Original subtitles

NARRATOR: Notre-Dame de Paris,

the most famous cathedral in the world,

attracts 15 million visitors each year.

attracts 15 million visitors each year.

- It's one of the most beautiful Gothic cathedrals,

perhaps the best loved and best known.

NARRATOR: An icon of French culture, heritage and history,

NARRATOR: An icon of French culture, heritage and history,

this monumental masterpiece is testament to the skill

and ingenuity of its medieval builders.

and ingenuity of its medieval builders.

- Gothic architecture is dynamic, it's always on the move.

NARRATOR: A superstructure in stone, wood and glass,

NARRATOR: A superstructure in stone, wood and glass,

Notre Dame de Paris is notable for its finesse, height and grandeur.

- 13th century Notre Dame was a really advanced construction site.

- 13th century Notre Dame was a really advanced construction site.

The designers pushed stonework to its structural limits.

NARRATOR: Over 860 years,

this sacred building has endured many dangers.

this sacred building has endured many dangers.

But the greatest devastation it ever suffered

was the fire on the 15th of April 2019.

- When wood burns, it doesn't stop.

- The firemen saved Notre-Dame.

NARRATOR: We'll see how, after surviving a catastrophic fire,

NARRATOR: We'll see how, after surviving a catastrophic fire,

the cathedral has been rebuilt.

How it was designed by its medieval builders,

and how they created enormous rose windows in its walls

and how they created enormous rose windows in its walls

at such dizzying heights.

- Stone is a heavy material, so it needs to be in perfect balance.

- Stone is a heavy material, so it needs to be in perfect balance.

- It's extremely difficult, physical work, and all done by hand.

- We're seeing shifts in the balance of just a few millimetres,

- We're seeing shifts in the balance of just a few millimetres,

so it appears to be a perfect design.

NARRATOR: While the authorities restore Notre Dame,

NARRATOR: While the authorities restore Notre Dame,

we'll use the latest technologies of the architects

and scientists on site to x-ray this stone shell.

Discovering the secrets of its construction,

Discovering the secrets of its construction,

and revealing how the genius of the medieval builders

created a cathedral which was built to survive.

created a cathedral which was built to survive.

NARRATOR: Of all Paris' iconic monuments,

for nearly 900 years, one has held a special place.

for nearly 900 years, one has held a special place.

The Cathedral of Notre-Dame.

This immense stone monument is the setting for Victor Hugo's most

This immense stone monument is the setting for Victor Hugo's most

famous novel - the one with the hunchback - and of many great events

of French history, including Emperor Napoleon the First's coronation.

of French history, including Emperor Napoleon the First's coronation.

Philippe Villeneuve is one of the world's leading experts

on the cathedral,

and in charge of its post-fire restoration.

- A building like Notre Dame is surprising.

It's incredibly light, and very daring.

It's incredibly light, and very daring.

- You feel that this monument's always been there,

part of the identity of Paris.

But actually it's the result of a complex plan.

But actually it's the result of a complex plan.

NARRATOR: Notre-Dame : a majestic superstructure,

a masterpiece of Gothic architecture

created by builders of the Middle Ages

created by builders of the Middle Ages

in France, on the Ile de la Cite, in the centre of the capital.

The cathedral is 127 metres long and 48 metres wide,

facing east-west.

Work begins in 1163, with the eastern part, the chevet,

extended after 20 years by the transept,

extended after 20 years by the transept,

then by a large nave completed around 1220.

This complex, resembling a Latin cross,

is closed at its western edge by an immense facade,

is closed at its western edge by an immense facade,

embellished by two square towers rising to a height of 69 metres.

embellished by two square towers rising to a height of 69 metres.

A spire is raised above the roof.

After a century and a half of work, the cathedral is finally complete.

After a century and a half of work, the cathedral is finally complete.

- It's magnificent,

a feat of architecture by people who really knew how to build.

a feat of architecture by people who really knew how to build.

It's not the Renaissance image of men who didn't really know

what they were doing.

- It needed project managers who were already thinking big,

- It needed project managers who were already thinking big,

about the resources to be put in place and the skills they needed.

Otherwise, they'd never have been able to finish it.

Otherwise, they'd never have been able to finish it.

(indistinct chatter) (siren wailing)

NARRATOR: In April 2019,

NARRATOR: In April 2019,

this Grande Dame in stone, several hundred years old,

was disfigured in a terrible blaze.

was disfigured in a terrible blaze.

- (bystanders chattering)

NARRATOR: In the space of a few hours,

flames ravaged the wooden structure,

flames ravaged the wooden structure,

the roof and then the spire, which collapsed into the main nave.

This dramatic fire

plunged Notre-Dame into a critical condition.

plunged Notre-Dame into a critical condition.

- Immediately after the fire, it's a scene of devastation,

- Immediately after the fire, it's a scene of devastation,

like it's been bombed.

- It's like a man cut open by shrapnel;

he might not have long.

NARRATOR: For weeks after the disaster,

nobody knew what would happen.

Fragile as a house of cards, the cathedral might well collapse.

Fragile as a house of cards, the cathedral might well collapse.

- The day after the fire, we were very worried.

We didn't know how the building would react.

We didn't know how the building would react.

- We knew what had collapsed. Would the rest follow?

NARRATOR: The French government launches a major restoration.

NARRATOR: The French government launches a major restoration.

Philippe Jost is chair of the body which controls Notre Dame,

and is coordinating the work of hundreds of craftsmen, architects

and is coordinating the work of hundreds of craftsmen, architects

and engineers.

Their aim after the fire:

to restore the cathedral to its former glory.

- Notre Dame's suffered a great trauma,

- Notre Dame's suffered a great trauma,

but is now being rebuilt using the same materials as before.

