When the fire broke out, Notre-Dame was undergoing restoration work and was covered with scaffolding. Miraculously, this did not collapse in the 800 Celsius heat, though many of the 50,000 poles melted, making dismantling the structure extremely difficult and perilous for the vaults underneath.
https://www.ft.com/content/7e81e6ee-d94c-11e9-8f9b-77216ebe1f17