All language subtitles for rebuilding.notre.dame.s01e02.1080p.webrip.x264-cbfm[EZTVx.to]

(mellow music)

On April the 15th, 2019,

the world's most famous cathedral was ravaged by fire.

The roof went up in flames,

and the lofty spire collapsed,

crashing down through the vaulted ceiling.

That day, Notre Dame de Paris became a ruin.

(mellow music)

Once the fragile building was stabilized,

the decision was made to rebuild the cathedral

exactly as it was before.

But where to start?

How do you reconstruct a day,

an 800 year old monument

when no one knows how it was originally built?

So began a remarkable challenge

in the very heart of Paris.

For the first time archeologists, historians, geologists,

and specialists in ancient materials,

as well as structural engineers,

and acoustic and digital specialists

explore the cathedrals and trails,

scan its vaults, and probe its foundations.

In the process, they reach spaces

that have been inaccessible for centuries,

rediscovering all the skills and techniques

of Notre Dame's original builders

for they hold the key to its reconstruction.

In their efforts to balance

both the earthly and the sacred,

the actual and the virtual,

the scientists and the architects

both share a single obsession

to bring beloved Notre Dame back to life.

(mellow music)

Built on the Ile de la Cite in the 12th century,

not Notre-Dame de Paris at the time of its construction,

is the tallest cathedral in the world.

Its dimensions, the artistry of its masonry,

and the form of its flying buttresses

show gothic architecture entering a new era.

Notre Dame resembles a gigantic stone vessel

in which the material gives way to the feeling of space.

I don't think there's any other cathedral

in France that's so elegant.

Everything is delicate.

The walls are thin, the highest windows are fine,

ever more light and ever less matter.

It's almost as if the material

has been stretched upwards to its limits,

yet at the same time, it hasn't been deformed.

The flying buttresses and the vaulted roof are all there,

with the wooden framework above,

and for eight centuries it hasn't butched.

This true masterpiece of harmony and balance

is the very essence of gothic architecture.

The cathedral as the result of a system

that makes it possible to hold together opposing forces.

Normally everything should constantly collapse

and that's what's so marvelous.

There's a point of pure harmony,

and that's a rare thing that's difficult to obtain.

It remains fragile,

but it's still a reality and it holds up.

This balance was totally destroyed by the fire

of April the 15th, 2019.

The spire collapsed into the vaults,

completely smashing them above the transept and the nave.

As for the lead roof and the framework

that disappeared in the flames,

they stopped weighing on the walls,

thus disturbing the stability of the whole building.

(mellow music)

Yet despite huge fractures

and much to the surprise of the architects,

the cathedral didn't collapse

in the days following the disaster.

We're very fortunate

that only the vaults are affected.

The flying buttresses held, the walls held,

the fire caused no structural damage at all.

What is the secret of Notre Dame's

exceptional resistance?

How did its builders design its structure,

and how should it be rebuilt

to guarantee the same stability it had before?

These have become the big questions

for the architects in charge of the restoration.

Indeed, it's impossible to rebuild the cathedral

without first solving the equation of the forces

that govern its structure.

A few months after the fire,

the flying buttresses were stabilized

with large wooden arches.

The vault was then incorporated,

now propped up like this

as it was at the time of its construction.

The cathedral looks like a medieval construction site again.

So a vast scientific investigation has begun.

Dozens of wood, metal and stone experts

are all examining the building,

probing its masonry,

analyzing its materials to understand how the vault,

the flying buttresses, the walls, the stones,

and the mortar all fit together,

as well as the frame,

that now vanished wooden architecture

that will have to be completely rebuilt.

Among them, the engineers of the materials

and structural mechanics group

led by Stephane Morel

have the job of unraveling the mysteries

of the forces at work within the building.

We started by separating the idea

of the vault from what supports and maintains it.

Let's say it's supports,

so we could explain particularly to the project manager

how the whole thing works.

The first question

to occur to the architects was,

where does the vault actually begin?

To find the answer Stephane Morel and his team

went to Notre Dame to get as close as possible

to these medieval vaults

that reach up to a height of 32 meters.

Observing just how the medieval vaults broke

following the disaster allowed Stephane Morel

to formulate a new hypothesis

on what he calls the structural limits of gothic vaults.