NARRATOR: The days after the tragedy revealed the considerable damage.

Two vaults had collapsed under the impact of the spire,

though the others had held up well against the flames.

Notre Dame's cathedral architecture sees the choir, transept and each

bay of the central nave covered by a succession of vaults on each level,

bay of the central nave covered by a succession of vaults on each level,

forming a massive stone casing.

During the fire, hundreds of tonnes of charred debris

During the fire, hundreds of tonnes of charred debris

from the roof structure fell on the upper vaults,

which somehow survived.

However, the collapse of the spire and the roof frame

However, the collapse of the spire and the roof frame

left three gaping holes in the overall structure

where the transept and the nave meet.

where the transept and the nave meet.

- The cathedral coped with the fire rather honourably,

as most of the building survived.

NARRATOR: The damaged vaults

have been restored to their original condition,

and all the others beautifully cleaned,

and all the others beautifully cleaned,

visible in the latest images of the restoration work.

And during the restoration,

Notre Dame revealed the feats of construction

Notre Dame revealed the feats of construction

that had remained hidden for over 850 years.

- What really impressed us was when we climbed up the scaffolding

- What really impressed us was when we climbed up the scaffolding

after the fire, after the vaults had been shattered,

and we could measure the thickness, or rather the thinness, of them.

and we could measure the thickness, or rather the thinness, of them.

You'd think that in order to hold,

a vault would have to be strong and thick,

but, in fact, Notre Dame's vaults are extremely thin.

but, in fact, Notre Dame's vaults are extremely thin.

Something we hadn't suspected.

- The infill of these vaults is no more than 15 cm thick,

- The infill of these vaults is no more than 15 cm thick,

which is normally difficult to tell.

When a medieval vault is still in place,

you can see it from below, of course,

you can see it from below, of course,

and possibly from above, but you rarely see its full thickness.

NARRATOR: In the way they were built,

the vaults of Notre-Dame demonstrate a mastery of height and finesse.

the vaults of Notre-Dame demonstrate a mastery of height and finesse.

- It's thin, almost like a drumskin, and vibrates when you walk on it.

- It's thin, almost like a drumskin, and vibrates when you walk on it.

- It's an extremely airy cathedral.

Everything is refined in Notre-Dame, that's what's so surprising.

Everything is refined in Notre-Dame, that's what's so surprising.

It's all light and delicate, like lace.

NARRATOR: How did the medieval builders design vaults

NARRATOR: How did the medieval builders design vaults

that were both delicate and strong?

Another Gothic monument, 56 kilometres north of Paris,

Another Gothic monument, 56 kilometres north of Paris,

may give us an idea.

Notre Dame de Senlis, at first glance,

Notre Dame de Senlis, at first glance,

looks different from the Cathedral in Paris.

But inside, the two structures share a number of architectural features,

But inside, the two structures share a number of architectural features,

especially the high vaults in the nave.

Historian Elise Baillieul specialises in Gothic cathedrals.

Historian Elise Baillieul specialises in Gothic cathedrals.

- These vaults have an aesthetic side, of course,

but also a practical one,

but also a practical one,

as they separate the inside of the building from the wooden framework.

- In the architecture of the Middle Ages,

- In the architecture of the Middle Ages,

they became more and more common as fighting fires became a priority.

The vault could prevent the burning roof beams from falling.

NARRATOR: Builders of all the cathedrals from the Middle Ages

used this type of stone ceiling.

used this type of stone ceiling.

In each case, the construction techniques are similar.

- So to build the vaults,

- So to build the vaults,

the walls are built first and then a kind of wooden scaffolding

in the shape of the vault is placed above them.

It's on this wooden scaffolding that the masons lay the stones

It's on this wooden scaffolding that the masons lay the stones

that will form the skeleton of the vaulting.

At this point, it's like a kind of magnificent shell.

At this point, it's like a kind of magnificent shell.

The system is fairly simple, but tricky to set up.

All it takes is a miscalculation in the balance of forces,

and when the scaffolding is removed, the vault comes crashing down.

and when the scaffolding is removed, the vault comes crashing down.

NARRATOR: The vaults built in this way are spectacular.

NARRATOR: The vaults built in this way are spectacular.

In the vast stone robe covering the cathedral,

In the vast stone robe covering the cathedral,

what is remarkable about the 13 vaults that overhang

the central bays of the nave, the choir

and the two arms of the transept, is their height:

and the two arms of the transept, is their height:

32 metres, a record for the 12th century.

- People had known how to build vaults like those in Notre Dame

since the end of the 11th century,

but the real technical challenge was getting them up to that height.

but the real technical challenge was getting them up to that height.

- They're much higher than any of their predecessors.

32 metres high and 13 metres wide.

32 metres high and 13 metres wide.

So it's really an architectural marvel.

NARRATOR: To build a vault 32 metres high,

NARRATOR: To build a vault 32 metres high,

the builders of Notre-Dame assembled a complex stone framework

made up of six pointed arches.

made up of six pointed arches.

Inside which are three arches that intersect at their centre,

known as ogives.

known as ogives.

The two diagonal ogives create the keystone,

the point of balance for the whole,

the point of balance for the whole,

while the third transverse ogive cuts the framework

into six independent sections,

which are then covered with a thin, slightly domed layer of stone,

which are then covered with a thin, slightly domed layer of stone,

known as vaulting.

- How can such a thin arch be so strong?

- How can such a thin arch be so strong?

- It resists because it has a double curvature,

curving both one way and the other.

- It's really the shape of an egg,

- It's really the shape of an egg,

and it's this geometric form that gives it great mechanical stability.

Because these vaults are capable of supporting very heavy loads.

Because these vaults are capable of supporting very heavy loads.

