to the original as possible.

The reconstruction of Notre-Dame gave rise to an architectural debate, mainly concerning the burnt spire that marks the intersection between the transept and the nave.

Some proposed a modern reconstruction, with glass and steel, arguing that the defining tower was only designed in the 19th century. However, a commission of experts decided that the reconstruction would aim to rebuild the cathedral as similar as possible to the damaged original.