Original subtitles

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.

- The team enlist the help of gravity.

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.