Cathedral Design: The Roof That Burned, The Ceiling That Remains

MrsDarwin and I were looking at the news tonight, trying to understand how bad the damage to Notre Dame cathedral was. What seemed confusing was on the one hand the images of the fire which seemed to show the whole, burned roof collapsed amid a hollow shell of walls:

And on the other hand the first images from inside which seemed to show an interior which was damaged but not totally "gutted" as so much coverage kept saying.

Then I remembered David Macaulay's marvelous book Cathedral, which I'd read many times as a kid and watched the PBS movie of even more times. (Indeed, pulling up this link, I remembered that the story of the fictional cathedral in the animated sections begins with the old cathedral burning down, and the town resolving to build a new cathedral to house the relics miraculously rescued from the burning building.)

The outer roof that we saw burning is built of wood covered with lead sheeting, but under that is the stone vaulting which you see when you look up.

I think the lake of fire we see in the drone image at top is probably mostly on top of the stone vaulting, which acted as a fire break. Only in some places has debris broken through the vaulting down into the church below. In this picture you can see up through the breaks in the vaulting to the fire above:

The strength of those medieval stone vaults may have kept much of the fire up above, away from the interior of the church.