Nova (1974–…): Season 47, Episode 16 - Saving Notre Dame - full transcript

Engineers race to rebuild the roof of the Notre Dame cathedral and secure the medieval structure within five years.

Are you wondering how healthy the food you are eating is? Check it -
Notre Dame de Paris...

A treasured icon
of Gothic architecture

and medieval engineering,

built from glass,


and timber over the course

of two centuries.

For 850 years,
this 226-foot-tall cathedral

has been an enduring symbol
at the heart

of French culture, and more...

Notre Dame is one of
humanity's greatest

artistic and architectural

Notre Dame is not just Paris.

It's France.

And beyond France,

it's the world.

But on April 15, 2019,

a disaster that threatens
to destroy it all strikes.

A massive fire
raging out of control...

Oh, my God!

Leaves the cathedral in ruins.

Now, an elite team of engineers,


and master craftspeople,

battle to save
this fragile structure

from a catastrophic collapse.

Out of tragedy,
an opportunity is born...


This is a dating fossil.

To solve archaeological

and understand the very fabric

of this medieval megastructure

like never before.

We can identify
each chemical element.

Can clues from the past

help save and rebuild
this landmark?

And can pioneering technology

prevent another disaster?

What we are producing today

will be the information usable
for the next generations.

"Saving Notre Dame"...

right now, on "NOVA."

Major funding for "NOVA"
is provided by the following:

The Cathedral
of Notre Dame de Paris...

an 850-year-old Gothic wonder.

It's the heart of France.

The distance from Paris
to all other places

is traditionally measured
from this iconic structure.

There is a continuation,

a historical continuation,

from the Middle Ages
to nowadays.

And it's very important
to build a kind of identity.

Notre Dame is one of
the monuments

which achieved this identity.

For Christians, it's a place
of worship, right?

And, and for those of us
with different beliefs,

it's one of just this incredibleartistic
and historic landmark.

You've had coronations there,

you've had the crowning of
Napoleon and King Henry.

There's just so much attached

to the cathedral.

But Notre Dame is much more
than that.

It's also a pinnacle
of medieval engineering.

The cathedral can hold
9,000 worshippers,

and its 100-foot tall walls

contain more than 32,000
square feet of stained glass.

The ceiling is a series
of domed Gothic vaults

that hold up the cathedral
from the inside.

A complex 550-ton web of timber

forms a cross-shaped roof,

topped with 1,300 lead tiles

and a 300-foot tall
central spire.

Wrapped around the church
are 28 flying buttresses,

limestone arches that brace
the walls from the outside.

And at the front,
two mighty towers,

with ten massive bronze bells

soar over 226 feet into the sky
over Paris.

The construction
took many generations.

Architecture was not learned
at the university,

so the architects and
allworkers learned mostly on site.

Along the way,
there were many setbacks.

In 1789, at the height
of the French Revolution,

anti-Catholic forces

destroy parts of the Cathedral.

A newly secular France
leaves Notre Dame

in a state of neglect.

But when Victor Hugo writes
"The Hunchback of Notre Dame"

in 1831, it sparks a $60 million

that tops out the cathedral
with a new roof

and a 750-ton
timber and lead spire.

Periodic renovations continue
to this day.

On April 15, 2019,

Notre Dame is wrapped
in 550 tons of scaffolding,

as workers begin

a $6 million operation
toshore up the cathedral's spire.

Notre Dame's rector,

Father Patrick Chauvet,

has finished evening worship.

His world is about to be
turned upside down.

I stopped here because

I really like Mado.

She offered me a drink, and
when she came back she said,

"Father, there's smoke above
the spire of the cathedral."

So I left my drink and went back
to check there was nobody

in the cathedral.

At 6:18 p.m.,
a sensor detects smoke

in the medieval roof timbers.

The system sends a coded fire
alert to the security team.

Instead of heading
straight for the roof,

a guard is dispatched to the
sacristy building nearby,

to check for a fire.

But he finds nothing.

He climbs up into
the church attic.

But by the time he gets there,
he's too late.

