♪ ♪ NARRATOR: Notre-Dame de Paris.

A treasured icon of Gothic architecture and medieval construction.

ELSIE OWUSU: The level of determination and the feat of engineering in those days, just extraordinary.

NARRATOR: But on April 15, 2019, disaster strikes.

A huge fire rips through the cathedral... (people exclaiming) NARRATOR: ...threatening to bring down the entire structure.

(revving) ♪ ♪ Now, master craftspeople and engineers battle to bring Notre-Dame back to life.

PHILIPPE VILLENEUVE (translated): From what we've done so far, it's clear to us that this cathedral will be extraordinary.

NARRATOR: Hundreds of workers are in an ambitious race to restore this medieval masterpiece in time for a grand reopening planned for 2024.

(speaking French): NARRATOR: Historians and scientists work together to analyze and reproduce Notre-Dame's architectural mysteries.

KARINE BOULANGER: It is a very unique opportunity.

It's a kind of a walking backwards in time.

♪ ♪ NARRATOR: They're revealing ancient technology hidden for centuries.

MAXIME L'HÉRITIER: We're dealing with unknown structures that are, so far, unique in Gothic architecture.

NARRATOR: And struggling to save the fragile structure of the building.

JEAN-DIDIER MERTZ: The loss of matter is catastrophic for us.

NARRATOR: Now, three years into this extraordinary five-year restoration project, can this team meet its ambitious deadline?

"Rebuilding Notre-Dame," right now, on "NOVA."

♪ ♪ ♪ ♪ NARRATOR: Paris.

A city of churches, basilicas, and almost 2,000 historic monuments.

On April 15, 2019, the 850-year-old Cathedral of Notre-Dame de Paris is undergoing a six-and-a-half-million-dollar renovation of the spire... ...when a fire breaks out inside the oak framework of the roof.

After 90 minutes, the 400-ton timber and lead spire gives way and crashes through the stone vaulting.

The fire destroys the roof and spire, and leaves three gaping holes in the vaulting.

The structure is significantly weakened, and there's an urgent concern.

If the remaining vaulting were to fall, it could trigger a catastrophic collapse.

PASCAL PRUNET (translated): We didn't know how the vaults, the walls, the buttresses would behave in the absence of the roof that had collapsed above.

So, we had to stabilize the structure.

NARRATOR: In the immediate aftermath, the focus is on protecting what has survived from further damage.

Engineers race to install supports beneath Notre-Dame's flying buttresses to shore up the fragile structure.

Meanwhile, water has saturated the vaults, adding weight to the weakened stonework, increasing the chance of a total collapse.

And without the roof, the building remains open to the elements.

Before they can make the structure watertight, workers have to remove 40,000 burned and melted scaffolding poles left over from the spire restoration.

To keep the stonework dry, the team builds a wooden platform.

On top, a lightweight aluminum frame covered with water-resistant sheeting forms a temporary roof that opens and closes on a system of rollers.

PRUNET (translated): It's an ingenious tool that protects against humidity and shelters these vaults, so they can dry.

NARRATOR: The rolling roof enables workers to lower construction materials through the central hole in the vaulting.

During all this work, the medieval masterpiece has been closed to worshippers and tourists alike.

WOMAN: Pretty sad to see it like this.

You can't get close to the building at all.

It's almost like you would walk by it and not necessarily go to it as a destination, like it once was.

NARRATOR: And what a destination it was.

Around 13 million people toured Notre-Dame each year before the fire.

They came to marvel at a building that pushed the limits of Gothic architecture.

WOMAN: I was very connected to it.

So I was quite broken after, you know, hearing about the fire.

NARRATOR: We still don't know how the fire started.

But three years later, the ambitious project to restore the cathedral has ramped up.

MAN (speaking French): MAN 2 (on radio): (speaking French) ♪ ♪ NARRATOR: The team here has more than tripled in size.

As many as 200 workers are now battling to bring Notre-Dame back to life.

VILLENEUVE (translated): We all really work with a lot of love and gratitude, and we motivate each other to meet the deadline and reopen the cathedral to the public.

NARRATOR: The French Senate has ruled Notre-Dame will be rebuilt exactly as it was before the fire.

Donors have contributed almost $900 million towards this effort.

