All language subtitles for BBC.Rebuilding.Notre-Dame.The.Next.Chapter.1080p.HDTV.x265.AAC.MVGroup.org

Notre-Dame de Paris.

A treasured icon
of Gothic architecture

and medieval construction.

The feat of engineering,
in those days - just extraordinary.

It's one of the most important
monuments in France.

But on the 15th of April 2019,
disaster strikes.

A huge fire
rips through the cathedral...

Oh!

Oh, no!

..reducing the roof and spire
to ashes.

For the last three years...

..our cameras have been following

teams of master craftspeople
and engineers,

as they battle to bring
Notre-Dame back to life.

Wow!

This is amazing.

I've been given
an astonishing opportunity

to visit the work site
here at Notre-Dame,

and this is such a critical time
for the team.

They've met the challenge
of making the structure secure,

and now they're moving on
towards rebuilding it.

Hundreds of workers here
are locked in an ambitious race

to restore
this medieval masterpiece

in time for a grand reopening,

ahead of the Paris Olympics
in 2024.

Ooh, what have you got here?

As we track their progress,

we'll also meet the historians
and scientists

working to solve Notre-Dame's
architectural mysteries.

Have you seen this before?
No. No, never.

It's a kind of walking backwards
in time.

What's this lady doing?
Is she riding upon a dragon?

They're revealing
ancient technology,

hidden for nearly 800 years.

We're dealing
with unknown structures

that are, so far,
unique in Gothic architecture.

..and struggling to save
the fragile fabric of the building.

The loss of matter
is catastrophic for us.

Three years into this extraordinary
five-year restoration project...

..this is the next chapter in the
race to save Notre-Dame Cathedral.

Paris.

A city of churches, basilicas,

and almost 2,000 historic monuments.

On the 15th of April 2019,

one of the city's most
famous buildings,

the 850-year-old
Notre-Dame de Paris,

was undergoing a £5 million
restoration of its spire.

A fire broke out inside
the oak framework of the roof.

After 90 minutes,

the 360-tonne timber-and-lead spire
gave way

and crashed through
the stone vaulting...

..causing catastrophic damage.

It's only thanks to the bravery
of the firefighters that night

that anything of the cathedral
survives at all.

And for weeks
and for months after that,

its future
still hung in the balance.

The roof and spire
are completely gone,

and there are three gaping holes
in the vaulting.

Teams of engineers
raced to install supports

to shore up the fragile structure.

But the cathedral
remained open to the elements.

Rainwater saturated the vaults,

adding weight to
the weakened stonework,

and increasing the chance
of a total collapse.

Before they could make
the structure watertight,

they had to remove 40,000 burned
and melted scaffolding poles

left over from
the spire restoration.

Now they've built
a temporary sliding roof -

it's called "The Big Umbrella" -
to keep the stonework dry.

The cathedral
looks very different today

than it did when I first visited.

I'm historian Lucy Worsley,

Chief Curator
at Historic Royal Palaces.

I first came here aged 16 -
French exchange trip.

Here's a picture.

Ah, so young!

And I remember going through

the doors of Notre-Dame

for the very first time.
It was breathtaking!

It was one of the formative moments
that set me on my course

to wanting to spend my working life
with historic buildings.

That's why, for me,
like for so many other people,

the night of the 15th of April 2019
was such a horrible shock.

We still don't know
how the fire started.

But three years on,

the ambitious project to restore
the cathedral is being ramped up.

In recent months,
the team here has almost quadrupled.

Bringing together ancient techniques
and modern engineering,

200 workers are now battling
to bring Notre-Dame back to life.

IN FRENCH:

The first time
I came here after the fire...

..it was devastating.

There was a real sense
of tragic loss. But now...

..things are changing.
It's like a new chapter's beginning.

The cathedral looks clean.

I can see many, many people
working away inside.

It's like the wounded old lady
is learning to walk again.

The French state has ruled

that Notre-Dame will be rebuilt
exactly as it was before the fire.

Donors have contributed £700 million
towards this effort.

But the task ahead is enormous.

The team must remove
tonnes of toxic lead dust

left over from the fire...

..then clean and restore
the fragile stained-glass windows.

