Reconstructing Notre Dame: All options open

Deutsche Welle: French President Emmanuel Macron has promised to rebuild Notre Dame within five years. Do you think this timeframe is realistic?

Thomas Eissing: If we're talking about the reconstruction of the roof alone, then in my opinion that is totally feasible. When it comes to reconstructing the entire internal structure — that requires some more thought. I haven't seen a damage report yet.

The roof's wooden framework, known as a truss, was completely burned. What cultural-historical significance did it have for Notre Dame?

The development of the French Gothic architectural style in the 13th century was closely tied to the cathedral structure, which led to the creation of a particular roof form in equilateral triangles. This meant roofs grew steeper, and they consequently had to be made stiffer to withstand the force of the wind, so the construction was more complex.

With the Gothic cathedral came the first real flourishing of carpentry in France, putting the country at the industry's forefront across Europe. French carpenters were early builders of what's called the "king post," a central vertical post in a triangular truss. They're not everywhere, but they are very special constructions, and they are used in the roof over the side aisles and the choir.

Read more: Notre Dame Cathedral: A symbol of France

Will the entire roof truss need to be torn down, or could it be used as the basis as for reconstruction?

Much of it seems to have collapsed, but I would try to salvage the pieces that better survived the fire because the historic technology used to join them together is important. Oak was used for the truss, and generally this type of wood tends to char on its outer layers without burning all the way through.

The support framework often survives as a shell, but this didn't happen in Paris, probably because of the combination of wood and lead. Lead has an incredible capacity to store heat, which then continues to feed a fire. That's why the fire in Notre Dame was so devastating.

It should be possible to get the right wood for reconstruction however. France has many oak forests, and it's not as though an exorbitant amount of wood would be required. The roof has a span of 13 to 14 meters (43-45 feet), and logs of that size can be found.

You are an expert on wooden construction, but what can you tell me about the condition of the stone structures? Can they still be used?

If stone is heated, its durability is impaired. The extent of the damage will have to be assessed.

The Cologne Cathedral, severely damaged during World War II, was reconstructed with a roof truss made of steel. Would this be a possibility for Notre Dame too?

The problem is that steel is much more sensitive material in fires. Wood alone doesn't actually burn so easily. A large oak beam first becomes charred on the outside, and that layer of charred wood actually has an insulating effect. Wood starts burning between 120 to 250 degrees Celsius (302 to 482 Fahrenheit), so it first has to go into a gaseous phase. If this doesn't happen, the wood can't burn. In comparison, fires cause steel to lose its strength and collapse, so it needs to be coated or outfitted with some sort of fire protection. 

So you would recommend rebuilding the roof truss with wood?

I would at least make this a point of discussion. It's not what's usually done to preserve historical monuments, meaning that when something is gone, then original material is lost.

But we are dealing with a very exceptional construction here. That's why I think it is appropriate to consider recreating the roof using some original pieces, so that people can once again experience the construction history and see how it originally looked.

Are there experts in the field of Gothic cathedral construction with enough knowledge to be able to adequately repair Notre Dame?

There need to be [building] plans, which in this case, there are. In the 1930s the roofing was measured precisely. A model of the roof truss also exists. This means we have very good templates, also for the juncture points. We actually know how the roof was built. The old building technologies can be mastered, and if the wood can be acquired, which I think it can be, then the roof truss can actually be reconstructed.

Read more: Opinion: Notre Dame will rise again

Around €800 million ($900 million) has already been donated to Notre Dame's reconstruction. In comparison, it cost roughly €182 million to rebuild Dresden's famous church, the Frauenkirche, which had been built in the 18th  century and was destroyed by Allied bombing in World War II. Can one actually compare these two projects?

I can't judge that. It is definitely easier to rebuild a church from the ground up, as was the case with the Frauenkirche, because you can calculate the risks and costs up front. It's a different situation in the case of Notre Dame: The stone church fortunately survived, though the detailed extent of the damage remains unclear.

What's decisive now is damage assessment, and particularly, in my opinion, the remaining structures: the vaults, walls, the wall capping and the buttresses. If these are still structurally intact, then I think anything is possible. Measuring and recording the damage will certainly take half to three quarters of a year.

Thomas Eissing is a professor at the University of Bamberg's Institute of Archaeology, Heritage Sciences and Art History and has expertise in wood and church roof construction.