Here are some photos of the structure at the top. Lots of wood.
http://www.notredamedeparis.fr/en/la-cathedrale/architecture/la-charpente/
I heard a reporter saying that the fire began in the area of the spire on top of the east end of the cathedral, which was part of the main renovation area. Although the exterior is of stone, the interior is filled with wood, I remember seeing that from a trip there many years ago.