NARRATOR: Cross vaulting was a major innovation of Gothic cathedrals,

creating lighter, taller structures that were nevertheless strong.

creating lighter, taller structures that were nevertheless strong.

So despite the fire in 2019, when many feared the worst,

So despite the fire in 2019, when many feared the worst,

most of Notre-Dame's vaults remain solidly in place.

- The whole of the medieval section burnt and fell on the vaulting.

The vaults are intact, a little burnt on the outside,

but structurally they held up.

but structurally they held up.

NARRATOR: These medieval vaults appear to float in mid-air,

but rest on a forest of arches and pillars

forming a stable support all the way to the ground.

forming a stable support all the way to the ground.

With this type of framework, the weight of the stone vault

is transferred entirely to the six load-bearing columns.

is transferred entirely to the six load-bearing columns.

Which means no walls in-between, and full-height openings.

The load is then spread from storey to storey down to the ground,

where 1.3 metre diameter pillars,

where 1.3 metre diameter pillars,

made up of a stack of circular blocks, act as support points.

The vault's cornerstone pillars are under particular pressure,

because they absorb the loads of two structures.

because they absorb the loads of two structures.

The fire put the strong pillars of Notre-Dame to the test,

The fire put the strong pillars of Notre-Dame to the test,

but they held up thanks to the skill of their medieval builders.

- The architects of the time, who we thought had no knowledge

- The architects of the time, who we thought had no knowledge

of load distribution and who didn't write down their calculations,

had an extremely acute sense of the overall balance.

had an extremely acute sense of the overall balance.

NARRATOR: What's striking about Notre Dame

is that strength is accompanied by a sense of style.

is that strength is accompanied by a sense of style.

- At Notre-Dame de Paris, what's amazing is the feeling

that when this 13th century architect

saw the harmony of the choir area,

saw the harmony of the choir area,

he said "I'm not changing a thing", and he carried on.

That's why the cylindrical pillars in the nave

That's why the cylindrical pillars in the nave

are identical to those in the choir.

NARRATOR: In this quest for solidity and aesthetics,

NARRATOR: In this quest for solidity and aesthetics,

the technical constraints under which the builders worked

make the cathedral even more remarkable.

Like in the upper sections, where the vaults were housed.

Like in the upper sections, where the vaults were housed.

- Here at Senlis, as at Notre Dame de Paris,

- Here at Senlis, as at Notre Dame de Paris,

these blocks are composed of a single stone block.

If you look closely, you won't see all the horizontal joints,

If you look closely, you won't see all the horizontal joints,

which simply result from

the superimposition of one block on top of the other.

Technically, it's a bit demanding.

It's an extra challenge, but in early Gothic architecture,

It's an extra challenge, but in early Gothic architecture,

there was a will to work with these smooth, even velvety effects.

NARRATOR: These one-piece blocks

are also remarkably strong support points.

Such pillars need a very resistant, high-quality stone,

but the builders knew where to find it.

but the builders knew where to find it.

Jonathan Truillet is head curator of heritage at Notre Dame.

- The men of the Middle Ages needed good quality stone

to build the cathedral. And they found it nearby,

to build the cathedral. And they found it nearby,

in what's known as the Lutetian basin,

either in surface quarries like the one behind us, or underground.

NARRATOR: The stone of Notre-Dame comes from limestone,

accumulated millions of years ago,

in what is known as the Lutetian Basin:

in what is known as the Lutetian Basin:

a geological area that extends around Paris.

Quarries from the Middle Ages were located in or near the capital,

Quarries from the Middle Ages were located in or near the capital,

but stone also arrived from further afield

via the Oise, Seine and Marne rivers.

via the Oise, Seine and Marne rivers.

In the cathedral district, a block of houses was even demolished

to create a six-metre wide and 76-metre long street,

to create a six-metre wide and 76-metre long street,

to make it easier to transport materials to the site.

Paris became the capital of France, and expanded at the same time.

Paris became the capital of France, and expanded at the same time.

The works started by the visionary bishop Maurice de Sully

transformed the lives of Parisians.

transformed the lives of Parisians.

These were the largest urban development projects

of the Middle Ages, and the quarries were working at full capacity.

of the Middle Ages, and the quarries were working at full capacity.

- This stone's been there for 45 million years,

and the men of the Middle Ages saw it just as we do now.

and the men of the Middle Ages saw it just as we do now.

To build the cathedral, they had to dig it from these hillsides

and transform it into blocks that could be used on the site.

It's a real challenge, which they did using quite basic techniques

It's a real challenge, which they did using quite basic techniques

and gives you an idea of the effort that went into building Notre Dame.

They used a pick to make a first quite deep hole,

to put in a metal wedge.

When you hit the wedge on this corner hard with a sledgehammer,

When you hit the wedge on this corner hard with a sledgehammer,

you can extract blocks about 20 cm in size.

- Everything was done by hand.

But they also had machines.

They had hoists, winches, and pulleys.

They had hoists, winches, and pulleys.

They even had cranes that could lift sometimes very large blocks weighing

several hundred kilos to great heights.

several hundred kilos to great heights.

NARRATOR: It would take the builders of the Middle Ages

less than a century to complete the initial construction.

less than a century to complete the initial construction.

With Notre-Dame,

they demonstrated an unprecedented mastery of stonework.

- The life of the cathedral is down to the quality of its materials.

- The life of the cathedral is down to the quality of its materials.

- Stone extracted from quarries is easy to cut when it's new,

- Stone extracted from quarries is easy to cut when it's new,

but when it's in a building, it hardens in contact with air,

so the stones become very resistant.

NARRATOR: Although the right choice for the building,

this high-quality stone was severely damaged by the heat of the blaze

this high-quality stone was severely damaged by the heat of the blaze

and the water hosed in by the fire brigade.