Wherever a vault has collapsed,

you can see that the lower parts always remain in place

even in a major disaster.

We think of a vault as starting at the level

of the capitals, but that's not really the case.

Mechanically, it starts a lot higher up.

So in Reims Cathedral whose vaults were bombed

during the first World War,

the lower parts, the load bearing piles

also remained in place

as if they were not structurally part of the vault.

(mellow music)

The work of the structure group

has highlighted the ingenious load transfer system

designed by the medieval builders.

A lot of what we think of as the vault

is actually part of the walls,

so the vault in the structural sense of the term

is much smaller.

This helped us to understand more about

how the vault support assembly works,

and especially the role of the load-bearing piles.

From inside the cathedral

they look like part of the vaults,

but in fact they're part of the wall.

The vaults seem to start very low,

but it's actually the wall there

and the vault itself starts much higher up.

(mellow music)

Those geniuses back then

didn't have engineers like we do.

They didn't have software or anything like that.

The fascinating thing is that

when you calculate the load distribution

of the vault pressure,

and the location of the flying buttresses,

they're placed exactly where they should be,

any higher or lower they wouldn't work.

Thanks to this system of flying buttresses

aligned with a load-bearing pile,

the damage was concentrated only on the vault.

(hymnal music)

by modules, by sections.

So each module has a certain autonomy.

If you remove one module,

the whole thing is at risk, but not entirely.

These independent modules

are sections grouping the vaults and flying buttresses.

They're perfectly balanced,

and they're repeated in order to form the backbone

of the building and guarantee its stability.

So we have a system of force runoff

that's incredibly logical and reasoned

so that the force which is very dangerous

and could destroy the whole edifice,

is directed to one side of it from bridge to bridge,

from articulation to articulation

until the whole building has bound together.

This trickle down principle of the forces

allowed gothic architects to slim down the walls,

and drill large openings to let light in,

And that's the marvel of an architecture

in which everything is linked together,

and that's quite a medieval concept

because in the Middle Ages they emphasize

those links of interdependence.

Contrary to our mentality today,

they were valued because they allowed a human being

not to be alone.

Man alone is in danger.

A stone alone is in danger.

So what creates solidity is of course the stone itself,

but especially its links of dependence to other stones.

These links of dependence

between the thousands of stones that make up Notre Dame

have a very concrete form,

the mortar used by masons to bind the stones together.

What role does it really play

in the stability of the building?

That's the question that the researchers

of the stonework group are asking.

They're collecting samples

from different parts of the cathedral,

chemist Jean-Michel Mechling supervises the operation.

A few weeks ago

we drilled just behind there into the walls

of the cathedral to collect mortar samples.

That's the thickness of the joint.

The thickness of the joint?
Yes.

And the idea is to get outta the borehole

as much information as possible.

So on site we use an endoscope

to see the entire length of the borehole,

and the initial results we're getting

on the mortars actually show a lot of regularity.

The mortars in the cathedral are very consistent,

so it's certain that these people knew perfectly

how to make mortars,

and always produce them in the same way.

The mortar samples

taken by Jean-Michel Mechling and his team

have been sent to the researchers

from the laboratories of the University of Lorraine,

and the Jean Lamour Institute.

Under the microscope,

the chemist has found both grains of quartz

and lumps of lime.

The 40 or so samples

that we've taken mainly from the vaults

are so far without doubt made from lime,

and that's quite logical and normal for the time.

It was mixed with sand from the sand,

and that's also pretty logical.

We wouldn't expect the sand to come from very far away.

These mixtures are very consistent

both in the lime sand ratio,

and the quality of the sand itself.

At Notre Dame,

the limestone walls are barely 60 centimeters thick.

They're thin, but they have proven to be incredibly strong

for eight centuries now.

So what role has mortar played in their structure?

The exact mechanical function of the mortar

is not yet fully defined.

We have some ideas,

and we're talking them over with some of the specialists

in structural calculations.

During the fire,

the mortar joints cracked at the level

of the medieval vaults.

(mellow music)

This phenomenon is of particular interest

to engineer Stephane Morel.

In his laboratory in Bordeaux, Southwestern France,

he's done experiments to reproduce how those cracks occurred

by exerting pressure on three limestone blocks

held together with mortar

of the same composition as that at Notre Dame.