The fire has been burning
for almost 30 minutes

and has spread across the roof.

And there was this horrifyingly

huge plume of smoke billowing up
out of it.

It was surreal.

I'd never seen anything
like that before.

You saw the fire trucks come up

alongside the cathedral
and ladders went up,

and the hoses came out,
you could see that the ladders

were just too small
for a building of this size

and the hoses were not nearly
big enough

for this kind of blaze.

It was tragic; the resources

that were available
were not going to be

what was needed to bring
this thing under control.

A lot of us realized

that this fire was just going
to ravage the cathedral.

We saw what was happening,

but we were powerless,

we could do nothing.

It really looked like
the end of the world.

It was so chaotic.

A delay in responding
to a fire of this nature

is absolutely critical.

A small fire burning locally
is a very different thing

than ten minutes laterwhen
all of the timber elements

are involved.

So in a situation like this,

five, ten, 30 minutes can make
all the difference.

This delay will have huge

As firefighters arrive on scene,

so does one of France's
chief architects

of historic monuments,

Rémi Fromont.

I managed to pass
the police checkpoint

and I joined the firefighters.

As the inferno rages
at the top of the cathedral,

Rémi risks his life
to venture inside

with the firefighters.

We did a tour of the cathedral
several times.

We checked the nave.

I saw the flames
and saw the blaze.

I gave them all the advice
that I could.

Within minutes, the firefighters
are pumping tons of water

into the roof space,

but to no avail.

To the horror
of the growing crowd,

the fire engulfs
the iconic spire.

The world watches helplessly

as the 750 ton oak and lead
masterpiece gives way.

Oh, my God!

Oh, my God...

That is awful.

When the spire

fell into the roof,
additional ventilation

will have caused
more oxygen-rich air

to be sucked in at the bottom
of the compartment.

That influx of oxygen

could have caused an increase
in the severity of the fire

within Notre Dame.

All of a sudden, there was
a huge, huge ball of fire

rising out of the cathedral.

It was spitting ash and
debriseverywhere, so we took shelter.

It was just devastating
to watch.

We were suddenly really aware

that of, of how easily this
whole thing could come down.

90 minutes after the fire

the entire roof of the cathedral
is ablaze.

Inside, it's become even more
dangerous for Rémi

and the firefighters.

Getting this fire under control
looks impossible.

The fire on the ground,

smoke everywhere,
a hole in the ceiling.

We were trying to understand
what was going on,

where the problems where,
check what had collapsed

and if there were other risks.

A southeasterly wind picks up

and pushes the blaze towards
the famous bell towers.

If the bell towers catch fire,

and the bells fall,

then they will smash through
everything below.

Inside the ingeniously

13th century north tower

a scaffold of wooden beams
holds eight bells.

The biggest weighing more than
four tons.

If the beams burn through,

they'll spark a fatal
chain reaction,

causing the bells to fall
like wrecking balls,

destroying the tower's
wooden backbone.

If the tower falls, it could
trigger a deadly domino effect

that brings down
the entire cathedral.

To avert this
catastrophic collapse,

the firefighters have no option

but to venture deeper inside.

"No doubt,

we must send
the firefighters in.

The cathedral must be saved."

We headed to the North tower

just when the flames
had reached the belfry.

The firefighters also knew it

We were guiding each other.

To douse the fire on the roof,
firefighters pump water

from the River Seine
and feed it to fire trucks

around the cathedral.

But to stop the towerscollapsing
they must send a team

into the burning structure.

Drop hoses in between the towers

and fight the fire spreading
from the roof.

But the steady wind
doesn't let up.

And despite their efforts, the
timber frame holding the bells

has caught fire and could
trigger the destruction

of the cathedral at any moment.

So the team must drag their
hoses to the top of the tower

and soak the timber frame
to prevent the unthinkable.

Throughout the night,

the fate of Notre Dame
hangs in the balance.

Eventually the firefighters
get the upper hand.

The flames have been beaten back
and only glowing embers

light up the night sky.

Nobody knows
how the fire started.

An investigation begins.