A total of 1,300 cubic metres had to be replaced.

A total of 1,300 cubic metres had to be replaced.

- After the 2019 fire,

we had to start rebuilding part of the cathedral,

we had to start rebuilding part of the cathedral,

so we asked ourselves virtually the same questions

as the builders of the Middle Ages.

Where do you find the stone necessary for the site?

Where do you find the stone necessary for the site?

How do you get it out? How do you transport it?

NARRATOR: Most of the quarries that supplied the site in the Middle Ages

NARRATOR: Most of the quarries that supplied the site in the Middle Ages

have now closed.

But today's builders have managed to find stones

But today's builders have managed to find stones

with similar properties near the capital,

such as in the Oise region.

- You can see at this height,

the first few centimetres is a type of crust that'll be removed,

because it's not good enough,

then 40-50 cm of stone that's what you need for the site.

What matters is how they look together and how durable it is.

What matters is how they look together and how durable it is.

You can't put two stones with very different

physical and chemical properties next to each other,

because you know they're not going to age well in the long term.

because you know they're not going to age well in the long term.

- We looked for stones with the same properties and qualities,

- We looked for stones with the same properties and qualities,

used in the same way.

- That's why we generally chose stone from the Lutetian basin.

NARRATOR: But it's not just the quality of the stone that determines

the strength of Notre Dame's protective shell.

Its vaults have been subjected to considerable pressure.

If they'd been too rigid, they would have shattered like porcelain.

If they'd been too rigid, they would have shattered like porcelain.

The material that allowed the vaults to absorb the vibrations

from the falling roof structure was key.

from the falling roof structure was key.

- You can't build large, towering edifices by just putting stones

- You can't build large, towering edifices by just putting stones

one on top of the other, like with Lego.

It just doesn't work.

- One of the special features of these large stone buildings is,

- One of the special features of these large stone buildings is,

of course, that the stones are fixed with mortar.

YVES: You can't just have a stone held by the one next to it.

YVES: You can't just have a stone held by the one next to it.

If there's no mortar, nothing to hold the stones together.

NARRATOR: The medieval builders knew that without mortar,

NARRATOR: The medieval builders knew that without mortar,

the building could collapse.

It's a simple, but essential ingredient.

- Mortar is a mixture of sand, lime and water,

which simply holds the structure together.

- Lime is limestone that has been put into a kiln.

The limestone disintegrates and becomes a powder, called quicklime.

The limestone disintegrates and becomes a powder, called quicklime.

The quality of the lime

depends very much on the quality of the limestone.

Like everything else,

Like everything else,

you need good ingredients to make a good dish.

NARRATOR: Mortar may only be a few millimetres thick,

NARRATOR: Mortar may only be a few millimetres thick,

yet knowing how it works is key.

In his laboratory in Bordeaux,

Stephane Morel and his team are trying to get an understanding

Stephane Morel and his team are trying to get an understanding

through a number of scientific experiments.

- This is a push and pull machine.

- This is a push and pull machine.

NARRATOR: Stephane has installed three blocks

identical to those in Notre-Dame's vaults, joined together with mortar.

identical to those in Notre-Dame's vaults, joined together with mortar.

These blocks have sensors to detect the slightest movement.

- Let's cut the joints and see what it does.

- Let's cut the joints and see what it does.

NARRATOR: Under the action of the press, cracks begin to form,

NARRATOR: Under the action of the press, cracks begin to form,

but only at the mortar joints.

- The blocks never break.

- The blocks never break.

This test clearly shows that the 'mechanical fuse'

in this assembly is really between block and seal.

NARRATOR: In other words, despite the pressure,

the stones have stayed perfectly intact.

STEPHANE: It's this lime mortar joint

STEPHANE: It's this lime mortar joint

that makes the masonry work so special.

- The building is a little plastic.

Obviously, only a bit, but it allows a certain flexibility

that's not normal in such a stone structure.

- Another important thing about mortar is that it evens things out.

- Another important thing about mortar is that it evens things out.

For example, in medieval times, the stones were cut in the same way,

For example, in medieval times, the stones were cut in the same way,

but not all of them were cut to within a millimetre of each other.

So applying a layer of mortar also evens out the next row of stones.

- And because of this, it will last for several centuries.

- And because of this, it will last for several centuries.

NARRATOR: The fire may be out, but the danger of collapse remains.

NARRATOR: The fire may be out, but the danger of collapse remains.

Although most of the vaults survived,

the build up of debris weakened the whole cathedral.

After standing delicately poised for eight centuries,

Our Lady was shaken and fragile.

Our Lady was shaken and fragile.

The subtle balance of such a tall, elegant structure

had been hard to achieve.

had been hard to achieve.

The builders used one of the greatest innovations

of Gothic architecture:

large stone arms extending from the building,

large stone arms extending from the building,

known as flying buttresses.

- Buttresses comes from the word "bouter."

Bouter in Old French means to push, to chase,

as in pushing the English out of France.

as in pushing the English out of France.

So it's an arch that pushes against this wall.

NARRATOR: The vaults of Notre Dame de Paris push downwards

NARRATOR: The vaults of Notre Dame de Paris push downwards

with the load taken up by the strong pillars inside the cathedral.

But these vaults also push sideways on the walls,

But these vaults also push sideways on the walls,

towards the outside of the structure,

which also has to be taken into account.

The balance comes from large flying buttresses

installed outside the cathedral walls.

28 in all.

28 in all.

- Ribbed vaults and flying buttresses are the images

of a Gothic cathedral we all have in our heads.

of a Gothic cathedral we all have in our heads.

- They're what you see when you're on a boat on the Seine,

or walking along the banks.