The blocks are equipped with sensors

that can detect the slightest movement.

Under the action of the press,

cracks in the mortar eventually form.

(sand crackling)

The blocks remain intact,

but the cracking takes place in the interface

between the stone and the joint.

The experiment reveals that the mortar

by cracking protects the integrity of the stone blocks.

It's a major asset of masonry structures

compared to those of ancient times.

With the first structures

back in Roman times,

there was nothing at all between the blocks.

The blocks had what's called dry joints.

They were arranged one on top of the other.

The problem with that

is that when you put a heavy load on them,

the surfaces and their points of contact

are never perfect,

and you pretty soon get breaks in the blocks.

The medieval builders understood

that in the absence of mortar,

it was the stone blocks themselves

that were directly subjected to stress,

and eventually broke with a passage of time.

When you put these blocks together

using a mortar joint,

the most fragile part of the assembly

is at the level of the joint,

and more particularly at the interface

between the joint and the block of stone.

This masonry is made up of materials that are very rigid.

A stone isn't flexible and neither is a joint,

but when you get cracks at these interfaces,

they allow small local displacement.

The combined effect of which taken together

at all the different levels and surfaces

allows the masonry to move quite a bit,

and that gives the overall structure a certain flexibility.

This is undoubtedly one of the explanations

for the durability of these masonry structures

that have been able to withstand several earthquakes

during their history,

and are still standing today.

(mellow music)

The cracks that appeared during the fire

are not so much a sign of weakness

as a proof of the building's resilience to shocks.

(mellow music)

Notre Dame is a relatively flexible cathedral

whose masonry has been moving for eight centuries

as it adapts to bad weather,

the flooding of the sand,

ground movements and the vibrations of the metro.

It's alive, it's dynamic,

it's sensitive, it moves, and then it moves back again.

When I walk around in here,

I can see pushing and pulling,

the forces, the reactions, all of that, it's great.

(mellow music)

So to restore all its mobility

to the cathedral,

the architects have decided to use mortar

instead of cement

using the same recipe as eight centuries ago.

As for the quality and consistency

of the mortars that were used,

in particular by Viollet-le-Duc,

well at one point they decided that cement was fine,

so they put in some cement joints,

and cement is worse than anything

because it not only has salt in it,

but it's harder than stone.

Unfortunately, we've gone back to things

that are more normal.

For the apps, in fact, for the whole restoration,

we'll be using lime and sand, nothing else.

(mellow music)

But using mortar in the joints

won't alone guarantee the stability

of such a towering edifice.

An exceptional discovery in the heart of the masonry

of Notre Dame relaunches the investigation.

With the frame and roof no longer there,

scientists have access to the upper parts of the building

for the first time.

Maxime L'heritier is an archeologist specializing in the use

of metal in gothic cathedrals.

At the top of the walls,

more than 30 meters above the ground,

he finds long metal staples concealed in the masonry

for more than eight centuries,

The frame rested on these staples,

that was one of the discoveries we made

on the use of metal at Notre Dame thanks to the fire.

The two main metals they used were iron and lead.

Scientists have identified several hundred

iron staples that connect the stones together

at the top of the choir, nave, and transect.

They decided to take some of them

from the damaged parts of the building.

One of the surprises

was the sheer quantity of staples

of different series of staples

that were used at all stages of the construction

of the gothic cathedral of Notre Dame.

There are enough in the nave

to hold all the sets of columns

that we found with the metal detector

along with close study of all the masonry

we can get to now with the scaffolding.

By methodically surveying

the different floors of the cathedral,

Maxime L'heritier and his team

have identified other staples in the galleries.

Again, the archeologists took some of them to date them.

He was thus able to understand their purpose,

which dates back to the time

of the construction of the cathedral.

So that's a surprise

to have so much metal in a cathedral

of the second half of the 12th century.

Even if some phases of construction,

especially up at the top

are from the very beginning of the 13th century.

It was quite a discovery

because until now we knew all about the use of metal

in cathedrals of the 13th century,

as in (indistinct) or in shaft.

But the cathedrals and big churches of the 12th century

show quite a limited use of iron,

just a few studs to hold up the columns.

