All language subtitles for Rebuilding Notre-Dame.s01e01.Inside the Great Cathedral Rescue

Notre-Dame de Paris, a treasured icon
of Gothic architecture

and medieval engineering.

Built from glass,

stone

and timber over the course of two
centuries,

it's one of the world's greatest
cathedrals.

- Notre-Dame is one of humanity's
greatest

artistic and architectural
achievements.

- This epic 90-metre-tall symbol has
stood

at the heart of French culture and
society for 850 years.

SPEAKS IN FRENCH

But on 15th April 2019,

disaster strikes.

PEOPLE SCREAM AND SHOUT

A massive fire rages out of control
and tears through the cathedral...

..leaving it in ruins.

Now, a team of master craftspeople and
elite engineers

battle to save this fragile structure

from a catastrophic collapse.

SHE SPEAKS IN FRENCH

Out of tragedy is born opportunity...

- Oh!

- ..to solve archaeological mysteries
and understand the very fabric

of this medieval megastructure like
never before.

- We can identify each chemical
element.

- Yes, it is important information.

- For the last year, our cameras have
had unique access

to follow teams inside Notre-Dame.

Can they decode clues from the past
and use pioneering technology

to rebuild this historic landmark?

This is the inside story of the first
year in the race

to save Notre-Dame Cathedral.

The Cathedral of Notre-Dame de Paris,

an 850-year-old Gothic wonder.

It's the heart of France.

Roads were traditionally measured from
this iconic structure.

- There was 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 for those of us with different
beliefs,

it's one of...just this incredible
artistic and historical landmark.

You've had coronations there,

you've had the crowning of Napoleon
and King Henry

and there's just so much attached to
the cathedral.

- But Notre-Dame is much more than
that.

It's also the pinnacle of medieval
engineering.

The cathedral can hold 9,000
worshippers

and its 33-metre-tall walls

contain more than 3,000 square metres
of stained glass.

The ceiling is a series of domed
Gothic vaults

that hold up the cathedral from the
inside.

A complex 500-tonne web of timber
forms a cross-shaped roof

topped with 1,300 lead tiles...

..and a 90-metre-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 inside,

soar over 68 metres into the sky over
Paris.

- The construction took many
generations.

Architecture was not learned at the
university.

So, the architects and all workers
learned mostly onsite.

- 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 £170 million restoration

that tops up the cathedral with a new
roof

and a 750-tonne timber and lead spire.

And periodic renovations continue to
this day.

On 15th April 2019, Notre-Dame is
wrapped in 500 tonnes of scaffolding

as the cathedral begins a £5 million
operation

to shore up the spire.

THEY SPEAK IN FRENCH

Notre-Dame's rector, Monsignor Patrick
Chauvet,

has finished evening worship.

His world is about to be turned upside
down.

HE SPEAKS IN FRENCH

At 6.18pm, 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 spread across the roof.

SIREN BLARES

- 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 realised that this fire

was just going to ravage the
cathedral.

- SPEAKS IN FRENCH

- 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 later when 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
repercussions.

As firefighters arrive on scene,

so does one of France's chief
architects of historic monuments,

Remi Fromont.

HE SPEAKS IN FRENCH

As the inferno rages at the top of the
cathedral,

Remi risks his life to venture inside
with the firefighters.

Within minutes, the firefighters are
pumping tonnes of water

into the attic space, but it's not
working.

To the horror of the growing crowd,
the fire engulfs the famous spire.

The world watches helplessly

as the 750-tonne oak and lead
masterpiece gives way.

- Oh, my God.

Oh, my God, that is awful.

- When the spire fell into the roof,

an 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.

- It was just devastating to watch.

We were suddenly really aware
that...of how easily

this whole thing could come down.

- 90 minutes after the fire begins,

the entire roof of the cathedral is
ablaze.

Inside, it's become even more
dangerous

for Remi and the firefighters.

Getting this fire under control looks
impossible.

A south-easterly wind picks up

and pushes the blaze towards the
famous bell towers.

Inside the ingeniously engineered
13th-century north tower,

a scaffold of wooden beams holds eight
bells,

the biggest weighing more than four
tonnes.

If the beams burn through,

they'll trigger a fatal chain
reaction.

The bells will fall like wrecking
balls

and destroy 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.

REMI:

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 towers collapsing,

they must send a team into the burning
structure.

Their mission -

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.

HE SPEAKS IN FRENCH

PEOPLE SING TOGETHER

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.

Toxic lead that covered the roof has
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 500-tonne scorched carcass of
scaffolding

could collapse at any moment...

..something unthinkable to those

tasked with preserving France's rich
cultural heritage.

SPEAKS IN FRENCH

Philippe Villeneuve is in charge of
historic monuments in France.

This is the cathedral that inspired
him to become an architect.

