Eric Wirth of the Guild of French Architects, has urged that the roof of Notre-Dame de Paris be reconstructed out of wood, for the sake of the building’s structural integrity. He said that even if metal or concrete roofs were lightweight, the building’s structure is built for a heavy roof, and that such Gothic cathedrals “stand up structurally because there is a large mass on the vaulted ceiling … they only work because the roof is heavy.”