Maxime L'heritier and his colleagues

discovered a veritable metal skeleton

dating back to the time of the cathedral's construction.

Nowadays, we wonder what this metal

is doing in this architecture, so particular to Notre Dame,

where from the 1160s they were planning to raise

very, very high vaults,

much higher than any of the buildings that preceded it,

and not just higher, higher, wider, and thinner.

Notre Dame was therefore

the first gothic cathedral site

where such quantities of iron were used.

It's a discovery that highlights

the little known contribution of blacksmiths and ferries

to this vast stone vessel.

Always working in the shade,

the better to see the color of the heated metal,

they forged one by one these thousands of staples

that are now hidden in the masonry.

(metal clicking)

The staples have been sent to the archeomaterials laboratory

to be analyzed by Maxime L'heritier,

and metal specialist Philippe Dillmann.

They are cut into samples,

then polished for a long time,

until this surface is as smooth as that of a mirror.

Through a microscope,

researchers will finally be able

to observe their structure,

and it's proving to be much more complex than expected.

So we've got these three bands

that have different levels of carbon content.

Here you see we have big deposits of it,

and here we have just as many,

but they're very small and up here there's carbon two.

These bands proved that the staple was forged

by welding together metal bars of different origin.

At this point,

we start to wonder if this is all material

that's been recycled,

and if this staple was made up of lots

of pieces of iron of different origins by a blacksmith

who recovered metal from different places.

What we've usually seen on other sites

is that 10, 20, even 30% of the iron we've analyzed

may have been recycled,

that it might have been welded together

with different compositions on either side of the wealth.

At Notre Dame, we've analyzed five or six staples

that all have welds, six out of six.

So we're going to analyze more of them

and as our study expands, it should confirm these results.

But at the stage we're at,

it's still raising a lot of questions.

(dramatic music)

At Notre Dame, we're finding 100% recycled iron

while this was probably

one of the richest construction sites

in the Kingdom of France at the time.

So what does this tell us?

What's the extent of this recycling,

if it is indeed recycling?

It could be due to faulty links

somewhere in the production chain

that led to the blacksmith

having to assemble several pieces of iron.

Was there a transport problem t

hat meant shipping small lots of one,

or one and a half kilos was easier?

Where did this iron come from?

Observation under an electron microscope

of the composition of the impurities

contained in the samples

and the deposits of slag,

the residues from iron ore smelting

provided a major clue.

The surface of the sample

is just a few square centimeters,

but there are a lot of deposits inside it,

maybe tens or even hundreds of thousands of them.

Here you can see that it's iron.

We've got aluminum, silicon, phosphorus.

There's some iron with a bit of phosphorus in it.

There's not much manganese.

It is then possible

to find in the staples traces of chemical elements

related to the manufacture of the iron

that composes them.

We will be able to compare

these trace element analysis

to the chemical analysis of the waste

found on archeological sites, the slag.

There's the same materials on all archeological sites,

so by comparing the composition of the slag

in all the sites of one particular area

along the send, for example,

which is a major thoroughfare,

or areas that are mentioned in the text,

we can compare them to find out if the iron in Notre Dame

is likely to have come from this region or farther away,

perhaps further along the sand.

(mellow music)

According to the analyses,

these staples were made of metal

from many different sources,

coming from the surrounding region and converging on Paris.

For economic or purely practical reasons,

that remains a mystery.

The structural reasons for using them

were most likely related to the construction phase.

We have this model here

that chose the completed building.

Throughout the construction,

the builders sought to use metal

to stabilize the structure,

and they chose to use staples to prevent displacement

and to counterbalance various forces.

That was probably the case on this peripheral belt

where it was feared that the lateral thrust of the frame

could cause slippage

in the last rows of stones,

more than 30 meters above ground level.

(mellow music)

The staples made it possible

to stabilize the walls of the building

throughout their construction until the last stage

that would fix them definitively,

the installation of the framework.

On these structures,

you really have to have significant vertical loads

for the masonry to work properly.

The joints need to work with significant vertical loads

to create more friction.

So the absence of the framework isn't such a good thing,

but anyway, it's going to be rebuilt.

This wooden super structure

consisting of more than 1000 oaks,

which crowned the entire building has now disappeared.