But for now,
the urgent question:

how damaged is the structure

and can it ever be rebuilt?

President Macron pledges
to restore the cathedral

in five years.

Tonight, I tell you
very solemnly,

we will rebuild this cathedral

Meanwhile, the world keeps vigil
for Notre Dame.

Daylight reveals the full extent

of the terrible destruction
wrought by the fire.

The oak roof and spire
are completely destroyed.

Tons of toxic lead
that covered the roof

have been sprayed into the air,

contaminating the site.

Burned roof timbers
cover the vaulting.

Three gaping holes
in the stone vaults

weaken the entire structure.

And the 550 ton scorched carcass
of scaffolding

could collapse at any moment,

something unthinkableto
those tasked with preserving

France's rich cultural heritage.

I'm in front of my cathedral,

which is in this state.

I need to work.

Phillipe Villeneuve is in charge

of historic monuments in France.

This is the cathedral
that inspired him

to become an architect.

I must have been five

or six years old.

My parents brought me here
one day,

like every child from Paris.

I was fascinated
by the architecture.

It stayed with me since.

Since 2013,
Phillipe has been responsible

for conserving Notre Dame.

It was the culmination

of a dream.

A dream come true.

Today that dream has turned into
a nightmare.

The stricken cathedral
is a giant house of cards.

If the stone vaulting collapses

the weight of the buttresseswill
push in the 100-foot walls.

And Notre Dame will be no more.

So Phillipe heads up
a rapid response team...

dozens of engineers,

architects, and scientists.

Their task is to prevent a total
collapse of the cathedral.

From the bottom of my heart,

I want to thank you all for your
dedication, your approach,

your passion.

You are doing a very difficult
job, which is essential

for the cathedral.

It's not only a difficult job,

it's also hazardous.

The crumbling stone vaults

and twisted scaffolding
make any visit

inside to investigate the
stability of the structure

extremely dangerous.

On the vaults we have
the problem of the impact

of the fire, but we will
alsohave to evaluate

the impact of the water used
to put out the fire.

And we can see from here
the inside of...

Go out.

The scaffolding is moving.


Motion sensors are installed
in the melted jumble

of scaffolding overhead.

These can be triggered
by gusts of wind...

a warning before a possible
full-scale collapse.

It's the alarm, because the
scaffolding has moved.

We must leave.

There are evacuations like this
each week; necessary,

but an impediment
to the urgent work

of stabilizing the structure.

It's very difficult to juggle
all these issues.

The problem is that we have to
take action very quickly.

But we need to consider

the reality of this building.

It's still in danger
of collapse.

We are still in the
stabilization phase

of the cathedral.

To avert a catastrophic

engineers could builda
steel skeleton inside the nave

to brace the walls.

Then, even if the vaulting
caves in,

the walls of Notre Dame
would stay standing.

But it's far too dangerous
for workers to erect steelwork

beneath the compromised

We cannot go under the vaults
because we don't know

whether they'll fall or not.

So, instead of bracing the walls
from the inside,

the team will build timber

under the buttresses outside.

Now, if the vaulting does fall
in, the buttresses can't push

on the walls, and they won't
come tumbling down.

They are very difficult because

no flying buttress is identical
to another.

They are made to measure.

Workers at this factory race
to cut

and assemble around 250 tons
of timber

to create the massive supports
Philippe's team needs

to prop up the vaults.

It's critical each support fits

beneath each flying buttress

to hold its weight.

Working around and inside
this space

is a logistical nightmare.

210 tons of lead cladding
covered the cathedral roof.

This was mostly melted during
the fire,

and now toxic lead dust covers
every surface.

The worksite is highly

Until the site is cleaned,

team members must wear full
protective clothing

to pass into
the contaminated zone.

When leaving site,

they undress,

discard all clothing,

carefully wash equipment,

then shower themselves.

Only then can they go back
to the clean area

even for a lunch break.

It's very difficult to endure
for the workers

who have had to deal with these
procedures for months.

These regulations
are not normal.

But this whole site
is not normal.