But the task ahead is enormous.

The team must remove tons of toxic lead dust that remains of the old lead roof left over from the fire, then clean and restore fragile stained-glass windows.

To reconstruct the roof identically, they must fill the three gaping holes in the stone vaulting, rebuild the timber framework from almost 1,000 oak beams, cover it with thousands of square feet of lead tiles, and raise the iconic 210-foot spire.

It's the largest restoration project in the cathedral's history.

VILLENEUVE (translated): I only have one goal-- repair and rebuild the cathedral, put the rooster on top of the spire, and say, "Mission accomplished."

NARRATOR: That's the plan.

But before they can even start work on this ambitious scheme, they must shore up the remaining damaged roof.

For hundreds of years, the arched stone vaults, made from thousands of cubic feet of interlocking limestone blocks, supported the roof above.

But now the vaulting is extremely fragile.

Repairing the arches could trigger a collapse.

To stabilize the structure, the team must install 52 timber support arches beneath the vaulting so they can rebuild safely.

♪ ♪ But installing them more than a hundred feet above the ground is no small feat.

♪ ♪ To do it, scaffolders have built a giant steel structure inside to allow workers to access virtually every corner of the immense cathedral.

RÉGIS JAEGER: It's a big job for our workers because it's a lot of material and the access is difficult.

We have 1,000 tons of scaffold.

It's a lot of tubes, big and heavy.

It's a big, big challenge.

OWUSU: An extraordinary amount of expertise has gone into creating a skeleton to sit inside the building so that the outer building can be rescued.

NARRATOR: It takes around 20 weeks to build up the scaffolding inside the cathedral in order to begin the installation of wooden supports, shaped to match the geometry of the vaults.

YVES MACEL (translated): This is where you need to be careful not to hit the scaffolding or hit the vault.

The challenge is to get everything inside without damaging anything, without putting people in danger.

You have to be very, very careful.

NARRATOR: Each temporary support arch is fitted with metal brackets to secure it in place.

Hydraulic jacks raise it to meet the underside of the vaulting.

MACEL (translated): Each support is unique and has its own specific location.

(drill whirring) NARRATOR: The arches connect to the rest of the supporting framework.

(translated): If the stones were to move, they would come to rest on our supports.

Our supports will stay there until the vaults are repaired.

♪ ♪ OWUSU: It's almost as if you have to design a building to keep the building safe.

What you would normally define as a temporary works has taken a level of ingenuity and a level of skill which is quite exceptional.

NARRATOR: It takes six months to install all 52 vaulting support arches.

(translated): The structure is no longer at risk of collapsing after it was weakened by the fire.

NARRATOR: Deputy director of operations Jonathan Truillet helps coordinate the work to bring Notre-Dame back from the brink.

♪ ♪ TRUILLET (translated): It's a technical and logistical challenge.

And if we don't respect our deadlines, we'll accumulate delays and never catch up.

NARRATOR: The deadline of 2024 for completion isn't just a hopeful wish.

This is when all eyes will be on Paris for the Summer Olympics.

A grand reopening of Notre-Dame will crown a year of celebrations.

TRUILLET (translated): Everyone must mobilize to reach this objective.

Even if it is ambitious, it is achievable, if we have the drive to accomplish it.

♪ ♪ NARRATOR: Fewer than three years remain to meet the deadline.

With the vaulting to rebuild, the roof and spire missing, and the site still contaminated by lead dust from the fire, the task is daunting.

TRUILLET (translated): We must work on multiple tasks at the same time, have several worksites within the worksite.

We must intervene both inside, to clean the site, and at the same time, above our heads to rebuild the vaulting and the roof.

NARRATOR: Above the stone vaulting, Notre-Dame's roof was completely destroyed by the fire.

Now one of the most complex challenges is to entirely rebuild the thousand-ton timber and lead roof.

Chief architect Rémi Fromont will oversee the reconstruction of the medieval roof structure, known as the forest.

FROMONT (translated): We're going to participate in the reconstruction of this absolutely emblematic, absolutely unique, absolutely magnificent, and absolutely iconic work, so it's very exciting.

It was one of the first great Gothic frameworks, extremely well-designed and ahead of its time, from a technical point of view.

