A Mansard or Mansard roof in architecture refers to a style of hip roof characterized by two slopes on each of its four sides with the lower slope being much steeper, almost a vertical wall, while the upper slope, usually not visible from the ground, is pitched at the minimum needed to shed water. This form makes maximum use of the interior space of the attic and is considered a practical form for adding a storey to an existing building. Often the decorative potential of the Mansard is exploited through the use of convex or concave curvature and with elaborate dormer window surrounds.
It was popularized in France by the architect François Mansart (1598–1666). His treatment of high roof stories gave rise to the term "Mansard roof" (toiture à la Mansarde). The spelling Mansard is not a correct form of the name. Sections of the Louvre, such as the central portico of the Richelieu Wing, display this style of roof.
At a time when French houses were taxed by the number of floors below the roof, this feature had the added benefit of exempting the upper floor from taxation. A revival of the Mansard occurred in the 1850s rebuilding of Paris. The style of that era in France is called Second Empire.
Under the influence of the Neo-baroque revival of the French Second Empire (1850–1870), the mansard became a common feature in many later 19th-century buildings in Europe and North America. Another revival of the style occurred in the United States and Canada during the late 1800s as one of any number of expressive forms adopted by Victorian architects. This style of roof became very popular in Back Bay, Boston, during the 1870s. In the Second Empire style, the Mansard roof was typically used to top a tower element, rather than across the full width of the building.
In congested sites in cities, a mansard enabled builders to keep a decently low cornice line, while incorporating a couple of extra stories within the apparent roof. Mansards may be seen on New York City's former Grand Central Hotel (1869).
Many fast-food restaurants, including most Pizza Hut and McDonald's outlets, also incorporate what appears to be a simple mansard roof, usually covered in a synthetic material. In reality, this is usually a façade, used to conceal air conditioning equipment and ductwork on top of a flat roof.