And at Notre-Dame, they're particularly expressive,

because they span both aisles.

because they span both aisles.

- These flying buttresses at Notre Dame are quite magnificent,

because they're so slender and fine. Everything is elegant.

because they're so slender and fine. Everything is elegant.

It's not heavy masonry like you see on other buildings.

NARRATOR: The buttresses of Notre-Dame are very well designed.

NARRATOR: The buttresses of Notre-Dame are very well designed.

Each lateral support is made up of several elements.

A very fine, single-flight arch spanning 15 metres.

A very fine, single-flight arch spanning 15 metres.

Then a wide post or abutment, topped by a pinnacle.

Then a wide post or abutment, topped by a pinnacle.

Combined, these three elements of arch, abutment and pinnacle,

allow the system to absorb lateral thrust from the vaults,

allow the system to absorb lateral thrust from the vaults,

while keeping it away from the load-bearing walls.

The buttresses of Notre Dame

may seem to have always been in place,

may seem to have always been in place,

but appearances can be deceptive.

- Architects, even in medieval times,

- Architects, even in medieval times,

sought to fix a number of defects in Notre Dame de Paris.

For example, they found that the buttresses weren't strong enough,

so they rebuilt them.

so they rebuilt them.

NARRATOR: Around 1300,

the flying buttresses were rebuilt for the first time.

And then again in the 19th century,

during a major restoration of the cathedral.

- Even if we think that buttresses were there from the outset,

we unfortunately don't know what they looked like.

They were certainly already very thin.

They were certainly already very thin.

They were already excellent flying buttresses.

NARRATOR: These spidery buttresses are essential

NARRATOR: These spidery buttresses are essential

to support the vaults from the outside.

Getting the balance right is difficult.

- It's a tricky balancing act.

Sometimes the vault pushes harder than the flying buttress

Sometimes the vault pushes harder than the flying buttress

and the wall bends outwards.

Sometimes the flying buttress is built too powerful in relation

to the vault and the wall moves towards the inside of the building.

to the vault and the wall moves towards the inside of the building.

NARRATOR: The buttresses are aligned with the pillars

NARRATOR: The buttresses are aligned with the pillars

to hold the vault in place,

A perfectly balanced troop of guards

to guarantee the stability of the building.

to guarantee the stability of the building.

- The amazing thing about Notre-Dame

is the high position of the buttresses,

which are extremely well placed.

which are extremely well placed.

When you balance the vaults and the buttresses,

you see shifts in the balance of just a few millimetres.

So it's a perfect design.

And that's quite astonishing,

given that current structures can move much more than that.

So it makes you wonder.

So it makes you wonder.

NARRATOR: Notre Dame's subtle balance was upset by the fire.

NARRATOR: Notre Dame's subtle balance was upset by the fire.

The damage and the debris on the vaults meant the flying buttresses

needed to take the load, and they threatened to break.

- My fear when the framework disappeared,

and especially when the vaults collapsed,

was that the buttresses would push the walls inwards,

was that the buttresses would push the walls inwards,

and like a house of cards, it would collapse.

The second risk was that if the vaults collapsed,

every single wall would collapse with them,

every single wall would collapse with them,

because of the thrust of the buttresses.

- Shoring it up was a project in itself,

and one that we did on a daily basis,

and one that we did on a daily basis,

looking to do it every way we could.

- That's why I had wooden supports

fitted under all the cathedral's buttresses in case they fell.

fitted under all the cathedral's buttresses in case they fell.

Fortunately, it didn't happen, but the supports are there just in case.

NARRATOR: Fortunately the buttresses,

rebuilt in the 19th century,

were able to absorb the impact without any obvious damage.

were able to absorb the impact without any obvious damage.

These high-performance devices mean architects could quickly start

rebuilding damaged areas.

rebuilding damaged areas.

Only four years after the fire, Notre-Dame has regained her poise.

- Now, standing here beneath it,

I'm happy because you can't see which arches collapsed.

And that for me is a victory. It makes so pleased.

And that for me is a victory. It makes so pleased.

NARRATOR: Notre Dame is gradually reclaiming her iconic appearance.

NARRATOR: Notre Dame is gradually reclaiming her iconic appearance.

And unexpectedly, the fire of 2019

has revealed the secrets of Notre-Dame's age-old solidity.

has revealed the secrets of Notre-Dame's age-old solidity.

After the blaze, scientists made a surprising discovery.

After the blaze, scientists made a surprising discovery.

Amidst the rubble, they found pieces of oddly-shaped iron.

Staples, weighing around three kilos.

Staples, weighing around three kilos.

At the prestigious Saclay research lab, work is underway

to get a better understanding of their use and role in the structure.

to get a better understanding of their use and role in the structure.

Mathilde Bernard and Maxime L'Heritier

are part of the research team.

are part of the research team.

- The lead on the spire had melted, because it melts at 300 degrees.

But the iron, which melts at 1500 degrees,

But the iron, which melts at 1500 degrees,

was found in fairly good condition in the rubble.

- They're like staples for paper, except a lot bigger and thicker.

- They're like staples for paper, except a lot bigger and thicker.

They have what are called little barbs on the legs of the staples,

They have what are called little barbs on the legs of the staples,

which will sink into the stone,

so that they stick properly to the lead seal and the stone.

NARRATOR: Researchers are carrying out chemical analysis

NARRATOR: Researchers are carrying out chemical analysis

to understand how these staples were made.

We're going to cut it up,

we'll make a few slices to see exactly what's inside the iron.

we'll make a few slices to see exactly what's inside the iron.

- Within a single staple, there are different types of impurities.

- Within a single staple, there are different types of impurities.

- The iron they had at the time wasn't very pure.