NARRATOR: During a research project in 2014, Rémi and his colleague measured the dimensions of every beam in the forest to create the first comprehensive survey of Notre-Dame's roof.

FROMONT (translated): Our surveys were very useful.

It's largely thanks to them that we're able to restore the roof of the cathedral identically.

NARRATOR: Rémi's team will need 850 oak trees to reproduce the Gothic roof trusses.

The cathedral's spire will be built from another 1,200 trees.

FROMONT (translated): The spire is a huge task to understand and restore.

It's quite dizzying to think that we're building a 210-foot-high wooden structure perched 115 feet above the ground on 13th-century masonry destroyed by fire.

We're not taking the easy way out, really not.

♪ ♪ NARRATOR: The spire was a 400-ton engineering masterpiece.

Hidden beneath the 16 copper statues and over 100 tons of lead tiles was a complex skeleton of oak beams, some as long as 65 feet.

The secret of its strength?

A dense lattice of oak tied into the rest of the roof supports the entire structure.

OWUSU: The spire came to be the embodiment of the building and of the Paris skyline.

Which took huge imagination and levels of engineering and creativity and architecture which is quite, quite exceptional.

NARRATOR: Reconstructing this wooden wonder to match the lost spire is no simple task.

FROMONT (translated): We'll rebuild identically, not because it's nice to build in a 13th-century style, but because we need the new roof to behave just like the old one.

Do it differently and the structure will behave differently.

The timber that was used originally was green, that's to say it was not dry.

So we'll use green timber.

NARRATOR: In the 13th century, hewing beams from hard, seasoned oak with simple hand tools was arduous work.

So carpenters cut their timber while it was still soft and green.

But building this way could be risky.

The medieval English town of Chesterfield may bear witness to the perils of building with green timber.

It's thought the 660-year-old crooked spire of the parish church could be due to beams that have warped as they've dried.

For Notre-Dame's spire, this precarious lean must be avoided at all costs.

FROMONT (translated): We need top-quality timber, perfectly straight, to avoid this kind of problem.

Working with green wood requires an extremely rigorous choice of tree.

NARRATOR: In public and private forests across France, the hunt for 2,000 perfect oaks for Notre-Dame's roof and spire begins.

One third of the country, 65,000 square miles, is covered by forests.

(translated): We're going to choose the trees.

NARRATOR: This crew of forestiers has their work cut out for them.

They must source 60 flawless oaks for the spire from an 8,000-acre forest.

(translated): We must inspect the tree from all angles, otherwise you'll never know if there is a defect.

NARRATOR: The oaks felled for Notre-Dame form part of France's annual forest management quota.

LOÏC EON (translated): That way.

But now we need to get a bit closer, because something is not quite right.

There's damage 20 feet up, so we can't choose this one.

It won't meet the specifications given for Notre-Dame's spire.

So, we need to look for another tree.

MAN (speaking French): NARRATOR: This oak was damaged as it grew.

These twisted fibers make it too weak for Notre-Dame's spire.

But on the other side of the clearing, another candidate emerges.

EON (translated): No damage, but we need to check the diameter.

We have a tree two feet in diameter.

AHMET CIRPAN (translated): In terms of felling it, there's no problem.

(revving) NARRATOR: Logger Ahmet Cirpan begins by making a cut that will direct the tree to fall into the clearing.

(translated): It will go between those two trees, here, in this direction.

I don't cut with my chainsaw at random.

Okay, now we can start the final cut.

(saw buzzing) ♪ ♪ Yes, that's why we can't have anyone getting in the way.

NARRATOR: Notre-Dame's medieval carpenters used markings to help them reassemble the beams correctly up on the roof.

♪ ♪ Today, this team attaches a barcode to each oak destined for the cathedral, so they can track it from the forest to its final position in the new spire.

It takes several months to complete the painstaking search to source and fell their target of 60 trees.

FROMONT (translated): We're going to restore and rebuild the missing parts, and that's something unique in a career.

We'll do it with all our heart, passion, and above all, know-how.

NARRATOR: Oak will form the backbone of the spire, but it will be wrapped in a heavy metal-- lead.

Although the spire was almost completely incinerated, its pinnacle survived the inferno.

Lodged in the vaulting stone, the team gently nudges it free... WORKERS (speaking French): NARRATOR: ...and carefully winches it down.