But, finally, five months later,

all 28 flying buttresses
are locked in place

and the walls are safe.

Now they can turn
to the next challenge...

secure the melted mass of

that hangs precariously
over the cathedral.

The scaffold weighs more
than a jumbo jet,

and only rests on
four spindly legs.

The team plans to wrap three
massive steel lattice beams

around it to tie the fragile
upper parts together.

Then they'll build more
scaffolding either side

and lay steel beams across it.

That way workers can get inside
the stricken scaffolding

to help cut off its
50,000 steel poles,

a truly Herculean task.

Only then can the team put up
a temporary roof

to protect them from the

while they rebuild Notre Dame.

It's going to be an extremely
dangerous operation.

The spire has disappeared,

but the scaffolding is still

It moves a bit,
but it's still there.

While engineers gear up
to remove the scaffolding,

architect Rémi Fromont

and Livio De Luca

begin a groundbreaking project
that will combine

the investigative work
with new scientific analysis.

Their ambition is to create
adata-rich model of Notre Dame...

a digital twin.

The digital twin will embed not
only the geometric structure,

or the visual appearance
of the cathedral,

but also all the scientific data
coming from the studies.

For example, you can click
on a stone in the vault

and access to all
the information

about its physical properties
such as the provenance,

but also the mechanical behavior

within the entire structure.

Luckily for Livio,
a series of highly detailed

laser scans of the cathedralhave
been conducted since 2006.

These are brought together in
this priceless 3D dynamic map

to show every stone, timber,
and iron nail in the structure,

across time, from the12th
century to the present day.

This is an unprecedented

The ambition is to collect all
the information from the past,

to pass it to the future.

There's very little first-hand

about the construction
of Notre Dame,

or the craftspeople
who built it.

In the wake of the fire,

new studies of the cathedral's

could unlock these secrets.

This new data, once included in
the digital twin,

will provide a blueprint for
the restoration and rebuild.

Inside Notre Dame,

scientists begin to gather data
and investigate the damage

to treasured statues,


and windows.

The cathedral's most fragile
wonder, its stained glass,

dates back to the 13th century.

36 windows circled the
lower level,

42 around the middle level,
and 43 around the upper level.

The three famous Rose windows

span up to 42 feet in diameter

and are made up of over
1,100 panels

of beautiful stained glass.

they survive the fire intact.

But the intense heat that melted

the cathedral's
lead-covered roof

means that much of the glasswork

is now covered in a layer
of toxic lead powder.

Removing it could damage
the delicate glass

and be harmful to restorers.

It was really painful to see
the catastrophe on the TV.

I was looking to see what's
happen around the windows

and it was, of course,
totally difficult

to have a good idea
of what's happened.

There is a before and after
15 April,

for historical monuments,
that's for sure.

Glass scientist Claudine Loisel

uses a handheld digital

to investigate the levels
of lead powder

on the stained glass.

She must then formulate
a strategy

to clean every single panel;a
vast decontamination program.

This window is in the back
of the cathedral,

in the lower level,
furthest from the inferno.

But it's still badly

Fortunately, these windows
havenot been cleaned for 100 years,

so the lead has settled on top

of a dust layer,
not on the glass itself.

The first thick layer of deposit
was, we can say

has a small protection
in one way.

So we have just to remove all
the deposit,

to clean these windows
from the 19th century.

Claudine examines deposits from
windows around the cathedral.

The samples reveal vital clues

about the spread of the
lead contamination.

After the spire fell,

the cloud of dust, lead,
and different particle,

push in the other direction,

so we are a little bit more
protected in this area.

The windows of the upper level,
in the path of the lead cloud,

have been most contaminated.

The team takes out and
transports these panels

to this special laboratory

where they experiment with ways
to remove the lead.

First, Claudine uses a precision
vacuum cleaner

to remove the hundred years
of dust

and most of the lead powder
along with it.

So this is a good way to protect
the conservator.

You can control the action,
the pressure on the glass

and also on the painting.

Then she uses water
and cotton balls

to remove the last of the lead.

Of course,

you need scientific evidence
that it's working.