On the site, workers are preparing to evacuate

the last vestiges of the roof framework of Notre Dame,

known as the forest,

in reference to the 1000 oaks that we use to make it,

and now just a cluster of charred beams.

It'll be rebuilt in wood,

the living material that the medieval builders used

throughout the cathedral's construction.

(metal clinking)

There was wood everywhere

at every stage of construction,

especially in the scaffolding and the bending,

as well as all the devices for hoisting materials.

And then of course, once the building was finished,

the roof had to be made,

and its framework was very important

because it ensured the protection of the building.

We still need this wood framework today.

Many other technologies might be easier to use,

but wood is what's needed,

if the building is to retain its human dimension,

it's human skin.

(mellow music)

How then to rebuild a heritage masterpiece

that's been lost forever?

How to plan it out, and what techniques to use?

Luckily only a few years before the fire,

the framework was the subject of a complete

and precise architectural survey

by two young architecture students.

One of them, Remi Fromont,

is now chief architect of the site,

in charge of the reconstruction of this framework

that he knows better than anyone.

I find it hard to believe

that our surveys become so important.

It was a very objective survey.

We drew everything we saw and we left nothing out.

So in fact, we did an almost archeological survey

of a framework.

They are extremely complete drawings

of each of the assemblies that make up the roof.

Thanks to the survey,

the medieval framework can be reconstructed identically.

But like all the rest of the cathedral,

the frame has undergone many evolutions and restorations

during its eight centuries of existence.

So Remi Fromont has entrusted archeologist, Frederic Epaud,

a specialist in medieval frameworks

with the job of reconstructing the history

of this lost heritage.

What's new about this study

compared to all the other structural studies

I've carried out is that we had to do a postmortem

on the framework.

We were starting from almost nothing

and on something that had disappeared.

We had to reconstitute the framework

to be able to study it.

So at first we were lucky enough

to have the 2015 survey by Remi Fromont

and Cedric Trentesaux.

It's all in there.

You can see the location of the pegs, the assembly marks.

There are a lot of annotations,

so it's a gold mine of information.

We contacted all the carpenters and researchers

who'd actually visited this framework,

and we managed to collect more than 900 photos.

The first objective

was to differentiate the medieval parts

from the great restorations of the 18th and 19th centuries.

Then we had to tell apart in the photos

wood from the 13th century and wood from the 18th century,

the differences are pretty tiny.

Sometimes it's the way the wood was shaped by the axe.

If a piece of wood was sawn,

then that made it easier because back in the 13th century,

all the wood was cut with an axe.

So if it was sawn, we knew it was a fairly recent repair.

And then there's the patina of the wood.

13th century wood doesn't age in the same way

as the wood they used in the 16th or 18th century.

All this information

along with the dating of the beams

carried out in the eighties

allowed a more precise reading

of the framework's eventful history.

(dramatic music)

A first framework was placed over the choir in 1185.

Then a second one was placed over the nave in 1215.

Finally in 1225, the framework over the choir

was completely dismantled,

the walls were raised by two meter 60

to the level of those of the nave,

and then the framework was completely reassembled.

This vast overhaul to raise the roof

puzzle the archeologist.

It's really a complete revision

of the initial project.

The old cornice, that is the upper part

of the 12th century masonry was at about this level.

Then in the 13th century, 30 years later,

they put a new cornice on top of it.

And behind this balustrade, there's even higher masonry.

And this masonry raises the roof

by almost two meters, which is huge.

So why was the whole choir completely redone?

To understand that you have to go

to the Collegiate Church of Mantes-la-Jolie near Paris.

(mellow music)

And more precisely to the attic

where one of the last gothic frameworks

still in place in France is to be found.

(mellow music)

The framework

of the Collegiate Church of Mantes

is almost the little twin sister

of that of the nave in Notre Dame de Paris

since it was built just four years earlier.

(mellow music)

The framework at Mantes is almost identical

with more or less the same dimensions.

Both have spans of 11 meters,

with a height of approximately nine meters 60,

or nine meters 80.

(mellow music)

On some beams, Frederic Epaud

has spotted an important detail

that highlights the relationship between the framework

of Notre Dame and that of Mantes.

Here we've got three marks

gouged in by a chisel with a curved blade.

Here there's five in one, which makes six.

