Rebuilding Notre Dame, Painstaking Battle Between New and Old

This spring will mark the 3-year anniversary of the devastating fire that destroyed the roof of the Notre Dame Cathedral in Paris. A day after the inferno, President Emmanuel Macron announced to the world that within 5 years France would “rebuild Notre Dame Cathedral, more beautiful than ever”. Since then, the ambitious project has run into a series of setbacks and unforeseen sidetracks. In addition to raising the money needed to rebuild, organizers have employed hundreds of scientists, historical experts, and tradesmen to secure the site and put a restoration plan in place. In December, France’s National Heritage and Architecture Commission gave the green light for most of the proposed renovations. Now the process of recreating a new Notre Dame, which imitates the old, is in full swing, with a goal of opening to the public on April 16, 2024.

Witnessing a Cultural Disaster

On the evening of April 15, 2019, hundreds of thousands of people around the globe began streaming live footage of smoke billowing above the roof of Notre Dame. Shortly before sunset, flames could be seen breaching the rooftop’s surface. Within an hour, they had engulfed the cathedral’s ornate spire and brought it crashing down, provoking a collective worldwide gasp of horror. Unbeknownst to onlookers, the seemingly delicate steeple was covered with 250 tons of lead. When it collapsed, it left a gaping hole in Notre Dame’s roof, through which tumbled hundreds more tons of stone, lead, and metal scaffolding.

Securing a Hazardous Work Site

In the days following the collapse, inspectors began the months-long process of determining how best to prepare the site for the extensive renovations to come. Complicating their task were two extenuating factors. First, the remaining structure was tremendously weakened and in need of immediate reinforcement. Second, most of the lead had been vaporized by the heat of the flames. Experts estimate that fire temperatures reached 800-900 degrees C (1500-1650 degrees F), lifting tons of lead particles into the atmosphere. Much of the airborne lead drifted away from the cathedral, eventually settling on the streets of Paris and beyond. But there were also high levels of toxic lead dust covering all parts of the cathedral and debris.

Notre Dame’s massive pillars of stone give it the appearance of strength but the structure is actually quite fragile. Engineers and architects have never fully understood how masons of the 12th and 13th centuries managed to build an edifice that’s height well-exceeds those of its contemporaries. The vaults of Notre Dame’s ceiling are relatively thin, ranging from 20 to 30 centimeters thick. The extreme heat of the fire removed an additional 1-2 cm, warped many of the stones, and re-opened cracks that had been repaired in the 19th century. With its roof in tatters, the structural integrity of Notre Dame’s walls was imperiled—much of the building in danger of collapse.

Before any significant work could begin, engineers needed to construct massive wooden supports to secure the flying buttresses. These graceful arches on the exterior of the building prevent the walls from collapsing outward by counterbalancing the weight of the heavy vaulted ceiling. With the ceiling in ruins, however, the weight of the buttresses threatened to push the walls inward. While the buttresses may have a uniform appearance to the naked eye, they had warped in varied and inconsistent ways over the centuries. Hence, each flying buttress had to be closely studied to account for myriad eccentricities before building the wooden brace that would prevent its collapse.

Another extremely precarious job was installing a tarpaulin over the roof to protect the interior from the elements. Thankfully, none of the stained glass in the building was damaged. However, certain windows were completely dismantled and will be reconstructed at a later date. If left in place, their weight and location could complicate or endanger ongoing work.

Ironically, one of the hardest tasks was disassembling the steel scaffolding which had been installed to restore the spire prior to the fire. Some pieces lay in a heap within the cathedral. However, the portions that remained standing were twisted, unstable, and fused together. As such, the scaffolding was no longer simple to dismantle. Motion sensors were installed throughout the tangled lattice. These triggered regular false alarms, forcing evacuations of the building until safe conditions were re-established and workers could resume.

Scientific Inquiries

After months of stabilizing the basic structure of the cathedral, clean up crews began working inside. A massive triage was underway. Archeological remains were carefully excavated from the rubble, catalogued, and shelved beneath large tents for further study. As a result, scientists and engineers are learning more about building techniques used by their 12th and 13th-century predessors. These discoveries not only inform history, but also influence the methods that will be employed for Notre Dame’s reconstruction.

One unexpected finding was the use of massive iron staples. Concealed behind the stonework’s facade, these staples connected stones in the vault of the cathedral, reinforcing and strengthening the roof. Longer bands of iron were discovered at the tops of the cathedral’s walls and elsewhere. This revelation provoked a detailed scan of other structures. Using sophisticated metal detectors, scientists gradually identified an elaborate skeleton of iron buried within the walls and columns of Notre Dame.

Another area under intense scrutiny has been the composition of various mortars used throughout the building. Scientists have lifted samples from different types of joints, different types of connected surfaces, and different centuries. The composition of each mortar sample provides clues about the origins of the materials that went into mixing it. Architectural historians are interested in determining whether different mortars were intentionally chosen to fulfill specific roles. The recipes, once revealed, are recreated in a lab and studied for strength, drying, and hardening properties. When portions of the vault (to name one example) are rebuilt, masons will use mortars that closely resemble the original mixes.

Where possible, archeologists have tried to puzzle various blocks of rubble back into their original positions relative to each other. They study never-before-seen surfaces in an effort to gather clues about the tools and techniques used by 12th and 13th-century craftsmen to cut and position each block. Such research will aid the restoration team in rebuilding the cathedral while following the original construction techniques as closely as feasible.

All Systems Go

Fortunately, France has received more than adequate funding to support this precarious and painstaking work. After the fire, donations flooded in from around the world, raising roughly 850 million euros—1 million euros for each year that the cathedral has been around. This sum is more than double the annual budget of the French Ministry of Culture. Less than one year after the project got underway, however, work ground to a halt as the coronavirus led France into a lockdown.

Despite the setbacks, Macron has held fast to his original goal of reopening in 5 years. Recognizing international interest in the project, the French president has called upon skilled tradesmen and technicians from around the world to help with Notre Dame’s reconstruction. The response has been positive, from professional mountain climbers that now swing from ropes to clean soot-covered stonework, to the American company Autodesk, selected to provide 3D modeling for the exterior landscape.

Until recently, the work has focused on securing the site, installing new supports and scaffolding, and clearing away debris. Now, however, with the approval of France’s National Heritage and Architecture Commission, renovations can proceed at full speed. Mass has been scheduled for April 16, 2024. Yet, experts and onlookers alike predict that restoration work will continue well beyond that date. One thing is clear, a small army of scientists, architects, archeologists, historians, technicians, tradesmen, artists, clergy, and others are committed to rebuilding Notre Dame in all of its former glory.

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Other Resources

• Le Monde, Les matériaux de Notre-Dame de Paris livrent leurs secrets
• Le Monde, Les travaux de sécurisation de Notre-Dame de Paris achevés, la phase de restauration peut commencer
• France24, Inside Notre-Dame: The challenges of restoring iconic cathedral
• Science, Saving Grace
• PBS Nova, Saving Notre Dame
• Friends of Notre Dame, NOTRE-DAME CATHEDRAL’S INTERIOR RENOVATIONS
• Smithsonian, France Approves Controversial Plan to Renovate Notre-Dame Cathedral
• BIM+, Paris turns to Autodesk for Notre-Dame design competition