L'HÉRITIER: We can still see the structure of the spire was made, with this fine lead sheets of a few millimeters thick that were used on the entire structure of the spire.

NARRATOR: They find six decorative lead roses attached to the spire section.

We will be able to study how this decoration was made.

Touching the spire that was just taken down from the vaults today, it's a magical moment.

NARRATOR: And there are more surprises to come.

Lead was decorative and kept the cathedral watertight.

But there's another metal used here that allowed Notre-Dame's masons to push the limits of Gothic stonework.

♪ ♪ Innovations such as flying buttresses to hold up the thin outer walls allowed medieval masons to build incredibly high, without needing massively thick walls.

As the team examines the structure closely, they discover metal throughout the cathedral that could unlock more of its architectural mysteries, from the nails that join timber beams to iron bars that brace and hold secure the stunning medieval stained glass.

L'HÉRITIER: I was amazed that there's so many iron in, in this building that was never truly studied before.

The staples that we see here, they're embedded in the, maybe the oldest part of Notre-Dame's masonry.

NARRATOR: These 18-inch-long iron "staples" secure the great arches beneath and prevent the stone blocks from collapsing under the enormous weight.

65 feet above, along the very top of Notre-Dame's walls, the destruction of the roof has revealed previously concealed ironwork that may have made the structure's height and slender form possible.

It's really exciting, because we're dealing with unknown structures on the top of the walls that are so far unique in Gothic architecture.

NARRATOR: Medieval builders may have worried that the top of Notre-Dame's tall, slender walls could be an Achilles' heel.

The weight of the roof could push the stones apart.

The destruction of the roof has revealed the builders joined these stones together with more than 500 staples, creating a ring of iron holding the walls together.

This engineering masterstroke has remained hidden under the roof of Notre-Dame for hundreds of years.

L'HÉRITIER: The staples, with the flying buttresses, are two ways of preventing the stones to, to collapse.

It's an ancient form, a form which is known since antiquity.

♪ ♪ NARRATOR: Ancient engineers used iron staples to lock the stones of giant megastructures into place.

These holes in the walls of Rome's Colosseum were once filled with iron staples that pinned the structure together.

But in medieval Paris, masons used this technology to revolutionize architecture.

L'HÉRITIER: It looks like, in Notre-Dame, we're trying to use ancient forms of reinforcement, such as the staple, in order to build a new form of architecture; really high, really thin Gothic structures, of which Notre-Dame is kind of the first true example.

♪ ♪ NARRATOR: At his lab, Maxime unlocks the secrets of each individual iron staple.

L'HÉRITIER: Like, we're acting as some kind of detectives, trying to find out the digital prints, the digital signature, of each of these staples and to try to rebuild their path from the workshop to the building site.

NARRATOR: Radiocarbon dating of organic material left over from the smelting process confirms the staples were installed in the early 13th century, when this part of the cathedral was built.

These are the oldest pieces of iron used in a Gothic church that we know of so far.

That's a huge discovery.

This is revolution in Gothic architecture.

No other Gothic monument had used iron in such a way before Notre-Dame.

NARRATOR: Maxime examines the microstructure of the iron.

Each staple was produced by welding together multiple pieces, suggesting that this iron was recycled.

L'HÉRITIER: The weld is the result of the mixing of scrap iron to make a brand-new iron staple.

We're maybe dealing with the richest building site at that time, and knowing that it might have used almost 90% recycled iron opens new perspective.

NARRATOR: The research shows recycling iron may have been common on the building site of Notre-Dame, shedding new light on medieval building practices.

The lab's electron microscope reveals further clues to the lengths that Notre-Dame's builders went to in sourcing the material.

L'HÉRITIER: What we discovered is that every single staple has a different chemical signature.

All these staples, they come from different iron that was made in different places.

It means that there's a truly active iron market in Paris, gathering iron from many, many different origins.

NARRATOR: These hidden iron staples may have also played a critical role in saving the building in the wake of the fire.

The staples were placed by the medieval master mason to reinforce the upper main walls.

They might have helped the walls to prevent collapsing during the fire.

OWUSU: As a conservationist, it's teaching us how expert these builders were in those days.