And what's interesting is that at Notre Dame on the nave,

so four years after this framework was made,

we find the same type of brand with the same type of tool.

So that's quite intriguing,

and even makes you wonder

if these were the same carpenters

that made the framework of Notre Dame.

There is however one made a difference

between the two frameworks.

Except that here in Montes

there was a design flaw,

a defect that's not to be found on the framework

of Notre Dame.

This defect was to cause a lot of damage

and would lead repairs that are still visible to this day.

And the big problem is that

we have a lot of weight right here on these beams.

And the beams couldn't take it

because they weren't thick enough.

There was nothing underneath to take up the weight

because there are vaults that are so close to the frame.

(mellow music)

It was because of these extremely domed vaults

that the frame couldn't be reinforced from below.

(mellow music)

These are the same domed vaults as at Notre Dame.

And the builders probably ran into exactly the same problem

when they built the first framework over the choir.

In the first version

of the framework over the choir of Notre Dame,

you can see that this transverse piece

was supposed to form the base of this large triangle,

but that wasn't possible

because of the prominence of the vaults

That's why 30 years later

on the nave and then on the choir,

they went back and raised the masonry

by almost two meters to sit the frame,

in dotted lines here, much higher up.

By raising his frame,

the master carpenter freed himself from the vaults,

and added an essential piece,

a crossbar located at the base of the triangle,

and brace from below with supports

to solidify the structure.

But then he had new challenges to face.

Exposing a frame with a steep slope

and one that at a height of more than 40 meters

is exposed to storms,

is not at all the same thing as building Romanesque frames

with very low slopes,

and that were positioned much lower.

You have to think of adapting the framework to the building.

The carpenters had to design a frame

that was both wind resistant

and compatible with very thin walls,

and they came up with an ingenious solution.

What's different about

the framework at Notre Dame

is that it's an experiment in triangulation,

an innovative technique at the beginning

of the 13th century,

which consists in transforming

the central piece into a spine,

so it will work in traction.

This central piece is compressed laterally

by two other beams,

so suspended like this,

the forces are drawn from the bottom to the top in traction.

Thanks to this suspension,

we will be able to converge a heavy load here at this point,

and that makes this framework that's perfectly balanced.

To achieve this triangulation,

the master carpenter of Notre Dame

will test different assemblies within the framework itself.

There are several ways

to achieve this triangulation in this framework here.

They were searching for the ideal static balance

in all the 13th century frameworks,

but the man in charge will have tested various assemblies

right here in this framework

until he found the most efficient triangulation possible.

So these trial and error experiments

will be reproduced identically

in the future framework of Notre Dame.

And the experiments showed

that over 850 years,

the whole structure has survived very well,

and that actually the basic form was excellent.

And that's really something.

It means that from the very start,

they had the idea of an initial form

that hasn't changed since, and it still works.

We'll be able to use it

when we're putting in the new frameworks.

The architects also decided

to rebuild the original framework of oak,

a material that's solid and long-lasting.

All that remained was to find the 1000 oak trees

they would need to do it.

95% of the wood used

in the frame of Notre Dame

was 25 to 30 centimeters at most in diameter.

These were trees with trunks that were relatively thin.

They were very slender and young as well,

at most 80 years old, on average between 60 and 80 years.

Contrary to popular belief,

the builders of the Middle Ages

didn't use centuries old oaks in their frames.

But on the contrary, young specimens that were tall,

fine and slender.

Rebuilding identically

isn't just reproducing a form.

It's also reusing techniques and know-how,

and it's also knowing how to reuse materials.

And that raises questions about

the quality of the wood that was used,

and what type of trees they were,

and what kind of forestry can produce wood like that?

It was in the forest of (indistinct)

in the south of France

that archeologist, Frederic Epuad

found the traces of a long gone type of forestry

that produced wood that was perfectly adapted

to building cathedrals.

It's like time travel

because this is a very special forest.

The oak composites are, I reckon about 50 or 60 years old.

I've never seen any others like them, it's really unique.

What you see here

is exactly what you would've seen

in a 13th century forest.

Very slender and fine oaks

How to explain this strange distribution?

60 years ago there was a big oak here

that was cut down low to the ground,

and from the stump that remained in the ground,

three shoots grew out,

and they became these three trees.

And these stump shoots grow together,

so they're in competition with each other.

