The news that Notre Dame Cathedral in Paris is on fire has come with an understandable uncertainty and concern about the demise of the entire structure. There are a couple of key points that haven’t yet made any of the major news coverage that are worth pointing out…
What’s been burning the last couple of hours is the timber roof that had rested on top of the cathedral’s stone vaults. Initial reports are that there was renovation being done in the structure’s attic, which makes sense given the presence of scaffolding in much of the footage, and it’s possible/likely that this activity led to the blaze. At least some of the roof, including the spire that collapsed, was actually part of the 19th century ‘restoration’ by Viollet-le-Duc. Its loss is catastrophic, but it would not be the first cathedral roof to be destroyed by fire–Rheims’ timber roof was destroyed in a conflagration caused by shelling during WWI.
That incident may provide some modest hope. Reims’ stone vaulting survived the shelling, though it was heavily damaged. The lost roof over Notre Dame, like those of the other great cathedrals, was primarily a weather covering, to keep rain and snow off of the vulnerable limestone vaults and mortar that form the main spaces of the cathedral below. It wasn’t structural, in that the vaults, piers, and flying buttresses that structure and surround those spaces below have functioned as support on their own–and, in fact, one of the major structural problems with gothic vaults was absorbing the thrust from the pitched timber roofs above them.
All of that structure–what we think of as the space of the cathedral–is stone, which doesn’t burn. So those news reports that have breathlessly reported on the ‘spread’ of the fire to these elements don’t have things quite right. Limestone does calcine and deteriorate under high heat, and the fragile vaults are vulnerable to falling debris from the burning roof. The quantity of water being used to extinguish the fire is, too, potentially damaging. So collapse is a real danger, but not necessarily inevitable.
Three possible outcomes. One, like Rheims, the cathedral could survive the roof’s burning damaged but intact, requiring a new roof (probably of lighter lumber or metal–like the repair of Reims) and repair to damaged but surviving vaults below. Two, falling timbers and the spire may have collapsed significant portions of the vaults below, which would require extensive new structural work involving scaffolding and years of craft-intensive labor. The collapse of choir vaults at Beauvais in 1284 was devastating but only partial. The worst case would involve weakened stone and mortar from the heat of the fire and the water from attempts to extinguish it and a broader collapse. If, for instance, the main towers were compromised, their ‘bookend’ effect on the vaults behind them would leave the nave without adequate buttressing in the long direction.
Update 7:45pm: From the first images of the interior it looks like at least one vault has partially collapsed, but most of the structure remained intact:
We’re getting to see some of the first images from inside Notre Dame Cathedral. President Macron: “‘I say to you very solemnly this evening, this cathedral will be rebuilt.” https://t.co/9D4jdaIsod pic.twitter.com/ZJ7ug4FUYt— Jim Roberts (@nycjim) April 15, 2019