Claudine uses x-ray spectroscopy

to determine exactly how many
wipes it takes

to bring the lead down
to normal levels.

So we can identify each
chemicalelement we have in the material.

Too few wipes
and the lead will remain.

Too many wipes and restorationwill
take longer than necessary.


now the analysis is finished.

After five wipes,
Claudine checks to see

if the glass is decontaminated.

Okay, we have different chemical
element... calcium, iron,

and if we want to see
the lead...

there is no lead!

After nine months we can see

a good solution, a good way
to clean and to preserve

the stained glass windows
from Notre Dame.

The upper level windows
were not only in the path

of the lead cloud,

but also closest to the inferno.

Claudine hunts
for hairline cracks

caused by thermal shock,

the rapid heating and cooling
of the glass.

These cracks is due to the fire.

This is a recent cracks

and this is typical
thermal shock.

It looks like the upper level
stained glass

will need to be painstakingly
glued back together.

But inside Notre Dame,

the lower level stained glass

to have survived unscathed.

And here we can see we have
a good stability,

adherence of the painting,

so there is absolutely no
thermal shock,

that's good news for us.

On site,

the teams of scientists

meet the engineers and

to share their findings.

Once Claudine's team
hasrestored Notre Dame's glasswork

to its former glory,
they may use

a radical new preservation

to safeguard it for future

It's being used on a huge scale
here, in northern England.

This is York Minster,

an 800-year-old Gothic

and home to the largest expanse
of medieval stained glass

in the U.K.,

the Great East Window.

It is one of the largest windows
ever made

anywhere in the medieval world.

We've got glass from the
12th right through

to the 18th century in quite
significant quantities.

And it is really our nationaltreasure
house of stained glass.

Engineers here are completing
a $12 million project

to protect York Minster's
stained glass

from harmful UV rays and the
corrosive effects of moisture.

In modern stained-glass

we're really doing as much as we
can to keep

both surfaces of the historic
stained glass dry and stable,

and that's where our ventilated,

environmental protective
glazing comes into play.

You can see that I'm almost in.

I think it's just
this last bit here.

Matt Nickels is installing
this new conservation system.

He slots a protective clear
glass exterior frame

into the window opening.

This goes into the original
glazing groove,

where the medieval glass would
have been.

This protective
glazingprevents corrosive condensation

from forming on the 800-year-old
stained glass

that will sit behind it.

The gap created means that

there's air circulation running

And when you've got air

it's regulating the temperature,

which means that there's less
moisture on the glass.

Each frame is custom made
and takes great skill to fit.

You don't want to make it
too small

because it's going to obviously
slide through.

No two windows
are gonna be the same.

With the outer panel installed,

they can reinstate the layer
of medieval glass.

They're actually in fairly good

considering that they're early
13th century.

There's always the worry

you're handling glass like this,

but you just got to make sure

that you're really,
really careful.

There's nothing quite like

seeing it with sunlight
behind it.

When you put it up like this,
it's quite magical, isn't it?

Techniques like this

offer a glimpse of how
scientists like Claudine

may eventually preserve
Notre Dame's glass.

This is the best way to protect
stained glass windows,

so it will be for sure an option

to protect the windows for
Notre Dame.

Had the vaulting collapsed

next to the windows,

the glass could have been
badly damaged.

But luckily, the stone vaulting,

which sits just under the timber
and lead roof,

protected the windows
from the inferno above.

When the architects of the
Middle Ages

constructed this vaulting,

they used it to separate the
timber frame of the roof

from the rest of the cathedral.

So the vaulting took the shock
of the falling timber

and the fire
and the firefighters' water.

The magnificent vaulting was
built to be resilient,

thanks to precise medieval

using over a thousand cubic
yards of limestone.

The arches work together
to support the roof

and stabilize the outer walls.

But the intense heat
from the fire

and the collapsing spire

took out 15% of
the stone vaulting.

Today, three 40-foot-wide holes

and several smaller gaps
mean the vaults could collapse

at any moment.

