Original subtitles

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.