The challenges for Notre Dame cathedral restoration efforts remain how to integrate any new material to the old existing structure, said a leading historian at Cambridge University.
Alan Macfarlane, historian and professor emeritus at King's College, Cambridge, said "In terms of the interior fittings and the roof and spire, it will be new – while the shape and stone body is the same – so it will be both the same Notre Dame, but also with new materials and buildings."
Whilst the cathedral's roof and spire were destroyed by the Monday-evening blaze, the main structure of the historic building, including two bell towers, was saved by a team of firefighters in a crucial time window of 15 to 30 minutes, according to the French authority.
Macfarlane said that the rebuild will likely be of the "unconvincing" kind seen with other such older structures destroyed and restored around the world; and neither will it be like neo-gothic replicas, which are "quite acceptable", for example the British House of Commons or King's College Cambridge.
"It will be authentic, for the shape is still there. Yet it will also take time to mellow and for people to accept that a building can both be old and new at the same time. Yet Notre Dame, like all great buildings, is largely seen through the imagination," he added.
In a televised address on Tuesday evening, French President Emmanuel Macron pledged to rebuild a "more beautiful" Notre Dame cathedral within five years before Paris hosts the 2024 Summer Olympics.
"We are a people of builders … We will rebuild the Notre Dame cathedral even more beautifully and I want this to be done within five years," Macron said. "I truly believe that it is up to us to transform this disaster into an opportunity to come together. It is up to us to find the thread of our national project – the one that made us, that unites us."
But experts are holding rather different opinions, suggesting the restoration work could take more than 10 years.
Eric Fischer, head of the foundation in charge of restoring the 1,000-year-old Strasbourg cathedral, which recently underwent a three-year facelift, told AFP the Notre Dame may take "decades" to rebuild.
"The damage will be significant. But we are lucky in France to still have a network of excellent heritage restoration companies, whether small-time artisans or bigger groups," he said, adding the ability to rebuild the colossal cathedral in a manner that respects its original form and character would depend on the plans, diagrams and other materials available to the architects.
They would need "a maximum of historical data or more recent data gathered with modern technology such as 3D scans" of the kind used in the restoration of the Strasbourg cathedral, he explained.
Notre Dame is considered to be the finest example of French gothic architecture, with an innovative use of rib vaults and buttresses, stained glass rosettes and sculpted ornaments. Construction of the church began in 1160 and continued over a century.
The cathedral is part of the "Paris, Banks of the Seine" site inscribed on the World Heritage List in 1991. The site includes bridges, quays and the banks of the River Seine, along the historical part of its course, between the Sully and Iéna bridges, the Ile de la Cité and the Ile St Louis.
Audrey Azoulay,director general of the United Nations Educational,Scientific and Cultural Organization,known as UNESCO,said that a rapid damage assessment would be carried out as soon as possible.
"Notre Dame represents a historically, architecturally, and spiritually, outstanding universal heritage. It is also a monument of literary heritage, a place that is unique in our collective imagination.
"UNESCO stands by France in safeguarding and rehabilitating this invaluable heritage," she said. "We are already in contact with experts and ready to send an emergency mission to assess the damage, preserve what can be preserved and plan short and medium-term measures."