To reconstruct the roof identically,

they must first fill the three
gaping holes in the stone vaulting

and rebuild the timber framework
from almost 1,000 oaks beams,

then cover it with
3,000 square metres of lead sheeting

before they can raise
the 65-metre-high iconic spire,

finally, to make
Notre-Dame whole again.

Beyond rebuilding identically,
for conservationists,

restoration is all about preserving
the story of a monument.

Old buildings matter to us

because they embody not just
our history, but also our future.

So there could be an argument
in conservation

that, if you're
conserving a building,

you don't need it
to be like the original.

You know, the story
of the building can best be told

by showing
where the differences are,

how the building has changed and
responded all across the centuries.

The challenge for the team right now

is to clear up
tons of toxic lead dust.

As the lead on the roof melted
and the spire collapsed,

it was sucked inside the cathedral.

I've worn lots of
different costumes,

outfits for telly over the years.

Never quite one like this before.

Workers must put on
full protective clothing...

OK.

..to pass into "the dirty zone".

The way on to the site
is actually...

..through the shower.
This is the clean zone,

and through there,
it's the dirty zone.

When you come out,
you come through the shower too,

because everything has to be washed.

All your equipment,
all your clothes,

and obviously yourself.

It's then just a short walk
into the cathedral itself.

Wow!

This is amazing.

I have never seen
this much scaffolding. Goodness me.

There's an army of people
marching about, doing things.

It's freezing - it's REALLY cold -

and it's dark
and there's banging and it's...

It's a bit like hell, actually!
SHE CHUCKLES

This is quite the opposite
of what a church is normally like.

There's a lift going up
into the nave. My goodness.

A lift made out of scaffolding.

Ah, bonjour, Jonathan.
Bonjour. Hello. Hello.

Deputy Director of Operations
Jonathan Truillet

coordinates the work to bring
Notre-Dame back from the brink.

Jonathan, there's a huge amount
of scaffolding in the cathedral.

What does it allow your team to do?

The scaffolding
not only allows access

to clean up the toxic lead dust

but also plays
another critical role.

Before the fire, the vaulting -

made from hundreds of cubic metres
of limestone -

supported the massive
timber-and-lead roof above.

But now the vaulting
is extremely fragile.

As the team
begins to repair the stonework,

it could trigger a collapse.

To avoid this,
one of their first tasks

is to erect 52 timber support arches
beneath the vaulting

so that they can rebuild safely.

Installing them 33 metres
above the ground is no easy feat.

Well, can we take a closer look?

Thank you.

The scaffolding enables the team
to build a temporary floor

so they can install
the timber supports.

The crane must lower
each 1.5-tonne support arch

down through the hole
in the vaulting.

Each support arch
is fitted with steel brackets

to secure it in place.

Hydraulic jacks raise it to meet
the underside of the vaulting.

With the temporary supports
in position,

it's safe for me to take
the lift to the top floor.

Ah!

That was a mistake, to look down.

Oh, look, here's the window.
We at the level of the windows.

Wow.

This is not normally how you
get to experience a cathedral.

The glass is so...

..rich.

So intense.

Ooh!

Bonjour. Bonjour. Bonjour.

We're now as high
as the lift can take us,

close to the underside
of the shored-up vaulting.

Oh, wow!

This is incredible.
It's like being in a timber barn.

This same technique was used
to build the vaults, 850 years ago.

The scaffolding and supports
will stay in place

until the vaulting, roof
and spire are rebuilt.

The scale of the support work is
just astronomical. It's incredible.

I notice there's quite a lot
of fire extinguishers up here.

Now these gigantic timber supports
are in place underneath the arches,

the cathedral is structurally safe -

it's definitely
not going to fall down,

which means that
the people working here

can move on to restoring
and rebuilding.

You can hear them hard at it
as I speak.

And this is going to be
a real race against time,

cos they want to reopen
the cathedral by 2024.

All eyes will be on Paris

when the Olympic Games
are held here in 2024.

President Macron has decreed
that the landmark

must be rebuilt within five years,

in time for a grand reopening
before the Games begin.

