A large part of the Notre Dame Cathedral in Paris was destroyed in the fire that broke out on April 15, 2019.

The fire started from the attic, destroying most of the cathedral's roof, including its iconic spire.

Investigations are still ongoing to determine the cause of the fire, but preliminary findings by the police point towards ongoing renovation works at the time of the blaze.

Donors from France and all over the world were quick in pooling more than 1 billion euros to support the restoration of the cathedral.

The president of France, Emmanuel Macron, also promised to rebuild the cathedral in five years' time.

However, this sum of money is not enough to restore Notre Dame Cathedral back to its former glory.

'The forest' of Notre Dame

The frame of the cathedral's roof measured 100m long, 13m wide in the nave (40m in the transept) and 10m wide.

According to Notre Dame Cathedral's website, about 21 hectares of oak trees were felled between 1160 and 1170 to create the frame.

These oak trees were very tall because they were between 300 to 400 years old.

Using such tall trees enabled builders to create a very tall roof for the cathedral.

As such, the attic was nicknamed "the forest".

Here are some photos of the roof before the fire:

Not enough tall trees in France

Forests owners are encouraged to donate their oak trees for the restoration since it is technically possible to restore the roof using the original methods.

The president of France's National Council of the Order of Architects, Denis Dessus, told BBC:

"We could rebuild an identical structure for Notre-Dame because it has been perfectly documented and the materials exist."

However, trees of this size are difficult to harvest in present-day France or even in the rest of Europe.

According to a research paper published in mid-2018, Europe has only 4 percent of its primary forest left where such large tall trees are found.

The vice-president of the French Heritage Foundation, Bertrand de Feydeau, said that trees of such sizes and quantity can no longer be found in France:

"If today we wanted to rebuild the structure of the cathedral with pieces of this size, we would not find them on the national territory."

The French might also consider using modern techniques and alternative materials such as steel to restore Notre Dame Cathedral so that the structure can last longer.

Dessus claimed that a lighter, more flexible structure can help the cathedral "to live for another 10 centuries".

Top photo collage from Ste Genevieve Catholic Parish and Notre-Dame de Paris.