That's normal in itself, because blacksmiths aren't going to try

and create works of art for something that's going to be

found inside masonry.

However, it has to be pure enough to withstand the stress,

because the more impurities there are,

because the more impurities there are,

the more brittle it's going to be.

NARRATOR: While combing the cathedral for other staples,

the researchers found something even more surprising.

There was a whole chain of metal that had been hidden for centuries

inside the stone.

inside the stone.

This chain reinforces the structure of Notre Dame de Paris

with thousands of staples at several key areas.

Researchers have highlighted three sets of staples,

at the very top of the walls,

at the very top of the walls,

and just under the framework of the nave, transept and choir.

These thick staples chain the blocks of stone together

These thick staples chain the blocks of stone together

with their curved legs driven into the stone and then sealed with lead.

- It was a surprise to see so much iron at Notre-Dame de Paris.

- It was a surprise to see so much iron at Notre-Dame de Paris.

NARRATOR: The researchers question when during construction

NARRATOR: The researchers question when during construction

they were installed.

They could be original,

or added during the major restorations of the 19th century.

- All the staples we've analysed show that, for the balcony,

we're in the middle of the 12th century and for the wall,

the beginning of the 13th.

the beginning of the 13th.

- It was the builders who really decided to install these staples

when they built the masonry.

NARRATOR: This recent discovery makes Notre-Dame

the very first Gothic cathedral built using iron.

- For a Gothic cathedral,

- For a Gothic cathedral,

this is the oldest systematic use of iron on a large scale.

The first time people had tried to fix stone blocks

The first time people had tried to fix stone blocks

from the top with staples.

NARRATOR: Before the fire, there had been little appreciation

NARRATOR: Before the fire, there had been little appreciation

of the structural role in the building's strength and durability

played by these fasteners.

- We're beginning to understand what they were thinking of

when they put in these staples,

particularly under the roof structure.

They thought it might push in one direction

They thought it might push in one direction

and the masonry in another,

so something needed to hold it in place.

- What did the builders think when they added these staples?

You can perhaps imagine them thinking,

as it's high up, it's likely to move.

as it's high up, it's likely to move.

And so they put iron there to prevent it from moving.

- There's an approach behind it,

- There's an approach behind it,

a philosophy where they wanted the building to stand the test of time,

and also be something grandiose for the age.

NARRATOR: A place of experiment and innovation,

this first iron lady was solidly built.

Despite their thick metal underpinnings,

the outer skirts of Notre-Dame are delicate.

Making such a lacy stone covering

was a real achievement by the builders.

Thanks to the thick pillars and buttresses

Thanks to the thick pillars and buttresses

that keep the vaults in balance,

the side walls are no longer load-bearing,

so can be removed to let in the light.

so can be removed to let in the light.

- There was a well-known phrase In the 13th century:

"Less stone, more light".

"Less stone, more light".

NARRATOR: The builders hollowed out the walls

to install countless windows with pointed arches,

to install countless windows with pointed arches,

and circular openings known as roses.

The stained glass is now completely free of the ashes from the fire.

The stained glass is now completely free of the ashes from the fire.

The walls of Notre Dame are pierced by three impressive roses,

on either side of the transept and on the main western facade.

on either side of the transept and on the main western facade.

- The cathedral is like a lightbox.

This light which is natural, which we experience outside every day,

This light which is natural, which we experience outside every day,

is in fact supernatural.

It somehow expresses the work of the divine on earth.

- At Notre Dame de Paris, we're lucky enough to have three roses

- At Notre Dame de Paris, we're lucky enough to have three roses

which, when they were built, were the largest in the world.

- The roses in Notre-Dame are 13 metres in diameter.

- The roses in Notre-Dame are 13 metres in diameter.

13 metres is the height of a four-storey building,

so you have to imagine you're standing in front of a glass wall

so you have to imagine you're standing in front of a glass wall

of that height, where there's only stone and glass.

NARRATOR: The three roses of Notre Dame

NARRATOR: The three roses of Notre Dame

are large, delicate and complex structures - and also very fragile.

- These are delicate structures that can lose their shape,

especially in horizontal areas, which can make them fall apart.

NARRATOR: But the roses have stood the test of time.

NARRATOR: But the roses have stood the test of time.

NARRATOR: How the builders of the Middle Ages

created these immense structures, so refined and complex,

created these immense structures, so refined and complex,

is worth a closer look.

Picking the rose on the facade of Notre Dame,

Picking the rose on the facade of Notre Dame,

in which a large circular stone frame

is built inside a square opening.

is built inside a square opening.

This frame is like a bicycle wheel, with 12 main spokes.

This frame is like a bicycle wheel, with 12 main spokes.

The main circle is reinforced by 12 secondary spokes,

to balance the weight of the structure.

to balance the weight of the structure.

The stained glass windows are then set in lead

inside this intricate stone assembly.

- Structurally, these elements are absolute masterpieces.

The roses are only held together by iron rods

which are inserted into two parts of the stone,

which are inserted into two parts of the stone,

and held together by lead.

And it's this lead that makes it possible to absorb

the force of the wind, thanks to its flexibility.

the force of the wind, thanks to its flexibility.

- It's a real achievement, with the widest, and most openwork roses

of the entire Gothic period.

NARRATOR: These architectural roses are majestic and impressive,

and they're also symbols of the religious world.

The Madonna and Child is in the centre

surrounded by the twelve tribes of Israel from the Old Testament.

Above, virtues and vices are represented in pairs,

Above, virtues and vices are represented in pairs,

such as patience and anger, charity and avarice.

And just below, the 12 signs of the zodiac,

And just below, the 12 signs of the zodiac,

as well as scenes from everyday life.