But with the Cathedral's vaults,
roof and spire to rebuild

and the entire site
still contaminated

by lead dust from the fire,
the task seems near impossible.

To make the site safe,

the team must remove
all the toxic lead dust.

Clara is one
of 40 decontamination specialists

meticulously cleaning
all 6,000 square metres

of Notre-Dame's vaulting, walls,
pillars and floors.

So this is to keep you safe?
It cleans the air? Yeah. Yeah.

And where does all the lead dust
that you've collected go?

It goes there, in my vacuum.

Oh, into the... Into the backpack?
Yeah. Yeah.

With this equipment,
we only work 2.5 hours at a time...

..and we need to stop take a break.

How long will it take
to clean the whole cathedral?

There are different phases
of cleaning. Yeah?

Now we're in the first one.

Only the first phase?
More cleaning to come?

Oh, yeah, yeah. Oh, my goodness.
It's only the very beginning.

The vacuuming of the lead dust
will take eight months.

Then restorers can move on
to deep clean the stone.

My dream was
to come here and work here.

It's an opportunity.
You...you never expect to be working

in one of the most important
monuments in France, so...

Your dream has come true! Yeah.

I think you deserve
a cup of coffee now.

Well, I've already had
three cups of coffee today!

THEY LAUGH

Go enjoy your break. Thanks.
You deserve it. Yeah.

Bye, Lucy. Ta-ra!

You know, it's obviously tragic
that the cathedral burnt,

but I hadn't really
thought about this -

it's given opportunities to
young people to learn new skills,

like Clara.

But wearing a vacuum cleaner
on your back

for two, four, six hours a day,
sucking up poisonous lead?

Hoo! That's tough work.

In places, you catch a glimpse
of how dramatically changed

Notre-Dame will be
after the cleaning.

Beneath the lead,

and centuries of smoke
from millions of candles,

lies pristine limestone.

This is how the cathedral looked
850 ago, and will again soon.

As the cleaning progresses,
craftspeople and technicians -

from mural specialists
and glass historians

to metal scientists
and carpenters - get to work.

These are France's foremost experts
in their fields.

In charge of guiding

this unprecedented concentration
of medieval knowledge

are chief architects Pascal Prunet
and Philippe Villeneuve.

But this team will need to
pull out all the stops

if they want to reopen
the cathedral in 2024.

In April 2019,
this was the heart of the inferno.

This is where the spire
fell down into the nave.

It's only when you get up here

that you appreciate
the scale of what happened.

And to rebuild that spire?
My goodness...

..they're going to need
a tremendous amount of wood.

Above the stone vaulting,

Notre-Dame's roof was completely
destroyed by the fire.

One of the most complex challenges
the team here now face

is entirely to rebuild
the 1,000-tonne roof structure.

Remi Fromont is the chief architect

tasked with reconstructing
the medieval roof,

which is known as "the forest".

During a research project in 2014,

Remi manually
measured the dimensions

of every beam in the forest,

to create a comprehensive survey
of Notre-Dame's roof.

This allows them
to replicate it perfectly.

Remi's team will need 850 oaks

to reproduce
the Gothic roof trusses.

The most intricate element -
the Cathedral's iconic spire -

will be built
from another 1,200 trees.

The spire was a 360-tonne
engineering masterpiece.

Hidden beneath 16 copper statues
and 140 tonnes of lead sheeting

was a complex skeleton of oak beams,

some as long as 20 metres.

The secret of its strength,

a dense lattice of oak
tied into the rest of the roof,

supports the entire structure.

The spire came to be
the embodiment of the building

and of the Paris skyline,

which took huge imagination

and levels of engineering
and creativity and architecture,

which is quite exceptional.

Identically reconstructing this
wooden wonder is no simple task.

The market town of Chesterfield

bears witness to the perils
of building with green timber.

It's thought the 660-year-old
crooked spire of the Parish Church

could be due to beams that
have warped as they've dried.

For Notre-Dame's spire,

this precarious lean
must be avoided at all costs.

In public and
private forests across France,

the hunt for 2,000 perfect oaks
for Notre-Dame begins.

These will form part of France's
annual forest management quota.

One third of the country, 65,000
square miles, is covered by forest.

This team of forestiers
has their work cut out.

