A mansard roof is a type of hip roof characterized by a double slope on all sides. This roof design features a steep slope at the base of the roof, with a shallower slope on top of the steep slope. The shallow slopes meet to form the roofline. In many cases, the shallow slope of a mansard roof is not visible from the ground, depending on how the roof is designed and installed. Variations include the traditional straight mansard roof, along with concave and convex designs.
This roof design was popularized by Francois Mansart, a French architect who worked in the Beaux Arts era in the 16th century. Mansard did not develop the design, but he did make it common, and people began to request roofs a la Mansart. English speakers apparently had difficulty with his name, corrupting it into “Mansard.” The Beaux Arts style became very popular in Britain and the United States, and numerous very fine examples of this style of roof can be seen in these regions of the world.
Mansart's reason for popularizing this style was not entirely aesthetic, although the soaring slopes of a mansard roof certainly do complement Beaux Arts architecture. He used the design to skirt French tax law. Under the tax codes of the time, people were taxed for the floors below the roof. With a mansard roof, one or more floors are actually tucked into the roof, and would therefore be exempt from tax. Property owners seeking to minimize their tax liability could use a mansard roof to enjoy some extra floors without paying for them.
The increased interior space under a mansard roof is one reason the design remains very popular. With a mansard roof, almost all of the space under the roof is usable, thanks to the steep slope on the bottom part of the roof. There is a great deal of headroom all the way out to the eaves, creating a fully usable floor which would be a cramped space under another style of roof. A mansard roof can also be further enhanced with dormers, which add light and more space to the design.
In areas with heavy snow in the winter, this roof design must be installed carefully. Snow can accumulate on the shallow slopes of the roof, potentially adding significant weight to the roof. If the weight grows to be too much, the roof may buckle or collapse under the strain. Accumulations of snow can also promote leaks, as water presses into the roof, freezes, and expands, creating numerous small cracks and other openings.