Cathedrals are complex structures. Art historian Barbara Schock-Werner knows this not only as an architect and monument conservator, but above all through her 13 years of experience as a master builder of Cologne Cathedral — as the person responsible for the largest Gothic cathedral in Europe. As an expert and coordinator of German aid to Notre Dame, she is closely following the reconstruction process after the major fire in Paris.

DW: Patrick Chauvet, dean of Notre Dame Cathedral, recently expressed his concern over the sensitive condition of the cathedral, stating that there is a 50% chance that the structure could collapse after all. How do you assess this prognosis?

The risk of further collapse is quite realistic. The vault is unlikely to collapse as a whole, but more parts could come down. What most people don't realize is that heat can also damage stone. Intense heat dissolves —  simply put — the structure of the stone.

The vault of Notre Dame was damaged by falling parts in three different places. Tons of burnt oak beams are still lying on the still standing sections of the vault; that's why no one is allowed under the vault since stones continue to fall from above. 

What is particularly delicate at this point in the reconstruction process?

At the moment, builders are currently removing the scaffolding around the crossing tower that was set before the fire for its restoration. This scaffolding was completely welded into place by the intense heat generated by the fire. Philippe Villeneuve, the chief architect of French monuments, is afraid that the balance that has recently been regained will be disturbed when the scaffolding is removed.

Also, it has not yet been possible to investigate to what extent the vault stones that are still up there were damaged by the fire. Even if the vault does not collapse, the investigations could reveal that the stones of the vault were so badly damaged by the fire that they will have to be replaced.

Only when these conditions have been clarified can a temporary roof be installed and the vault protected again. So far, the colleagues in Paris have been lucky that the vaults have not been exposed to storms and other weather stress.

What other hurdles remain in the reconstruction process?

Philipp Villeneuve has said that the upper 20 centimeters (7.8 inches) of the outer walls have been annealed, and investigations are currently being carried out. If the stones there were badly damaged by fire, the entire upper edge will have to be replaced.

That would delay the process even further. And of course the ubiquitous lead dust poses a health risk to people. Working quickly in the interior is  also impaired since it can currently only be done when wearing protective masks.

Speaking of delays… French President Macron appeared quite ambitious when he announced that the cathedral would be rebuilt by 2024. How realistic do you think that is?

Well, it depends on the condition of the vault. Our Parisian colleague also wants to close it [the roof] as soon as possible so that the interior is preserved — to make it accessible to the public again.

The entire restoration of the cathedral — cleaning it inside and out, patching the windows, cleaning the walls, restoring the organ and so on — will certainly take more than five years.

But if people focus intently on the work and no major disasters occur, the interior could be reestablished in five years. Philippe Villeneuve works very efficiently and carefully. It is not always so easy to do so, given the political pressure.

You are organizing the reconstruction aid for Notre Dame from the German side and you serve in an advisory capacity. How can you help concretely at the moment?

I have offered that Germany could help with the restoration of the clerestory windows that have already been removed. I have just sent this offer to France and hope to receive a response within the next two months as to whether French colleagues would consider this useful. If so, we could step right in.

There are now all sorts of designs in discussion for a rather modern reconstruction approach, with some of them being quite bizarre. Which one is your favorite?

My favorite is the old spire. Eugene Viollet-le-Duc's crossing tower, rebuilt in the 19th century, was a masterpiece of neo-Gothic architecture. I would restore it exactly as it was. Of the designs that are in circulation, the one by Sir Norman Foster with its stainless steel and crystal crossing tower seems to me to be the most appropriate as it most closely complements the building. But I am actually in favor, as is my Parisian colleague, of reconstructing the original.

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In your 13 years as master builder at Cologne Cathedral, you were also known for your commitment to innovation, for example with the cathedral windows by Gerhard Richter, which were inaugurated in 2007. Are there modern elements that you could also imagine in the context of the reconstruction of Notre Dame?

I'm reticent about that; that's up to the French. Whether one builds a modern work of art inside, an altar or a memorial by a modern artist that reminds one of the catastrophe... I can certainly imagine that.

And that would be a good addition since Notre Dame was robbed of almost all of original altars during the two revolutions. It would make sense.

Architect and art historian Barbara Schock-Werner is a consultant for the protection of historical monuments and was master builder of the Cologne Cathedral from 1999 to 2012. She is often consulted as an expert for historical church buildings. German Minister for Culture and Media Monika Grütters commissioned her to coordinate German aid for the reconstruction of Notre Dame in Paris.