All language subtitles for Rebuilding Notre Dame - 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 3-D 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.