Since 2013, Philippe has been in
charge of conserving Notre-Dame.

The stricken cathedral is a giant
house of cards.

If the stone vaulting collapses, the
weight of the buttresses

will push in the 33-metre-high walls
and Notre-Dame will be no more.

So, Philippe assembles a rapid
response team,

dozens of engineers, architects and
scientists.

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

It's not only a difficult job,

it's also hazardous.

The crumbling stone vaults and twisted
scaffolding

make even venturing inside

to investigate the stability of the
structure

extremely dangerous.

SHE SPEAKS IN FRENCH

ALARM BLARES

SHE SPEAKS IN ENGLISH

Motion sensors are installed

in the melted jumble of scaffolding
overhead.

These can be triggered by gusts of
wind

or signal a full-scale collapse.

There are evacuations like this each
week that slow progress.

To avert a catastrophic collapse,
engineers could build

a 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 structure.

So, instead of bracing the walls from
the inside,

the team will build timber frames
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 SPEAK IN FRENCH

Workers at this specialised factory
race to cut and assemble

around 225 tonnes of timber

to create the massive supports

Philippe's team needs to prop up the
vaults.

It's critical each support fits
perfectly

beneath each flying buttress to hold
its weight.

Working around and inside this space
is a logistical nightmare.

200 tonnes of lead cladding covered
the cathedral roof.

This was mostly melted during the fire

and now toxic lead dust covers every
surface.

The work site is highly contaminated.

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.

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 massive scaffolding

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.

This 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 elements
while they rebuild Notre-Dame.

While engineers gear up to remove the
scaffolding,

architect Remi Fromont and Livio De
Luca

begin a ground-breaking project

that will combine the investigative
work with new scientific analysis.

Their ambition is to create a
data-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 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 behaviour
within the entire structure.

- This 3D dynamic map will show every
stone, timber and iron nail

in the structure across time from the
12th century

to the present day.

- This is an unprecedented project.

The ambition is to collect all the
information

from the past to pass it to the
future.

- There's very little first-hand
information

about the construction of Notre-Dame,

or the craftspeople who built it.

In the wake of the fire, new studies
of the cathedral's materials

could unlock these secrets.

This new data captured 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, murals and
windows.

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

dates back to the 13th century.

36 windows circle the lower level, 42
around the middle level

and 43 around the upper level.

The three famous rose windows span up
to 13 metres in diameter

and are made up of over 1,100 panels
of beautiful stained glass.

But the intense heat from the fire

that melted the cathedral's
lead-covered roof

means that today, much of the
remaining glasswork

is covered in a layer of toxic lead
powder.

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

I was looking to see what's up and
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 15th April
for historical monuments,

that's for sure.

- Glass scientist Claudine Loisel uses
a hand-held digital microscope

to investigate the levels of lead
powder on the stained glass.

She must then formulate a strategy to
clean every single panel.

It's a vast decontamination programme

that will make the glass safe for
restorers to begin work.

This window is in the back of the
cathedral in the lower level,

furthest from the inferno,

but it's still badly contaminated.

CLAUDINE SPEAKS IN FRENCH

Fortunately, these windows have not
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,

as 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

pushed 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 hoover up the 100 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 meticulously remove the last of the
lead.

- Of course, you need scientific
evidence that it's working.

- Claudine uses X-ray spectroscopy to
measure exactly how many wipes

it takes to bring the lead down to
normal levels.

- So, we can identify each chemical
element we have in the material.

- Too few wipes and the lead will
remain.

Too many wipes and restoration will
take longer than necessary.

- OK, now, this analyses if it's
finished.

- After five wipes with the distilled
water,

Claudine checks to see if the glass is
decontaminated.

- OK, we have different chemical
elements,

calcium, iron.

And if we want to see the lead, there
is no lead.

- CLAUDINE LAUGHS

- 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.

- This is one window from the choir

and we have two panels from the middle
of the...bay.

But here we have a crack.

- The hand-held digital microscope

helps Claudine see deep into the
crack.

If it's caused by the fire, it will be
clean and fresh.

If it's older, it will be full of
residue.

- We can see the well-defined line of
the crack.

We have no residue,

so, we can say this crack is due to
the fire.

This is a recent crack 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

appears to have survived unscathed.

- And, yeah, we can see we have a good
stability

around of the painting.

So, there is absolutely no thermal
shock.

So, that's good news for us.

- On site, the teams of scientists
meet the engineers and architects

to share their findings.

CLAUDINE SPEAKS IN FRENCH

Once Claudine's team has restored
Notre-Dame's glasswork

to its former glory,

they may use a radical new
preservation technique

to safeguard it for future
generations.

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

This is York Minster,

an 800-year-old Gothic masterpiece

and home to the largest expanse of
medieval stained glass in the UK,

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 national treasure
house of stained glass.