They must source
60 flawless oaks for the spire

from this 8,000-acre forest.

On the other side of the clearing,
another candidate emerges.

But it must wide enough and
tall enough to form a spire beam.

Lumberjack Ahmet Cirpan
begins by making a cut

that will direct the tree
to fall into the clearing.

CHAINSAW WHIRS

Notre-Dame's medieval carpenters

etched Roman numerals on their beams

to reassemble them
correctly up on the roof.

Today, this team attaches a barcode

to each oak destined
for the cathedral,

so they can track it from the forest

to its final position
in the new spire.

It takes the forestiers
several months

to complete their painstaking search

to fill their quota of
60 trees for the new spire.

Notre-Dame's builders did
not only innovate with timber.

They also pushed the limits
of what could be made with glass.

More than 3,000 square metres
of spectacular stained glass

circle the cathedral
in three levels.

One of the things I really remember

about coming to Notre-Dame
for the first time

was the otherworldly light
from all the stained glass.

Today, most of the natural light's
been blocked out of the cathedral

by this huge scaffolding structure.

But there's a silver lining.

The scaffolding gives experts
the chance to get up close

to Notre-Dame's astonishing
rose windows.

This is the first time
it's been possible in 160 years.

It is a very unique opportunity

because we won't see them again
in the same way, never.

When you're so close from the panels

you have a kind of intimacy
with the work of art

and even you can see sometimes the
mistakes of the medieval painters.

The three rose windows date
from the 12th and 13th centuries

and together they're made up
of 1,100 panels.

Protected by the stone vaulting,

they survived the fire unscathed.

These kaleidoscopic wonders

are filled with depictions
of ancient legends,

biblical scenes

and saints performing
incredible feats.

The panels have been restored
multiple times over 800 years,

as they've been damaged.

Like time capsules,
they contain messages

passed down across the ages.

They also hold
many unsolved mysteries.

Glass historians Karine Boulanger
and Elisabeth Pillet

hope this access will help them
reveal the secrets of

the largest window in Notre-Dame,
the gigantic South Rose Window,

measuring 13 metres in diameter.

They're busy mapping
every shard of glass.

Karine, what's this lady doing?
Is she riding upon a dragon?

It's Margaret.

She's tamed the dragon.
She's tamed the dragon? Yes.

Oh, I can see, she's telling it,
"Down, dragon, down!"

That's fabulous, I love it.

And you can tell me
how old this window is, can't you?

This is your superpower.
Yes, in a way.

It's 13th-century glass but there
are some restorations, of course.

You see, the colour of
the glass is different.

So I'm guessing that...

that bit there,
that looks like it's quite dark -

is that medieval? Yes, it is.

All the red is medieval?

Yes, but if you look at this red
here, you see how different it is.

Oh, so that's not medieval?

No, it's 19th-century. You see,
this one is more translucent.

It's like a jigsaw puzzle. Exactly.

It's not only
the subtle differences in colour.

The style of painting on
the stained glass changed over time.

Medieval glass painters applied
a solution of copper

and iron oxides to the coloured
glass before it was baked

to harden the paint.

The style evolves. A 13th-century
painter won't paint the same way

as a 15th-century painter,
obviously.

It's the same time for
the 19th century.

What causes the medieval glass
to go cloudy like that?

It's linked to the composition
of the glass itself.

In order to make glass,
in the 13th century you used potash

and it causes corrosion.

That's potash over eight centuries,
corrodes like that. Yes.

It's like a time-limited artwork,
isn't it? Yes.

Potash, a potassium-rich
blend of plant ashes,

reduced the melting point of
the ingredients used to make glass.

By the 19th century,

sodium carbonate combined with
calcium oxide were used instead,

and produced more stable glass
that didn't corrode.

This factory on the banks
of the Loire in Southern France

is one of the last places

in the world that can produce

stained glass using
medieval techniques.

We produce glass for
major historical buildings

as Versailles or the White House.

Workers start by mixing
sand, metal oxide for colour,

sodium carbonate and calcium.

They heat the mixture
to 1,300 degrees Celsius

and build up layers of the molten
glass on the end of a blowing pipe.