There is no light from the side.

And since the forest here is very dense,

they'll be forced to grow very quickly to find the light.

And because there's no light getting through

into this thicket,

there won't be any side branches.

That's very good because in 50 or 60 years,

there'll be adult trees 10 meters tall

and with no knots in them, ideal for construction.

So the builders of the Middle Ages

knew how to cultivate the right trees

in order to supply their big cathedral construction sites.

With this kind of forestry,

it took three or four hectares at most

to supply a whole cathedral site.

So of course it goes against all the preconceived ideas

such as that we had to raise entire forests

to make cathedral frameworks,

and that it took extremely thick and very old trees.

This just isn't the case.

Forestry in the 13th century was closely linked

to the construction techniques of the day.

Back then, most frameworks were made

with squared off wood cut with an axe.

These very slim trees

were therefore pruned to a minimum,

and the bark was removed,

while preserving the natural shape of the trunk.

Squaring off was kept to a minimum

to preserve the heart of oak at the center of the beam.

And to preserve the wood itself

by following its thread

and leaving in all its sinuous curves.

Each beam in the framework of Notre Dame

corresponded to an entire tree,

and respected its sometimes curved shape.

It's no doubt in reference to the 1000 trees

that composed that frame that it was called the forest.

There's nothing better

in terms of resistance.

And it's also perfectly adapted to the frameworks

of gothic cathedrals.

We have structures like Notre Dame

that are exposed to winds or hurricanes

to a height of 40 meters.

So we need wood that's flexible,

and resistant, and that doesn't break.

And the scans of Notre Dame have clearly demonstrated

that the framework itself has bent.

The trusses have bent like that,

but they haven't broken.

None of the wood has broken.

None of its parts have come undone,

and that's directly linked to how these forests

were managed and how they were pruned.

We really have a perfect match

between the mechanics of the frames

that were intended across the centuries,

an understanding of the forest,

and a technical know-how perfectly adapted to the wood,

a total osmosis between nature, technique,

and the construction itself.

Frederic Epaud's investigation

has revealed the great mastery

shown by the medieval builders

Such an exceptional heritage will

Such an exceptional heritage

will therefore be rebuilt

as closely as possible to the original.

We don't restore identically,

the exact term is that

we make a reconstruction or a restitution.

We will reconstitute the frames

in a state that we think is a coherent state.

as they were back in the 13th century,

And we won't be putting back anything

that's been repaired since.

The final architectural element

that crowned a building, the framework,

also contributed to the overall flexibility

of its structure.

So with buildings like this it's a balance.

I wouldn't say that its unstable

but it's always standing like that.

so there must also be some flexibility above it.

The frames are just laid on the masonry,

they're not sealed to it at all.

The masonry moves and

the frame moves another way

because it's another material.

and all that has its own life.

A fabulous balance of forces

that the builders have created by pushing ever further

the limits of the material.

The tensions were pushed really very far

by the elevation, by the height, by the length.

It is in fact everything possible was exaggerated

to the breaking point,

to the point where if you went any further it wouldn't hold.

There's this extraordinary moment

when you create a work of art,

when you say, let's stop here.

It's still holding, and I won't go any further

because the whole thing's already a miracle.

And perhaps it's all thanks to science and thought,

but let's not tempt the devil.

So for me, harmony isn't a utopian vision,

it's a very concrete experience

of opposing forces in balance with each other.

Notre Dame de Paris

was a masterpiece of structure and proportion.

The builder's quest for evermore space and height

in order to let in the light.

(mellow music)

And we sometimes forget that architecture

isn't limited to its materials,

although they're necessary of course,

for the building to hold,

but art's about using those materials

to create the immaterial.

To produce that empty space

that will be a place of gathering,

a place of welcome, and a place of light

By losing its light and its acoustics

during the fire,

Notre Dame lost its immaterial dimension,

that part of the sacred that the whole world mourned,

a shared emotion that was symbolized by its iconic spire,

which no longer overlooks the rooftops of Paris.

How to restore their light

to these stained glass windows

covered with a thick lead dust,

and restore its acoustics

to a partially collapsed building?

How to restore the iconic status

to the world's most famous cathedral?

In short, how to give back Notre Dame its soul?

(mellow music)