The team collects, storesand
catalogues the fallen stone

in this tent, located
alongside the cathedral.

They may be able to use
some of this stone

to reconstruct the vaults.

But it's clear they'll also need
to source new stone.

Notre Dame is made up of many
different types of limestone.

Medieval masons chose hard
limestone for the towers,

pillars, and outer walls
tobuild tall and hold up the roof.

For the sculptures, they chose
dense, fine-grained limestone,

that can be carved with
great detail.

And for the vaults they selected
softer, more porous limestone

that's light but strong.

If the team rebuilding
the vaults

pick a limestone
that is too heavy,

the new vaults may not last as
long as they should.

Geologist Lise Leroux

investigates what quarry this
stone came from.

We have some blocks coming from

the collapse of the vault
for study.

This detective work will help
the team source

replacement stone that
sharesidentical mechanical properties.

We have to verify.

The fallen vaulting stone
contains a rare micro-fossil

called orbitolites complanatus,

a kind of plankton.

Fossils like this are found in
just one layer of rock.

This will make sourcing
new stone

of the same type even trickier.

Can they use this geological

to discover the original source
of the vaulting stone?

To find out, Lise and fellow
Notre Dame scientist

Claudine Loisel venture deep
beneath Paris.

Hidden under the city streets is
a rich source of limestone,

a vast labyrinth of
quarry tunnels.

Lise and Claudine
enter this maze

two miles south of Notre Dame in
the famous Catacombs.


In the late 18th century,

the quarries were given a
different purpose

and they housed bones from
old cemeteries,

which were inside the towns.

Cemeteries which were closed at
the end of the 18th century

for sanitary reasons.

Among the bones,

Lise and Claudine find traces
left by the medieval miners.

Here, the block's been removed

and we're left with this trace.

They then square off the sides,

and use it to build Notre Dame.

And the strata height here,
itdictates the height of the block

that can be extracted.

The blocks we see at Notre Dame
have this height.

So the quarry itself puts
a constraint

on the construction
of Notre Dame.

We have life and we have death.

Well, yes.

The upper level of the quarry

holds hard limestone with

large, well-preserved fossils.

These fossils are

more characteristic of
the limestones

used for the pillars,
the arch in Notre Dame.

But not for the vault.

Lise and Claudine hope to find

a match for
the soft vaulting stone

in the lower level
of the quarry.

Now to look if we can find

the specific micro-fossils.

I'm not sure, because
the surface is very rough

and it's not so clear because
ofall of the state of the surface.

The limestone here is softer,

but Lise cannot see a match
for the rare micro-fossil

found in the Notre Dame
vaulting sample.

So, back in the lab,

she takes a closer look
at a sample of limestone

from the lower level
of the quarry.

These little fossils...

this one, this one,

this one...

are, in fact
some planktonic fossils,

which are called foraminifera.

It's not the fossil signature
she's looking for.

But then...


This one here
is orbitolites complanatus.

This little planktonic fossil
is a dating fossil,

which match with the stone
coming from the vault.

It's a stratigraphic indicator,

characteristic from
the Middle Lutetian,

which is a geological
age of deposit.

Lise confirms the origin of
the Notre Dame vaulting stone.

It's quarried
from the deepest seams

of limestone beneath Paris.


But what about the harder

used by medieval masons to build

Notre Dame'sload-bearing
pillars and arches?

Another micro fossil signature
confirms the origin

of this type as well.

The arches are built
with a hard stone...

with a resistant stone,
to support the vault.

And the vault itself

is logically constructed

with a lighter,
more porous stone.

And in the quarry located
in Paris,

we have this two kind of stone.

Medieval masons knew exactly
how to exploit

the varying
mechanical properties

of the limestone for Notre Dame;

knowledge passed down
through the generations.

Sourcing more of the
correct stone won't be easy...

the old quarries
are no longer active.

But engineers now know

what limestone to look for...

this will help them find a match
in quarries outside Paris.

Stone is not
the only raw material

that will need to be replaced

as engineers reconstruct
Notre Dame.

The timber roof
was also a medieval wonder.