The glass-blower forms
a sphere from the red-hot mass.

He rolls it to maintain this shape,

which is critical
to form an even thickness of glass.

It's manual know-how.

There is absolutely no machines

and the glass-blowers use
their sense, their feeling

to blow one glass sheet.

They swing the seven-kilo ball
of glass in a four-metre-deep pit,

so it elongates to form a tube.

Herve has blown glass here
for more than 33 years.

Once cooled,
they cut the cylinder...

..and send it to a special furnace
where it's unrolled.

Extreme heat of 750 degrees Celsius

and a wooden block

smooth out the glass
and minimise imperfections.

They carefully inspect each pane

and remove any rough edges.

Every glass sheet is different

and it has the spirit
of the glass-blower.

Losing this patrimoine
and know-how would be a disaster.

The factory marries
these ancient techniques

with the latest technology

to reproduce
stained glass accurately.

We can fit perfectly
with the old colours

by using tools like
spectrophotometry, like X-rays,

and this allows us to know exactly

what are the elements
that are in the glass

and reproduce it for the future.

By analysing samples of
ancient glass to learn

the chemical composition,

they can match its unique
combination of metal oxides.

This team is ready with the skills,

passed down through the centuries,

to reproduce any of
Notre-Dame's stained glass

that may be beyond repair.

Until the 20th century, when
glass was too badly damaged,

they had to replace it.

There were restorations
every 50 years or 100 years,

so a stained glass window is
always a mixture of original glass,

here, 13th-century glass,

and restoration from
every century, almost.

As the historians map
the South Rose Window,

they uncover an unusual trend.

This window should contain glass
from many restorations

spanning almost 800 years.

But they're finding only
original 13th-century glass,

glass installed during the 19th
century or panels containing both.

Where has all the other glass gone?

We are finding lots of things.

They altered quite strongly
the design of the panels.

The roses are possibly
extensively restored

and we have to study them again
to understand what has been done.

Karine and Elisabeth must work fast

while they still have
this unprecedented access

to the South Rose Window.

They will continue
their investigations

to try to solve this mystery.

Just above the South Rose Window

is one of three holes
in the vaulting,

created as the spire collapsed.

Incredibly,
the top section of the spire

survived the inferno.

Its slender pinnacle is
lodged in the vaulting stone.

The team gently nudges
this spire section free...

..and carefully winches it down.

We can still see the structure of

how the spire was made
with these fine lead sheets

of a few millimetres thick

that were used on
the entire structure of the spire.

Six decorative lead roses remain

attached to
the spire's lead sheeting.

We will be able to study
how this decoration was made,

touching the spire that was just
taken down from the vaults today.

It's a magical moment.

The fire has given scientists
and historians an opportunity

to look deep into
the fabric of the structure.

Notre-Dame pushed
the limits of Gothic architecture.

Advances in stone
construction techniques,

such as the introduction of flying
buttresses, allowed medieval masons

to build incredibly tall and thin.

As the team examines
the structure closely,

they discover metal hidden
throughout the cathedral

that could unlock more
of its architectural mysteries.

From the thousands of nails
that joined timber beams

to iron bars that
brace and hold secure

the stunning medieval stained glass.

I was amazed that there is
so many iron in this building,

that was never truly studied before.

The staples that we see here,

they're embedded in maybe the oldest
part of Notre-Dame's masonry.

These 45-centimetre-long
iron "staples"

secure the great arches beneath

and prevent the stone blocks
from being pulled apart

by the enormous forces.

20 metres above, along
the very top of Notre-Dame's walls,

the destruction of the roof

has revealed previously concealed
ironwork that may hold the secret

to this cathedral's
gravity-defying height.

It's really exciting because we're
dealing with unknown structures

on the top of the walls

that are so far unique
in Gothic architecture.

Medieval builders
may have been worried

that the top of
Notre-Dame's tall, slender walls

could be an Achilles heel.

The weight of the roof
could push the stones apart.

The destruction of the roof

has revealed the builders
tied these stones together

with more than 500 staples,

creating a ring of iron
holding the walls together.

This engineering masterstroke has

remained hidden under the roof of
Notre-Dame for hundreds of years.

