It was the roof "frame" and the structure of the spire that burned. They were covered in lead, great stuff for the job from the 1100s (when the building was underway) through the 19th century (the last major restoration): it's waterproof, doesn't burn, and sheets of it are easily joined together. Hundreds of tons of lead -- and the environmental laws and rules in the EU aren't any softer on lead contamination than the ones in, say, California. Expect to hear more about that in coming weeks.
The trusses themselves were a "queen-post" design, fairly strong for their weight and with an open space in the center, allowing for access (as seen in this CNN article). The roof mainly keeps water away from stone arches (six-part rib vaults) below it, arches that carry compressive force from the buttresses on each side. When news articles talk about the danger of interior collapse, damage to those limestone arches is a big part of the worry; while I snarked on Facebook about the kind of idiocy that manages to set a stone building on fire, the heat of structure fires is a great danger to limestone architecture.
I'm not finding a lot on the interior structure under the spire in a quick search. The dramatic interior photograph that has shown up everywhere seems to show a great lump of debris right under the crossing and it could be very bad.
At the base of the spire was a group of statues: the twelve Apostles, in four groups of three, each group preceded by one the animals symbolizing the four evangelists, and all of them -- all but one -- facing out towards Paris. The lone exception? St. Thomas, patron saint of architects. He was looking up at the spire -- with the face of Eugène Viollet-le-Duc, mastermind of the extensive 19th-century restoration. All of the statues had been removed only days before the fire; no doubt both the saint and whatever records Viollet-le-Duc left are going to be getting a lot of attention in coming months and years.
This isn't the first time Notre Dame has been badly damaged; the 19th-Century work largely addressed damage caused during and in the aftermath of the French Revolution. Nor is it the first time the modern world has had to address this kind of disaster: Reims Cathedral was reduced to ruins in a very similar manner during World War One. Restoration started immediately after the war and the cathedral re-opened twenty years later.
As of this morning, over $339 million had been pledged by private donors to rebuild the cathedral of Notre Dame. During the fire, brave firefighters rescued irreplaceable artifacts. As dreadful as things look, it is not lost. It will be rebuilt.
BUILDING A 1:1 BALUN
6 months ago