A mansard is a roof whose lower part slopes much more steeply than the upper part — almost like a wall with windows in it, and then a flatter roof on top. That steep lower slope lets you put real rooms in the roof, not just a dark attic. Architects and estate agents say mansard roof; you also hear mansard as the adjective for that shape. It is a specialist word, not a casual one. It is named after the French architect François Mansart, though the spelling of the roof and the spelling of his name are not quite the same. Though actually, people sometimes point at any fancy old roof and call it a mansard; the tidy meaning is that two-pitch shape.