The staples with
the flying buttresses

are two ways of preventing
the stones to collapse.

It's an ancient form, a form
which is known since antiquity,

but it looks like in Notre-Dame

we're trying to use ancient forms of
reinforcement such as the staple

in order to build
a new form of architecture,

really high, really thin
Gothic structures,

of which Notre-Dame is
kind of the first true example.

Maxime uses pioneering technology

to unlock the secrets of
the iron staples.

We're acting as
some kind of detectives,

trying to find out
the digital prints,

the digital signature of
each of these staples

and try to rebuild their path from
the workshop to the building site.

Radiocarbon dating confirms

they were installed in
the early 13th century,

when this part of
the cathedral was built.

These are the oldest pieces of
iron used in a Gothic church

that we know of so far.

That's a huge discovery.

This is a revolution
in Gothic architecture.

No other Gothic monument
had used iron in such a way

before Notre-Dame.

Maxime reveals
the microstructure of the iron.

Multiple pieces of iron have been
forged together to form each staple.

The weld is the result
of the mixing of scrap iron

to make a brand-new iron staple.

We're maybe dealing with the richest
building site at that time

and knowing that it might have
used almost 90% recycled iron

opens new perspectives.

The team has shown that the
recycling of iron

may have been commonplace on
the building site of Notre-Dame,

shedding new light on medieval
building practices.

The lab's electron microscope
reveals further clues

to how far Notre-Dame's builders
went to source the iron.

What we discovered is that every

single staple has a different
chemical signature.

All the staples, they come from
different iron that was made

in different places.

It means that there's a truly
active iron market in Paris,

gathering iron from many,
many different origins.

These hidden iron staples
may also have played a critical role

in the aftermath of
the fire of April 2019.

The staples were placed by the
medieval master mason

to reinforce the upper main walls.

They might have helped the walls to
prevent collapsing during the fire.

To create the most advanced
church of its time,

builders of Notre-Dame turned to
an ancient construction technique,

only now fully understood
in the wake of the fire.

As a conservationist, it's teaching
us how expert these builders were

in those days.

It's a testament to their
technical competence

and their vision that they put in
these structural elements,

which have preserved
the building for us.

Their knowledge might have kept
the cathedral standing.

But the damage to the vaulting
wreaked by the fire

is shocking to see.

For the restoration of Notre-Dame,

it might just be the stone of the
vaults that's the biggest problem.

The thing is it got soaked with
water,

firstly from the firefighting,

and then because the roof
was missing for months

and the rain came in.

They've got a temporary roof up now
and it's starting to dry out.

But even that has presented
an unforeseen problem.

As the stones dry,

salts are crystallising on
the underside of the vaulting,

and they're breaking off the outer
layers of limestone.

All the damage in this part and on
the vault is the result of the salt.

You can see the loss of matter
is two or three cm.

This is catastrophic for us.

In the 18th and 19th centuries,
restorers cast sacrificial layers

of plaster on top of
the vaulting in case of a fire.

This protected the
stonework from the heat

of the 2019 inferno, but not from
the water used to fight it.

This, along with months
of rainwater,

drew salt from the plaster
into the porous limestone.

As the moisture evaporates,
the salt crystallises

and forces the limestone apart,

destroying the inner surface
of the vaulting.

To find the solution,
you have to remove the salt.

Remove the salt? And as the
salts are soluble salts,

you will have to you to use
water.... Use water.

..and some other tricks.

To extract the salt,

the team will coat the vaulting
with a paste of clay,

sand and purified water,

called a poultice.

The water from the poultice
is drawn into the stone,

where it dissolves the
harmful salt crystals.

As the clay of the poultice dries,

it draws the salty water
out of the stone,

saving the vaulting
from further damage.

The poultices will be removed when
they will all have dried.

Workers chisel away
the mortar between the stones

to allow the poultice
to reach deep into the blocks.

Next, they load
the sticky mixture

into a compressed air gun
and spray it into every crevice.

Finally, they carefully
smooth the poultice

across the face of the
vaulting stone.

And you can see that it follows

very, very closely the surface
of the stone.

