As flames rose from the timber roof of the historic Notre Dame de Paris, the world watched the 850-year-old landmark’s tallest spire and roof succumb to the heat. Although citywide fires are mostly a thing of the past, these ancient structures standing among us through the centuries, remain vulnerable—with Notre Dame standing as an extraordinarily tragic example.
Before Monday's devastating fire, Notre Dame wasn't in the best of shape. For decades, the cathedral showed many signs of aging through cracking, and the fire yesterday likely started because of a $6.8 million restoration project.
Now, Notre Dame will need much more in order to return to its former glory, but it will take more than money. Renovation crews will need vast of amounts of information—modern and historical—to accurately recreate the cathedral’s iconic Gothic architecture.
And at least in this one instance, Notre Dame finally catches a break.
A Cathedral Under Threat
It’s surprising at first glance that a structure like Notre Dame, with its large, imposing stone walls, could even catch fire—but it’s what’s on the inside that counts.
While a masonry vaulted building is inherently safer in a fire than an all-timber building, the wooden roof above the vaults is needed to make the structure waterproof, says John Ochsendorf, MIT professor in the departments of architecture and civic and environmental engineering and current director of the American Academy in Rome.
“Such roofs are typically framed in oak and covered with lead sheeting,” he says. “The largest timbers are more fire resistant because of their size, but the smaller timbers can burn more easily...pigeon droppings and construction debris that accumulated on the vaults over centuries can add fuel. Such a roof is extremely dry and is vulnerable to fire as a result.”
There's a lot at stake when it comes to rebuilding Notre Dame. The 850-year-old Gothic building's 295-foot spire dated back to 1345, and the cathedral remains one of the first—and finest—examples of the flying buttress architectural style. Because of this, researchers all over the world have studied, and most importantly, meticulously documented Notre Dame.
And none more so than the late Vassar College Art History professor Andrew Tallon, who tirelessly 3D laser scanned the entire Notre Dame cathedral and . Although Tallon passed away from brain cancer in 2018, his work wasn’t lost. Ochsendorf, who worked with Tallon on the project, says “such a 3D model can provide crucial information about the geometry and materials.”
Tallon’s laser scan process used a Leica ScanStation C10 laser beam mounted on a tripod to engulf an area with a scan, measuring distances for every surface the laser hit. That continuous mapping created a 3D image of the cathedral that Tallon once said was accurate to within five millimeters. Ochsendorf says Tallon was able to make those scans work visually by pairing them with a panoramic photograph from the same spot, creating a near-realistic image of the space.
Over five days in Notre Dame, Tallon repositioned the scanner roughly 50 times, capturing more than one billion points of data. Tallon then created an accurate recreation by stitching together the photographs over 3D modeling, serving as a high-resolution digital blueprint for the building.
Here's Tallon talking to National Geographic about his process in 2015:
Michael Davis, chair of architectural studies, professor of art history at Mount Holyoke College in Massachusetts, who spent time with Tallon during the process and has also done extensive surveying of historic structures, says the great virtue of laser scanning is it “creates or documents with great precision the building as it stands.”
“Laser scanning can measure places and surfaces with tremendous accuracy that you could never hope to get to in person, such as the curvature of a flying buttress,” Davis says. “I think the laser scanning offers a really useful document of the state of the structure. You can see how it is behaving, if it is out of plumb, if everything is where it should be.”
Davis says pairing that data with rigorous researching into the history of a building that took shape over multiple centuries can create give an accurate picture.
Finding the Medieval Materials
While having the exact measurements is a huge benefit for whoever takes on the project of rebuilding Notre Dame, there’s also one other conundrum seeing as modern France is not medieval France.
"It is not easy to procure oak timbers of the size needed. Centuries ago, large oak elements were harvested by royal carpenters from the king’s forests, were they could grow for centuries,” Ochsendorf says. "Today it is very difficult to find solid oak timbers of the dimensions that would be needed. These elements could be 2 feet by 2 feet in cross sections and could be 20 to 30 feet in length."
Another big question is deciding exactly what Notre Dame you are going to build?
"[Notre Dame] is a kind of beautiful Frankenstein of all these different parts, 12th, 13th, 14th and 19th century parts" Davis asks, "so is that the one we are going to restore or is this an opportunity to undo some of the restorations of the 19th Century where we think they got it wrong and rectify errors we have identified?"
The most recent example of a similar rebuild happened at the York Minster in England where a 1984 fire burned off much of the roof across the transverse space in the middle and along with wooden vaults.
“If you walk into the building today you couldn’t tell that anything has happened. They completely rebuilt the building,” Davis says. “Some of the structural details may look a little new and fresh for a medieval building.”
During that restoration effort, officials installed an elaborate fire-suppression system, adding in modern technical measures to an otherwise ancient structure, a feature Notre Dame renovation crews would be smart to install as well.
Notre Dame Lives
For Notre Dame, any estimation of a rebuild length must wait until the extent of the damage is known but the effort will most certainly be measured in years, if not decades. While the masonry of the vaults helped keep the building from collapse, it's unknown if the fire’s heat rendered stone in the cathedral’s upper reaches fatally compromised.
“When they get up there to the upper levels of the wall, is the stone just going to turn to powder?” Davis asks. “How much of the structure that looks today like it survived is actually going to be viable for reconstruction?”
Then comes the effort to find and match materials as closely as possible. Davis says that during the 19th century, officials worked through the archives of the historical monuments office to find detailed plans for locating original stone from Paris quarries to match the existing structure as best they could. While more than 150 years of modernity has changed Paris considerably, finding local stone is still worth the effort.
If renovation crews can find these chunks of Parisian stone and massive oaks, the work of Tallon could provide a digital blueprint for rebuilding the timber roof and soaring spire. The cathedral Tallon so admired for its unrivaled perfection could soon be saved by his own work.
But through the heartbreak of watching history burn—streamed to millions around the world—French President Emmanuel Macron painted a vision of hope, saying Notre Dame will rise again.
A bit of 21st-century laser scanning may just save the medieval heart of France.
You can donate to the restoration of the Notre Dame by visiting the , , or the .