It's a testament to their technical competence and their vision that they put in these structural elements which have preserved the building for us.

NARRATOR: But Notre-Dame's marvels go beyond the walls and roof.

The great cathedral's medieval builders also pushed the limits of what could be made with glass.

The three rose windows date from the 13th century, and together, they're made up of over 1,100 panels.

Protected by the stone vaulting, they survived the fire unscathed.

♪ ♪ These kaleidoscopic wonders are filled with depictions of biblical scenes and saints.

The scaffolding gives access to these rose windows so experts can decode their secrets.

BOULANGER: It is a very unique opportunity, because we won't see them again in the same way, never.

(in French): CLAUDINE LOISEL: NARRATOR: Glass scientist Claudine Loisel and historians Karine Boulanger and Elisabeth Pillet are working on the largest window in Notre-Dame.

The gigantic south rose window measures almost 42 feet in diameter.

They're busy mapping every shard of glass.

PILLET (translated): It's a really big job.

In fact, I think at the beginning, when we looked at this rose window, we had no idea of all the questions it would raise.

NARRATOR: They hope to build a complete picture of how the window has evolved over centuries of restoration.

The team must first identify what is original 13th-century glass and what is glass from subsequent restorations.

BOULANGER (in French): (in English): You see, there is a difference between this yellow and this one.

This one is more translucent.

It's 19th-century glass.

This one is 13th-century glass.

The difference in colors results from the composition of the glass, which was different between the medieval time and 19th century.

NARRATOR: But there's a problem baked into the original medieval glass: it's slowly decaying.

LOISEL: On the older glass, you observe much more corrosion process in the glass composition.

This glass composition was more sensitive to the environment.

NARRATOR: In the 13th century, glassmakers used potash.

Made from burnt wood and ferns rich in potassium, potash reduced the melting point of the ingredients used to make glass.

By the 19th century, sodium carbonate combined with calcium oxide was used instead, and produced more stable glass that did not corrode.

This factory on the banks of the Loire River in France is one of the last places in the world that can produce stained glass using medieval mouth-blowing techniques.

SIMON BALLAGH: We produce glass for major historical buildings as Versailles or the White House.

NARRATOR: The team starts by mixing sand, metal oxide for color, sodium carbonate, and calcium.

They heat the mixture to over 2,000 degrees Fahrenheit and build up layers of the molten glass on the end of a blowpipe.

The glassblower forms a sphere from the red-hot mass, rolling it to maintain this shape, which is critical to form an even thickness of glass.

It's manual know-how.

There is absolutely no machines, and the glassblowers uses their sense, their feeling, to blow one glass sheet.

HERVÉ GRIMAL (translated): Glass is a living material.

It takes a long time to get to know it, to feel the material at your fingertips.

NARRATOR: The glassblowers enlist the help of gravity.

They swing the 15-pound ball of glass in a 13-foot-deep pit to elongate the ball into a tube.

GRIMAL (translated): It's a profession where there's weight and there's heat.

So you have to be strong, tough at times.

It's a very demanding job.

♪ ♪ NARRATOR: Hervé has blown glass here for more than 33 years.

GRIMAL (translated): For us, it's about always having the right length, the right diameter, and the right thickness of the glass, too.

NARRATOR: Once the cylinder has cooled, they make a single cut... ...and send it to a special furnace, where it's unrolled.

Extreme heat of nearly 1,400 degrees Fahrenheit and a wood block smooth out the glass and minimize imperfections.

GRIMAL (translated): The goal is to try to get a very even thickness, to achieve the perfect sheet.

NARRATOR: They carefully inspect each pane and remove any rough edges.

BALLAGH: Every glass sheet is different, and it has the spirit of the glassblower.

Losing this patrimoine and know-how would be a disaster.

♪ ♪ NARRATOR: The factory marries these techniques with the latest technology to accurately reproduce stained glass.

BALLAGH: We can fit perfectly with the old colors by using tools like spectrophotometry, like X-rays.

And this allows us to know exactly what are the elements that are in the glass and reproduce it for the future.

GRIMAL (translated): We'll be making glass for the restoration of Notre-Dame in Paris, which will be a high point in my career.

Well, it will make for a nice resumé!

NARRATOR: The team's knowledge of historic techniques, combined with modern technology, enables them to reproduce any of Notre-Dame's stained glass from any century.