And what we recommend is that the
poultice should not be thicker

than half a centimetre
to one centimetre,

otherwise there is a risk
that it falls down.

Do you have enough time?
You have to open the cathedral.

Can you have it there
as long as you need to?

If you wish all the salts
to be extracted,

we need to have a very slow
process until the vaults are dry.

And this will take time, much time.

So we have to get the
cathedral ready in 2024.

I understand those time constraints

because it's a worldwide
known symbol

and this building has
to live again.

The poultice may stay in place until

the missing vaulting
has been rebuilt

and the cathedral is
permanently watertight.

It's the end of another
long day on site.

Notre-Dame falls silent once again.

Well, this is quite
a remarkable experience

because I've got the deserted
cathedral to myself.

It's extraordinary.

And just over there, I can still see
the remains of a burned beam...

..which is a reminder

that amidst all the activity
that goes on in the daytime,

this is where the
fire actually started,

this is where nature was
out of control.

The medieval timber roof,
known as "the forest",

was built section by section
over almost 100 years,

as the cathedral grew beneath.

Hand axes were used
to craft each individual beam

for a specific position in
the roof structure.

The fire,
which started in the forest,

took just hours to reduce
this medieval masterpiece

to ashes.

The team will soon begin
an unprecedented challenge,

to rebuild the forest
in a matter of months.

SPEAKS IN FRENCH

Notre-Dame's spire
is relatively modern,

built in the 19th century.

These beams were cut with saws.

So, it's down to the French sawmills

to transform 1,200 of
the oaks into flawless beams

with the precise dimensions
needed for the new spire.

IN FRENCH:

Francois Feillet runs this
sawmill in Normandy

that processes more than 30,000
tonnes of timber each year.

It's one of 45 sawmills across
France that has answered

the call to cut the beams
for Notre-Dame's new roof.

The team wastes no time in
getting to work on the beams.

First stop, the debarker.

This machine excoriates
the outer layers,

removing the loose bark.

Now the three-tonne tree trunk
enters the cutting shed

and rolls on to the saw carriage.

The band saw blade is a
1,000 lb high-speed ribbon of steel

that runs at 43 metres per second.

This is a laser-guided
precision operation.

Francois removes slices,
just 3cm thick,

to trim the beam down to
the exact dimensions

requested by the Notre-Dame
architects.

The beams from Francois' sawmill
are stacked,

ready to join more
than 1,000 others,

coming from across France
for Notre-Dame's new spire.

IN FRENCH:

Simplement.

Inside Notre-Dame,

the first chance in
160 years to get up close

to the vast South Rose Window

has revealed a puzzle the glass
historians need to solve.

We should be finding glass
from all different periods -

we know the window's been
restored lots of times.

But the glass all seems to be either
original 13th-century glass

or relatively modern
19th-century glass.

Where's the rest of it gone?
It's a bit of a mystery.

The chief suspect is
architect Viollet-le-Duc.

In the 1840s, he was tasked

with breathing new life
into Notre-Dame.

At the time, it was not
the beloved building we know today.

The cathedral was ransacked
during the French Revolution.

Statues of kings on
the facade were decapitated

and it was used as a warehouse,

lying derelict
and unloved for decades.

Over 20 years,

Le-Duc reinstated the statues
of the facade -

he rebuilt the sacristy,

he designed hundreds of new
gargoyles

and raised the ill-fated
65-metre tall spire,

a replica of the medieval original.

Viollet-le-Duc, God bless him,

would have been what we consider to
be a star-chitect, you know?

He was a man who knew his mind,

he was a man who was
highly respected,

really determined and saw himself
as a powerful leader.

During his restoration,

Le-Duc removed all glass

in the South Rose Window
that was not original

and replaced it with modern glass.

Elisabeth is also finding that he
made brutal changes

to some of the
original glass panels.

What do you observe in this one,
Elisabeth?

ELISABETH CHUCKLES

IN FRENCH:

He's lost his toes! His toes...
ELIZABETH CHUCKLES

And part of his dress and
he's lost his aureole, his halo.

Absolument.

IN FRENCH:

I've come to the Paris Mediatheque
of Architecture and Heritage.

These archives may hold clues

to why Le-Duc made such dramatic
changes to the glass

in the South Rose Window.

