John Stamper, associate dean and professor of architecture at University of Notre Dame. 

He is an expert in the history of Notre Dame de Paris. 

Comment: 

"Notre Dame Cathedral was the most important symbol of the French monarchy in the twelfth and thirteenth centuries, marking the transition from Early to Late Gothic architecture with the invention of the flying buttress and the change from six-part to four-part ceiling vaults.  

The two most critical questions today concern the safety of the great rose windows in the transept arms, which date from the 1250s, and whether or not the stone vaults of the nave and transepts have collapsed."