BOULANGER: A stained-glass window is always a mixture of original glass and restoration from every century, almost.

Until the 20th century, when a glass was too badly damaged, we had to replace it.

NARRATOR: While mapping the south rose window, they uncover an unusual trend.

They expect to see glass from multiple restorations spanning eight centuries.

But they're finding original 13th-century glass, glass installed during the 19th century, with some panels containing both.

BOULANGER: We are finding lots of things.

They altered quite strongly the design of the panels.

NARRATOR: While the glass team has this unprecedented access, they must work fast to solve the mystery of why the south rose window only has 13th- and 19th-century glass.

♪ ♪ The clock is ticking for the architects and restorers.

To meet the challenge of reopening Notre-Dame in 2024, the workforce here has increased dramatically.

Up to 200 people pass through the site each day.

♪ ♪ But the lead dust that coats every surface makes operating here potentially dangerous.

Protective clothing is essential.

Blaise Gomis is part of a team dedicated to safeguarding workers from the deadly effects of lead poisoning.

Without them, this huge operation would grind to a halt.

GOMIS (translated): Between the polluted zone and the clean zone, there's us.

You have to go through us.

We take names so we have a count of the people on site.

Then, after they finish their job, when they leave, they must go past me again.

(in French): WOMAN: GOMIS: WOMAN: GOMIS (translated): Lead, as you know, is harmful.

So they must be equipped.

We give them overalls, underwear, boots, and helmets.

(suit zipping) ♪ ♪ And when they leave, they take showers to make sure they eliminate as much lead as possible.

NARRATOR: To make the site safe, the team's next challenge is to remove all the toxic lead dust.

They tackle the cleaning zone by zone, eventually decontaminating the entire cathedral.

But this operation kicks dust into the air.

(vacuum whirring) Workers in an area being cleaned must wear heavy-duty breathing equipment.

Clara Dupuydauby is one of 40 decontamination experts that use special vacuums to meticulously clean every inch of Notre-Dame's vaulting, walls, pillars, and floors.

With this equipment, we only work two hours and a half at a time, and we need to stop to take a break.

We go have lunch.

And two hours and a half, and we go home.

NARRATOR: Vacuuming the lead dust will take eight months.

Then restorers can move on to deep-clean the stone for the first time in its history.

Inside Notre-Dame, it's already possible to get a sense of how dramatically changed the cathedral will be after the cleaning.

Beneath the lead and centuries of smoke from millions of candles lies gleaming limestone.

This is how the cathedral looked 850 years ago and will again soon.

VILLENEUVE (translated): Here are the stages.

PRUNET (translated): First step, second step...

Dirty, intermediate, and final.

That's great.

(drill whirring) NARRATOR: The restorers working at Notre-Dame are among France's foremost experts in their fields.

♪ ♪ Guiding this impressive concentration of medieval knowledge are chief architects Pascal Prunet and Philippe Villeneuve.

VILLENEUVE (speaking French): (translated): Let's go and meet the cleaning team.

(translated): The stone's changing color.

We're very satisfied with that.

But we know this is only the first stage of the cleaning.

(translated): But after, we'll be able to work without masks, so that's the goal.

VILLENEUVE (translated): When we look at this vaulting, it's clear to us that this cathedral will be extraordinary.

So it's all enormously energizing.

NARRATOR: But this team still has a big job ahead if they want to reopen the cathedral in 2024.

VILLENEUVE (translated): It's a completely crazy deadline.

But despite everything, we continue to work so it can be met.

♪ ♪ NARRATOR: To meet the deadline, the team cannot afford any unexpected delays.

But an insidious threat to Notre-Dame's stonework is developing.

With the loss of the roof and no protection from the elements for almost two years after the fire, the exposed stone vaulting was repeatedly soaked by rain.

♪ ♪ Now protected by the temporary roof, it's drying out.

But as the stones dry, salts are crystallizing on the underside of the vaulting, breaking off the outer layers of limestone.

All the damage in this part and on the, on the vault is a result of the salt.

You can see the loss of matter is two or three centimeters.

This is catastrophic for us.

NARRATOR: In the 18th and 19th centuries, restorers cast layers of plaster on top of the vaulting in case of a fire.