It was constructed from
25,000 cubic feet of timber,

cut from 52 acres of oak...

that's approximately
1,300 trees.

For this reason,
it was known as "the forest."

Every single oak
in Notre Dame's forest

was handpicked for
the physical properties needed

in the roof structure...

from dense straight oak
for pillars,

to curved oak
for support arches.

But the fire burned
every beam in the forest.

Today, this intricate 550-ton
timber jigsaw lies in ruins.

We thought this sublime roof
would be here forever.

It was a big puzzle with beams

from different periods,

all the way back
to the 13th century.

And to see it suddenly

all burned, all mixed up...

Oh, it's very emotional.

It's very difficult.

Almost 60 tons of the
precious roof timber

still lie precariously
on top of the vaults.

Despite the destruction,

every single beam holds
the history of Notre Dame.

It has deep
archaeological value.

It's vital that workers
forensically record

the position
where each beam fell,

before they remove them.

This helps them determine

where it originally sat in
the roof structure.

Now, these highly trained
rope access technicians

gear up to catalogue and clear

the charred timber
on the vaults.

It's not possible
to walk on the vaults,

because the structure
is very precarious.

They needed to

create a way
to access with ropes.

We need to wear a special mask

because of the lead dust
that we might inhale.

We label the timbers

and we mark them with a code

that the architects will

be able to identify.

The team has their work
cut out...

there are thousands
of separate pieces

of timber to catalog.

We are working day and night.

We have a lot of work to do.

They've already extracted

around 4,000 pieces.

Timber scientist
Catherine Lavier

begins painstaking
detective work to reveal

how Notre Dame's vast forest

was originally assembled

and could be rebuilt today.

Some pieces were
very well-preserved

because as you see here,
with different faces

and another piece of wood is
coming here,

with a wooden joint
here to assemble them.

And it's rather typical
from the medieval period.

And here,

you have a mark,

of carpenters.

So they are sure that this piece
with this piece are together.

It's very important
for carpenters.

They prepare
the wood on the ground

and after that,

they go to the roof
and reassemble again.

Every carpenter
has his own way to mark,

but in general it's based on

the Roman numbers,

but we can find some differences
between teams of carpenters.

We were very surprised
to find that

because I thought
everything will be destroyed.

And, finally, not.

The tree rings of the timbers
conceal further clues.

Each ring represents
one year of growth;

a time capsule of information

about the life of
the tree in that year.

Catherine analyzes core
samplesfrom Notre Dame's roof trusses.

She measures each ring
to reveal the secret story

of some of
the original oak trees

the structure was made from.

This screen shows the size

of each ring I measured.

At the start of its life,

you see it has
very, very large rings,

which correspond
to very rapid growth.

Next, it looks like it

some more dramatic events,

some difficult years,

here, when the rings
are very thin,

This could be because of
too much rain,

not enough sun,
and not enough nutrients.

And then,
the life of the tree continues

until it's cut down,
around its 96th year.

Catherine is gaining new insight

into the types of trees
best suited to rebuild

the complex forest
of Notre Dame.

This extraordinary challenge
will require

around 1,300 oak trees,

craftspeople versed
in the lost art

of medieval carpentry practices,

and a blueprint for possibly
the most geometrically complex

timber structures in Europe.

The one person who can unlock

the lost forest's
geometrical secrets

is architect Rémi Fromont.

In 2014,
Rémi spent the entire year

mapping every inch
of the timber.

It was a magical place

to go in there;
there was a smell.

There was a very special
atmosphere of light.

We still had the traces of tools
also on the woods.

It sometimes seemed like
they only left yesterday.

We are collecting photographs,

3D point clouds,

and the physical
and chemical characterization

of all the materials.

The fire at Notre Dame

triggers a race across France

to 3D scan historical monuments,
inside and out.

These represent
a digital insurance policy

to preserve French heritage.

The laser bounces off
each contour in the room.

The machine then measures

the time it takes
for the laser to return.

Millions of measurements

form a cloud of data

called a "point cloud."