- There's a lot of care that goes into things that you can't see,

- There's a lot of care that goes into things that you can't see,

because the Gothic was not just built to be seen by the faithful

or by pilgrims, but for God, for beauty itself.

NARRATOR: The lace-like stone and glass roses suffered in the fire,

but have been preserved, thanks to the clever design

but have been preserved, thanks to the clever design

and the quality of the materials and building techniques.

At every stage, the builders aimed to protect

At every stage, the builders aimed to protect

the superstructure from the dangers it faced.

And in particular, from an insidious enemy:

And in particular, from an insidious enemy:

water.

- Water takes everything with it.

It seeps into the masonry, into the stones, into the mortar.

It seeps into the masonry, into the stones, into the mortar.

It comes into contact with metal reinforcements like iron staples.

And when an iron element rusts, it shatters the stone.

It's an enemy you can't ignore.

It's an enemy you can't ignore.

NARRATOR: To protect themselves from this enemy,

builders turned to a highly resistant material that could be

builders turned to a highly resistant material that could be

moulded to any complex shape:

lead.

The steeply sloping roof, pitched at 55-degrees,

The steeply sloping roof, pitched at 55-degrees,

is entirely covered with five millimetre thick lead sheets.

1326 of them.

1326 of them.

- Why use lead as a roofing material?

In the Middle Ages, it was relatively rare and expensive,

In the Middle Ages, it was relatively rare and expensive,

reserved for the most prestigious monuments.

So lead was used on this cathedral,

because it was one with strong connections to royal power.

because it was one with strong connections to royal power.

NARRATOR: The building is also watertight,

thanks to an ingenious drainage system.

thanks to an ingenious drainage system.

Rainwater first runs off the roof before draining through gutters,

cleverly hidden inside the flying buttresses.

cleverly hidden inside the flying buttresses.

- And so to accentuate these drains,

gargoyles were built in the 13th century.

gargoyles were built in the 13th century.

Stone sculptures.

- To give the impression from below

that the water's being spat out of the gargoyle's mouth.

that the water's being spat out of the gargoyle's mouth.

NARRATOR: The gargoyles,

these grimacing monsters hanging out into the void,

give Notre Dame its distinctive outline.

give Notre Dame its distinctive outline.

The large number of them divides the flow of water into smaller amounts.

But on the evening of 15 April 2019,

But on the evening of 15 April 2019,

the entire roof of Notre Dame went up in flames.

Once the fire had been put out, the urgent task was to protect

Once the fire had been put out, the urgent task was to protect

the stone of the cathedral from more water damage.

- The water from the fire brigade wreaked havoc,

seeping everywhere into the walls.

The first thing I remember when I went back into the cathedral

The first thing I remember when I went back into the cathedral

was the sound of water running down the walls.

And with bad weather to add to that,

I immediately asked the rope crew to put up some tarpaulins.

I immediately asked the rope crew to put up some tarpaulins.

The fire happened on Monday.

All the vaults were protected by the Friday.

And then, on Friday, it rained.

And then, on Friday, it rained.

NARRATOR: With the stone now protected for a time,

NARRATOR: With the stone now protected for a time,

the roof needed to be rebuilt,

and above all, the wooden framework that supports it.

But that's not easy when nearly the whole structure,

But that's not easy when nearly the whole structure,

'Notre Dame's forest', has gone up in smoke.

Luckily for the rebuilders, one of the architects in charge

Luckily for the rebuilders, one of the architects in charge

of restoring Notre-Dame, Remi Fromont, had meticulously surveyed

the medieval roof structure before it burnt down.

the medieval roof structure before it burnt down.

Between the 13th century and the fire of 2019,

Between the 13th century and the fire of 2019,

the highest vaults of the nave, transept and choir

were covered by thousands of oak timbers,

a very dense framework that earned it the nickname of 'The Forest'.

The Forest's backbone

is a series of linked triangular structures called trusses.

is a series of linked triangular structures called trusses.

Each truss, 10 metres high and around 14 wide,

Each truss, 10 metres high and around 14 wide,

is a set of horizontal, vertical and diagonal struts

built to make the structure resistant to the force of the wind.

built to make the structure resistant to the force of the wind.

- How does the wooden frame work? It's based on the triangle.

The advantage of a triangle is that you have a shape that doesn't move.

The advantage of a triangle is that you have a shape that doesn't move.

If you have wind loads coming from the side,

the triangle won't be able to go out of shape.

NARRATOR: At 35 metres up, wind is a big problem.

To wind-proof the structure,

the Gothic carpenters had to put in twice as many wooden triangles.

the Gothic carpenters had to put in twice as many wooden triangles.

- The art of complex carpentry was invented between the 12th

and 13th centuries in France.

and 13th centuries in France.

- You had a first triangle giving the roof its shape

and was also structural.

Underneath it a second small triangle, a little lower,

Underneath it a second small triangle, a little lower,

that gave even more rigidity to the whole framework.

It's a framework within a framework.

A bit like Russian dolls that fit together,

A bit like Russian dolls that fit together,

so that if one part breaks, you always have a second

to take up and redistribute the stress and keep it working.

Hence, the extreme longevity of these frameworks.

Hence, the extreme longevity of these frameworks.

NARRATOR: The triangular shape partly explains

how Notre Dame's structure is so strong,

and why it remained intact until the 2019 fire.

and why it remained intact until the 2019 fire.

But it's also down to how it was built.

- These were extremely modern,

technically advanced structures for their time,

and it was probably one of the first times for that size of building

and it was probably one of the first times for that size of building

that an assembly was used we still use today:

the mortice and tenon joint.

A mortice and tenon is where a plug part fits into a socket part.

The tenon is centred on the socket part,

which means it can't come out laterally.