- Engineers here are completing a £10
million project

to protect York Minster's stained
glass from harmful UV rays

and the corrosive effects of moisture.

- In modern stained glass
conservation,

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.

- Right, I 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.

- And this goes into the original
glazing groove

where the medieval glass would have
been.

- This protective glazing prevents
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 through,

and when you've got air circulation,
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 going to be the
same.

- With the outer panel installed,

they can reinstate the layer of
medieval glass.

- They're actually in fairly good
condition

considering that they're early 13th
century.

There's always the worry whenever
you're handling glass like this,

but you've 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 a
stained glass window.

So, it will be, for sure,

an option to protect the windows from
Notre-Dame.

- By an incredible stroke of luck,

the three spectacular rose windows of
Notre-Dame,

that date from the 13th century,

survived the fire intact.

- It was a great relief to see all the
panel in place,

in good stability.

So, it was really, "Phew, everything
is OK."

- 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.

The magnificent vaulting was built to
be resilient,

thanks to precise medieval
craftsmanship

using an estimated 800 cubic metres of
limestone.

The arches work together to support
the roof

and stabilise the outer walls.

But the intense heat from the fire and
the collapsing spire...

..took out 15% of the stone vaulting.

Today, three 12-metre-wide holes and
several smaller gaps

mean the vaults could collapse at any
moment.

The team collects, stores and
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 medieval masons never intended the
stone to be subject

to an 800-degree inferno, then soaked
with tonnes of water.

Now, without a roof to protect it,

areas of the vaulting are exposed to
the elements.

Stone scientist Jean-Didier Mertz
forces mercury at high pressure

through samples of the damaged
vaulting

to calculate the porosity of the
stones.

This tells him how sponge-like the
stonework has become

one year on from the fire.

- The damage produced some cracks due
to the fire.

With these cracks, it is possible to
the water

to go in and to fill all the cracks.

- Jean-Didier confirms the surface of
the vaulting stones

are covered with micro cracks,

dramatically increasing the porosity.

Greater porosity means the vaulting
can absorb more water

than they thought, potentially
swelling the blocks.

As the blocks dry, they then shrink.

Jean-Didier has dried this vaulting
stone under lab conditions.

He knows the block holds two kilos of
water

and takes six months to fully dry.

- The vault of Notre-Dame, it is not
exactly the same

because the cathedral is not
completely sealed.

And with new rainwater

and the repeated cycle of swelling and
shrinkage,

it is a real decay...

..the weathering of the material.

- Even if some fallen vaulting stone
can be reused,

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 to build 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
picks 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 shares identical mechanical
properties.

- We have to verify...

- The fallen vaulting stone contains a
rare microfossil

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
fingerprint

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.

- Ah!

- Ah, oui!

- 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.

- Amongst the bones, Lise and Claudine

find traces left by the medieval
miners.

- D'accord.

Mm.

- The upper level of the quarry holds
hard limestone

with large, well-preserved fossils.

- These fossils are more
characteristic of limestones

used for the pillars,

the arch in Notre-Dame.

But not for the vaults.

- Lise and Claudine hope to find a
match for the soft vaulting stone

in the lower level of the quarry.

- We now...to look if we can find the
specific microfossils.

I'm not sure because on the surface
it's very rough

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

- The limestone here is softer,

but Lise cannot see a match for the
rare microfossil

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...

- Oh!

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
Oligocene,

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.

- Conclusive.

- But what about the harder limestone
used by medieval masons to build

Notre-Dame's load-bearing pillars and
arches?

Another microfossil 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 in itself is logically
constructed

with a lighter, more porous stone.

And in the quarry located in Paris,

we have these two kinds 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 700 cubic
metres 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 hand-picked

for the physical properties needed in
the roof structure,

from dense straight oak for pillars,

to curved oak for support arches.

But the fire consumed every beam in
the Forest.

Today, this intricate 500-tonne timber
jigsaw lies in ruins.

SPEAKS IN FRENCH

Over 50 tonnes of the precious roof
timber 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.

These highly trained rope access
technicians

catalogue and clear the charred timber
on the vaults.

THEY SPEAK IN FRENCH

- 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 catalogue.

- We have a lot of work to do.

That's why we're working day and
night.

- 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 analyses core samples from
Notre-Dame's roof trusses.

She measures each ring to reveal the
secret story

of the very oak trees the structure
was made from.

SHE SPEAKS IN FRENCH

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 complex timber structure in
Europe.

The one person who may possess the key
to unlock

the lost Forest's geometrical secrets
is architect Remi Fromont.

In 2014, Remi spent the entire year
mapping every inch of the timber.

- We are collecting photographs...

- 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 will combine
with Remi's 2014 survey

in Livio's digital twin for
Notre-Dame.

- What we are producing today will be
probably the information

usable for the next generations.