In 2016, researchers used
this same technology

to create a full point cloud

of Notre Dame's
lost timber roof structure.

This remarkable 3D scan
willcombine with Rémi's 2014 survey,

in Livio's digital twin
for Notre Dame.

What we are producing today

will be probably
the information useful

for the next generations.

The team now has
the data they need

to rebuild the timber roof
with the exact same geometry.

The new oak needed

could come from
forests like this.

Almost a third of France

is covered with forest.

Oak is a vital
strategic resource

throughout theMiddle
Ages and the Renaissance.

Vast forests are needed
tobuild cities and expand navies.

This is the Château de
Beaumesnil in Normandy.

It's a
National Historic Monument,

built on the site of

an 1,100-year-old castle.

It was built in seven years.

It's something extraordinary
for just seven years' work.

The château has seen
better days.

The curved beams
that hold up the roof

are close to collapse
and must be replaced.

The wood grain has been
cut through.

This weakens the support beam.

And then you see that the beam
is completely eaten away.

The wood is degraded,
eaten by the fungus.

The restoration work here

requires much of
the same skill and knowledge

it will take to rebuild
Notre Dame's lost forest.

The timber has been chosen
so the curve of the grain

perfectly matches
the curve of the new beam.

If you get a straight tree,

which has a straight grain,

if you cut a curved piece,
piece of wood inside of this,

so here is the fiber,
so it can break, right there.

But if you take
the tree that's curved,

the fiber is like this.

So it cannot break.

You keep all of thestructural
strength of the tree.

The carpenters use

an original beam as a template

to mark out the new beam
on the oak.

The carpenters who built
Notre Dame

would be familiar with
the tools this team uses

to hew the raw timber.

So after you split
most of the wood,

you use a broad-axe.

They have a single bevel,

long cutting edge,
and the handle is offset.


if you're working,

as you go down, your hand here,

you see I'm not
hitting this sharp edge.

For skilled carpenters,

cutting Notre Dame's
roof timbers with axes,

compared to a modern sawmill,

will take roughly
twice the time;

possibly too long.

This curved oak will be

one of ten the team
needs to install

as part of
the château roof restoration.

It sits alongside this
400-year-old original beam.

This one was cut

probably 1635, '37,

and then this one 2020.

I hope our ancestors are
happy with this.

Just like the Notre Dame beams,

the Château's
original beam holds

messages from
the old carpenters.

It's extraordinary to find
all these marks.

It's very old
and at the same time,

it looks like
it was done yesterday.

French craftspeople

have the oak,

they have the skills, and
they have the plans required

to reconstruct Notre Dame's
vast forest of roof timbers.

It's over a year since the fire
ravaged Notre Dame cathedral,

and the investigators
have not pinpointed

the cause of the blaze.

Immense challenges and
uncertainties still lie ahead.

The building
is not yet out of danger.

Over the next 12 months,

engineers must remove
the melted scaffolding

and seal the cathedral roof
to make it watertight,

then stabilize
the weakened vaulting.

It's a monumental task.

And rebuilding
the entire cathedral

could take much longer than

the five years decreed by
President Macron.

Faced with such a drama,

thankfully there's hope.

We need faith for this project.

It's this building itself
that generates this faith...

even for atheists...
And that's something magical.

Architects around the world

have unleashed
their imaginations

to submit grand plans for what
the new spire above Notre Dame

could look like...

from mirrored roofs
with kaleidoscopic pinnacles,

and vast solar panels
powering nearby buildings,

to stained glass edifices
thatwill light up the Paris skyline.

However Notre Dame is rebuilt,

the unique collaboration
of architects and scientists

is rewriting how we understand

the very fabric of
this magnificent cathedral.

I think the fire in some ways

helped remind a lot of people

what an important part
of our sort of shared history

and shared culture this is.

Soon, a complete digital twin
of Notre Dame

should allow future generations
of craftspeople

to maintain, protect, and
faithfully rebuild Notre Dame,

preserving this world treasure
for all time.

I have only one obsession...

save the cathedral,
resurrect it,

and reopen it to the public.

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