NARRATOR: To understand the techniques used by the carpenters

NARRATOR: To understand the techniques used by the carpenters

of Notre Dame, you don't have to travel far from Paris.

55 kilometres to the west of the capital is Mantes-la-Jolie.

55 kilometres to the west of the capital is Mantes-la-Jolie.

And its collegiate church known as Notre-Dame's twin.

Built at the same time,

experts believe it could have been a lab for the builders.

experts believe it could have been a lab for the builders.

Luckily, the church still has its original Gothic roof structure.

- This framework has a lot in common

with the one that covered the central nave of Notre-Dame-de-Paris.

with the one that covered the central nave of Notre-Dame-de-Paris.

It's also a medieval framework, with the same structure

and roughly the same age.

There is clearly a kind of conversation

There is clearly a kind of conversation

going on between the two sites.

NARRATOR: Today you can still see traces of this conversation

left by the 12th-century builders.

- You can very clearly see these assembly marks

- You can very clearly see these assembly marks

left by the carpenters.

This framework is first assembled on the ground, so the parts are marked

and numbered by the carpenter, using his tools of course,

and numbered by the carpenter, using his tools of course,

and this same sign will be found on all the parts assembled here.

It's not at all out of the question that the same carpenters

It's not at all out of the question that the same carpenters

and the same workers could have been working on both sites.

NARRATOR: But although the same carpenters

NARRATOR: But although the same carpenters

certainly worked on Notre-Dame de Paris and Mantes la Jolie,

there's a key difference between the two buildings.

there's a key difference between the two buildings.

In Mantes, there is a fault you don't find

on the Notre Dame roof structure.

- What's interesting to see here is the closeness.

Even the contact between the outside of the vault and the top.

I mean, this master section, this horizontal part of the structure.

I mean, this master section, this horizontal part of the structure.

It's not great.

So, at Notre Dame de Paris,

what they tried to do in the decades that followed,

was to move these main parts of the structure

away from the vault and raise them up.

So the side walls were lifted to raise up the whole thing.

So the side walls were lifted to raise up the whole thing.

NARRATOR: At Notre-Dame, the framework above the choir

NARRATOR: At Notre-Dame, the framework above the choir

was initially built flush with the vaults,

but 40 years later, it was completely dismantled

to raise the two load-bearing walls by 2.6 metres.

to raise the two load-bearing walls by 2.6 metres.

The new framework was then rebuilt on these two walls

The new framework was then rebuilt on these two walls

with a large transverse beam at the base of the triangular truss

to balance and solidify the structure.

- This allows us to have a ridge line between the nave and the choir

that are at the same level,

and to have roofs that are not offset but also aligned.

and to have roofs that are not offset but also aligned.

And so the overall design of the cathedral is very consistent.

NARRATOR: The master carpenters of Notre-Dame

NARRATOR: The master carpenters of Notre-Dame

have produced a masterpiece, helped by their experience

of the collegiate church at Mantes-la-jolie.

Today, their successors are working to restore

the cathedral to its former glory.

The cathedrals' emblematic spire, already rebuilt in the 19th century,

would also need to be built again.

- What could be better than the Viollet-le-Duc spire?

What architect had as much genius as him?

NARRATOR: This elegant spire needed great skill to build.

NARRATOR: This elegant spire needed great skill to build.

Exposed to the wind, it also absorbed a great deal of pressure.

- The enemy is the wind.

When it hits the rigid spire, the spire will try to escape the wind,

When it hits the rigid spire, the spire will try to escape the wind,

by lifting itself up.

And as it lifts, to escape the wind, it will twist.

So once, twice, three times, then the fourth twist can be fatal,

So once, twice, three times, then the fourth twist can be fatal,

and it could fall.

NARRATOR: This is exactly what happened

NARRATOR: This is exactly what happened

with the spire of the builders in the Middle Ages.

Too badly damaged, it was eventually dismantled.

In the 19th century, the architect Viollet le Duc built a new spire.

His priority was to strengthen the fragile structure

His priority was to strengthen the fragile structure

and make it more stable against the wind.

Not an easy task.

At the heart of the structure, at the foot of the spire,

he created a particularly strong base

he created a particularly strong base

using the four existing pillars of the transept.

The four posts of his spire were fitted into the pillars,

The four posts of his spire were fitted into the pillars,

making them very solid.

This base supports the spire.

- The whole base of the spire is really fixed against the masonry,

so it can't warp.

So that's what we call the stool. It's like a stool you sit on.

So that's what we call the stool. It's like a stool you sit on.

NARRATOR: Above this stool,

the spire is made up of several crucial elements:

the rigid base with a vertical shaft

the rigid base with a vertical shaft

which becomes a long framework up to the top of the spire.

This double-reinforced framework then rises into the sky,

This double-reinforced framework then rises into the sky,

with ever-thinner wood beams.

This spire, a masterpiece of complex construction,

This spire, a masterpiece of complex construction,

was something new and extraordinary, and all lost in the fire.

Except for the 12 copper statues,

removed for renovation a few days before the tragedy.

The construction secrets of the builders of the Middle Ages,

The construction secrets of the builders of the Middle Ages,

pushing the limits of stone and wood, have been revealed

at the largest restoration site of the 21st century.

at the largest restoration site of the 21st century.

- The extraordinary thing about this restoration

is that all the skills to restore the cathedral

have been found in France.

- We're lucky to have these amazing carpenters

- We're lucky to have these amazing carpenters

whose work is on a par with those of the 13th century.

NARRATOR: This medieval masterpiece, renovated to the same high standards

NARRATOR: This medieval masterpiece, renovated to the same high standards

and using the same techniques as before,

can now be passed on to future generations.

can now be passed on to future generations.

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