- The team now has the data they need
to rebuild the timber roof

with the exact same geometry.

And the 500 tonnes of new oak needed

could come from forests like this.

Almost a third of France is covered
with forest.

Oak was a vital strategic resource

throughout the Middle Ages and the
Renaissance.

Vast forests were needed to build
cities and expand navies.

Felling and carving oak for
Notre-Dame's roof

requires carpenters skilled in using
medieval tools and techniques

like this team who keep the old ways
alive to this day.

- We rebuild buildings using only hand
tools

and no power tools at all.

Using this kind of tool, you don't
apply your vision on the tree.

You...you have your work influenced by
the shape of the tree itself.

- Today, carpenter Leo Rousseau has
the honour of felling his first tree

using techniques from the Middle Ages.

- Ah, yeah, it's like everything, it
takes practice.

You don't have to go to the sports
room at night.

You don't want your tree to fall into
another trees,

because you want to use the other
trees for the next generations.

So, the idea is to create here a hinge
that gives you

the direction where you want the tree
to fall.

And then you cut from the back for the
tree to fall.

- THEY SPEAK IN FRENCH

Leo now has a taste of what it took
for the Notre-Dame carpenters

to source the 1,300 oaks for the roof
timbers.

- I like the feeling that you have to
put so much effort into one tree.

It gives you a little bit of respect

on what you're going to do with that
particular beam or...

Yeah, it's a different feeling.

- Carpenters are using the old ways to
restore

the Chateau de Beaumesnil in Normandy.

It's a national historic monument

built on the site of an 1,100-year-old
castle.

SPEAKS IN FRENCH

The chateau has seen better days.

The curved beams that hold up the roof
are close to collapse

and must be replaced.

SHE SPEAKS IN FRENCH

The carpenters here face many of the
same challenges

that Notre-Dame restorers will need to
overcome

when they rebuild the cathedral'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

and then you get a curved piece of
wood inside of this...

So, here is the fibre so it can break
right there.

But if you take the tree that's
curved,

the fibre is like this, so it cannot
break.

You keep all of the structural
strength of the...of the tree.

- They use an original beam to produce
a template

and 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, we would start with these axes.

You know, it's a long handle, which
gives you a long swing.

Because if you want to make notches
cutting deep in the wood,

so you want a long handle to give you
momentum and use the weight

of the tool itself to basically do the
job.

And after you've split most of the
wood,

you use a second type of axe, which is
called a broad axe.

They have a single bevel, long cutting
edge

and the handle is offset.

So, if you're working as you go down,

your hand here, you see I'm not
hitting the...this sharp edge.

- For skilled carpenters, cutting
Notre-Dame's roof timbers with axes

compared to a modern sawmill would
take roughly twice the time.

This curved oak will be one of ten

the team needs to install as part of
the chateau roof restoration.

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

HE SPEAKS IN FRENCH

Just like the Notre-Dame beams, the
chateau's original beam

holds messages from the old
carpenters.

French carpenters have the oak,

they have the skills

and they have the plans required to
reconstruct

Notre-Dame's vast Forest of roof
timbers.

While the architects consider options
for rebuilding the roof,

they must also work out what
innovations to engineer

into the structure to safeguard it
against future fires.

Scientists here at the University of
Edinburgh School of Engineering

use scale models to investigate

how fires spread through timber
structures

like the Forest of Notre-Dame.

- Our job is to understand the
underlying physics

and then to use our understanding of
the physics to create strategies

by which people can be more safe

and assets can be more safe from fire.

- As Luke ignites a single beam

in this model of a timber roof
structure...

- Ignition.

- ..it becomes clear how quickly the
energy radiates

to the timber beam alongside until it
too catches fire.

This phenomenon is called reradiation.

- In a structure like a roof at
Notre-Dame,

what we had is a collection of timber
members in a nice pattern,

all kind of looking at each other as
they burn.

And that fact that they're looking at
each other,

they're radiating at each other,

is what causes the fire to grow and
escalate

in such a dramatic way.

If you create a gap or a firebreak

within your network of timber
elements,

you can prevent the combustion from
occurring

on the far side of the firebreak.

- To stop the spread of fires like
this,

today, engineers build fire-separating
partitions

into the roofs of historic buildings.

These firewalls are erected inside the
roof of York Minster.

In the event of a fire,

they should prevent the reradiation
and transfer of energy.

Innovations like this could help
safeguard Notre-Dame's new roof

against future threats.

One year after the fire that ravaged
Notre-Dame Cathedral,

investigators have still not
pinpointed the cause of the blaze.

And 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 they can stabilise the weakened
vaulting.

It's a monumental feat, and rebuilding
the entire cathedral

could take much longer than the five
years decreed by President Macron.

Architects around the world have
unleashed their imagination

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 that will
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...

..as many times as necessary.