Notre Dame is not stable and urgently needs reinforcing. That is the conclusion of an initial assessment of damage caused by the fire which destroyed its roof and spire on the night of 15 April.
The collapse of a part of the vaults has severely reduced the safety of its structural system, which, in the case of a Gothic cathedral, does not rely on the heavy mass of the walls, as in classical architecture, but on discharging weight through clustered columns, external flying buttresses and counter-supports—a structural “exoskeleton” that until now has been extremely effective and resilient.
Paolo Vannucci, a mechanical engineer at the University of Versailles, has modelled the engineering of the structure and shown that the walls of Notre Dame could collapse under the pressure of wind speeds higher than 90km per hour, while before the fire they could withstand winds of up to 220km per hour, so they urgently need temporary supports. The stability of the vaults is also causing concern because the stone has been weakened by the fire and the weight of tons of water and the molten lead of the roof covering.