This protected the stonework from the heat of the 2019 inferno, but not from the water used to fight it.

This, along with months of rainwater, washed salt from the plaster into the lower layers of the porous limestone.

As the moisture evaporates, the salt crystalizes, and forces the limestone apart, destroying the inner surface of the vaulting.

We don't have the original surface of the, of the stone.

It is a real problem from an historical point of view.

NARRATOR: Geologist Véronique Vergès-Belmin will use a technique to draw the salt out from the stone.

We need to use a material that can absorb the salt and extract them through capillary forces.

NARRATOR: To extract the salt, Véronique's crew will coat the vaulting with a paste of clay, sand, and purified water known as a poultice.

The water from the poultice is drawn into the stone, where it dissolves the harmful salt crystals.

As the clay of the poultice dries, it draws the salt water out of the stone, saving the vaulting from further damage.

The poultices will be removed when they will all have dried.

(tool whirring) NARRATOR: The restorers chisel away the outer layer of mortar between the stones to allow the poultice better penetration.

(machinery running) Next, they load the sticky mixture into a compressed air gun and spray it into every crevice.

Finally, they carefully smooth the poultice across the areas affected by the salt damage.

VERGÈS-BELMIN: You can see that it follows very, very closely the surface of the stone.

And what we recommend is that poultice should not be thicker than half a centimeter to one centimeter.

Otherwise, there are risk that it falls down.

We need to have a very slow process until the vaults are dry.

But this will take time, much time.

We have to get the cathedral ready in 2024.

This building has to live again.

NARRATOR: The poultice may stay in place until the new roof is built and the vaulting is permanently protected from rain and snow.

Notre-Dame was built over the course of a hundred years, section by section, during the 12th and 13th centuries.

As each new segment of the cathedral was constructed, another section of timber roof, known as the forest, was built to cover it.

Hand axes were used to craft each individual beam in the medieval roof structure.

The fire, which started in the forest, took just hours to reduce this medieval masterpiece to ashes.

The team will soon begin an unprecedented challenge to rebuild the forest in under two years.

FROMONT (translated): We're going to reuse techniques that are extremely similar to what was used, because it's technically necessary, and if we don't do that, the wood will behave differently.

NARRATOR: The spire lost in the fire was erected in the 19th century.

These beams were cut with saws.

So today, Rémi's carpenters will use modern saws to cut the new spire beams.

FROMONT (translated): We have extremely rigorous rules that are the highest that can be had in carpentry, because Notre-Dame is absolutely exceptional in terms of wood quality.

NARRATOR: This sawmill in Normandy processes over a million cubic feet of timber each year.

It's one of 45 sawmills across France that has answered the call to cut beams for Notre-Dame's spire.

(translated): For us, it's a chance to prove our dynamism and show that French forests can help rebuild this structure-- one of the jewels of France.

I do this as a form of philanthropy-- it's for free.

NARRATOR: The team wastes no time in getting to work on the beams.

First stop: the debarker.

♪ ♪ This machine strips off the outer layers, removing the loose bark.

Now the one-ton tree trunk enters the sawmill and rolls onto the saw carriage.

FEILLET (translated): On this joystick, we have buttons that allow you to do operations like log loading or what is called slabber chipping.

The slabber is the first machine before the blade, which shreds away the outer part of the log.

NARRATOR: The bandsaw blade is a high-speed loop of steel that runs at 140 feet per second.

François removes inch-thick slices to trim the beam down to the dimensions requested by the Notre-Dame architects.

♪ ♪ FEILLET (translated): What I enjoy most is discovering the wood.

Each tree is unique.

Ultimately, it's a game of strategy you play with each tree.

It's never the same thing-- it's something new every time.

♪ ♪ NARRATOR: The beams from François's sawmill are stacked, ready to join more than a thousand others coming from across France for Notre-Dame's new spire.

(translated): A sawmill like ours, we will not do anything like this again in our lives.

We'll go down in history for having modestly contributed, like everyone working on Notre-Dame, to restore this cathedral-- our cathedral.

♪ ♪ NARRATOR: Inside Notre-Dame, the first chance to get up close to the gigantic south rose window in 160 years... (speaking French): NARRATOR: ...has revealed a mystery.