Ooh, what have you got here?

Oh, wow!

Oh! The spire... The spire!

Have you seen this before?
No, never.

So I guess your colleagues, the
architects, will be looking closely

at this to get clues for how
it should be.

Oh, look, is this Monsieur Le-Duc?

No, I don't think so. No?

I think it's Monsieur Le-Duc,
he's put himself in the drawing.

THEY ALL CHUCKLE

The windows of the tribune...

And the higher windows.

Look, look, look,
it's the famous...! Gargoyles.

It's the famous gargoyles.
Here they are!

Gargling, hee-hee.

What creature do you think that is?

The gargoyles not only
frighten away evil demons,

they also protect Notre-Dame's walls
by directing rainwater

away from the cathedral.

That folder is a complete
treasure trove -

it's full of these really beautiful
drawings

by Viollet-le-Duc,

showing his plans for Notre-Dame,

and it's such a treat to see them -
not only because they're beautiful,

but because this folder contains
such important information

about France's idea of itself

that's captured in
Notre-Dame Cathedral.

But it's Le-Duc's plans for

the South Rose Window
that may hold the key

to unravelling its mysteries.

All this part here was destroyed...
Yes. ..in the 19th century. Yes.

They threw it away.

Viollet-le-Duc recognised that it
was an original medieval design,

but, for him, maybe it wasn't
the ideal 13th-century rose -

that's why he changed the design.

So, he looked at a medieval window
and he thought,

"No, that's not medieval enough"?

SHE CHUCKLES
Yes. In his point of view, yes.

I think it was that.

But such radical changes to
the glass panels

suggest Le-Duc was motivated
by more than mere aesthetics.

After hours of research,

the team makes a
stunning breakthrough.

It sounds like you're coming
across completely new information,

is that fair?

In fact, we just discovered
that there was

a change of structure of the rose -

there was an iron reinforcement
in the centre of the rose,

but, obviously, it wasn't enough,

so Viollet-le-Duc put it further
away from the centre.

When he removed the ironwork here,
he had to change some of the panels.

Le-Duc's vision for a more
medieval South Rose,

combined with the need to beef up
the iron reinforcement,

required him to change the glass.

But the historians find evidence
he actually altered

the entire stone
structure of the window.

It's like two different windows...

IN FRENCH:

In fact, he
slightly...turned the rose...

Ah... So it's gone like this

and he's sort of gone,
"E-e-e-r-r-r...".

CHUCKLING

So, why did he do that?
That's a big change, isn't it?

Le-Duc's restoration was sweeping.

He removed a smaller
structural ring of iron

and replaced it with a bigger ring,

to strengthen
the core of the window.

He removed all trace
of previous restorations

to replace them
with panels of new glass.

And he turned the whole window
through 15 degrees...

..to make it structurally stronger.

These two drawings in the archive
have been revelatory, for me,

because they've helped me understand
why Viollet-le-Duc may have

messed with the structure
of the window.

This showed what it was like before,

and you see this vertical axis -
here we've got panes of glass.

That's not structurally very safe,
but after, in this drawing,

he's put in a big, strong stone arm

that's going to hold the
whole thing together.

I mean, he wasn't just doing
things for aesthetic reasons,

he was also helping Notre-Dame
to withstand the centuries.

Thanks to the scaffolding,

this team is painting
an intimate portrait

of how the South Rose Window,

one of the wonders of this
cathedral, evolved to survive.

It has been such a privilege
to spend time in this wounded

but wonderful cathedral.

And it's clear how utterly dedicated
the architects, the restorers,

the scientists are who've been
tasked

with bringing it back to life.

I'll be amazed if they do
make their deadline of 2024.

But do you know what? There's
no shortage of ambition here.

This experience marks another
chapter in my relationship

with Notre-Dame... Oop.

..a building that has inspired
me and countless others.

I've seen the vaulting secured,

the interior cleaned
of toxic lead dust

and the mysteries of
its windows solved.

Next time, I'll be back to witness
the rebuilding of the vaulting,

the new roof and the spire
under construction.

And maybe, just maybe,

the reopening of this astonishing
building to the world once more.