In the 19th century, Napoleon III undertook to modernise Paris, where people were constantly flocking. Baron Georges Eugène Haussmann, prefect of the Seine, destroyed entire districts that were unhealthy and dark. He laid out wide avenues that he lined with opulent stone buildings topped with Mansart-style roofs which allowed for housing in the attic. He had the roofs covered with grey zinc sheets, a material that was light to carry, to guarantee better waterproofing. More than 70% of the roofs in Paris are covered with zinc. The most bourgeois buildings remained faithful to slate.