We only have 13th-century glass and 19th-century glass.

In the 19th century, they removed everything that wasn't 13th century, and then they had to do new panels if a panel was missing.

♪ ♪ NARRATOR: The chief suspect for these radical changes to the window is architect Eugène Viollet-le-Duc.

In the 1840s, he was tasked with breathing new life into Notre-Dame.

At the time, it was not the beloved building we know today.

The cathedral was ransacked during the French Revolution, statues of biblical kings on the façade were decapitated, and it was used as a warehouse, sitting derelict and unloved for decades.

Over the course of 20 years, le-Duc reinstated the statues of the façade, rebuilt the sacristy, designed hundreds of new gargoyles, and raised the 210-foot spire.

OWUSU: Viollet-le-Duc, God bless him, would have been what we consider to be a starchitect, you know?

He was a man who knew his mind, he was a man who was highly respected, really determined, and saw himself as a powerful leader.

NARRATOR: And he used his power to make some puzzling changes.

During his restoration, le-Duc removed all glass in the south rose window that was not original and replaced it with modern glass.

Elisabeth is also finding that he made significant alterations to some of the original glass panels.

(translated): Look at this little martyr-- it looks like she was cut.

She's missing the colored lines around the edge.

The halo is slightly cut here, and her feet are cut, too.

Maybe she has been moved within this window.

(speaking French) NARRATOR: Why did le-Duc alter the window?

Was he trying to impose his own design?

♪ ♪ The glass experts hunt for clues here, at the Paris Médiathèque of Architecture and Heritage.

They hope le-Duc's plans for the south rose window shed light on his thinking.

BOULANGER (speaking French): NARRATOR: As they dig into the archives, they make a breakthrough.

BOULANGER: We just discovered that there was a change of structure.

Before Viollet-le-Duc, there was an iron reinforcement in the center of the rose, but obviously it wasn't enough.

So Viollet-le-Duc put it further away from the center.

And when you removed the ironwork here, he had to change the form of the panels.

NARRATOR: Le-Duc's restoration was sweeping.

♪ ♪ He removed a smaller structural ring of iron and replaced it with a bigger ring to strengthen the core of the window.

He removed all traces of previous restorations to replace them with panels of new glass.

And he rotated the whole window 15 degrees to make it structurally stronger.

The problem must have been that the medieval rose wasn't stable in its axis.

That must have been the problem.

That's very interesting.

♪ ♪ NARRATOR: Le-Duc's major changes to the south rose were motivated by more than mere aesthetics.

Strengthening the window has helped preserve this masterpiece.

We don't come across this kind of new information every day.

♪ ♪ NARRATOR: Thanks to the scaffolding, these historians and scientists are painting an intimate portrait of how the south rose window, one of the wonders of this cathedral, evolved to survive.

♪ ♪ Three years into the ambitious five-year restoration project, the team at Notre-Dame has already stabilized the structure, the process to free the cathedral from the toxic lead is underway, and two "test chapels" have been fully restored.

MARIE PARANT (translated): We are very surprised by the quality of the materials.

They used beautiful pigments, gold of very good quality-- a very beautiful technique.

NARRATOR: Here, mural painting and stonework restoration techniques have been tested.

They will be replicated throughout the rest of the building.

PARANT (translated): In the long term, it's to optimize the restoration of all the other chapels.

We're very happy.

We feel that we've played our part.

It's really the first step towards the complete restoration of Notre-Dame.

♪ ♪ NARRATOR: But there's still a long way to go.

Rebuilding the vaulting, the roof, and the spire identically will be a monumental task.

(translated): It's extremely ambitious work, considering the schedule.

(translated): We have a huge responsibility to the generations of today and the future.

♪ ♪ NARRATOR: Meanwhile, historians and scientists are rewriting our understanding of the very fabric of this medieval wonder.

♪ ♪ It opens new perspective.

That's a huge opportunity.

OWUSU: Tragic as the fire was, I think it took something like that to make us understand just what an absolutely amazing work of collaborative genius that building was.

WOMAN: Notre-Dame is Notre-Dame.

(chuckling): We definitely need it back.

♪ ♪ NARRATOR: The last chapter of this extraordinary endeavor has begun: to return Notre-Dame to France and the world.

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