The Mansard roof gets it's name from architect Francois Mansart who popularized it in the 1600's in France. A mansard roof has two distinctly different slopes on each side. The lower portion of the roof has a very steep pitch often with dormers attached, while the upper portion has a low slope, just enough for water runoff to occur. Typically speaking the low slope portion of the roof cannot be seen from ground.
Mansard roofs offer so much attic space it is often used as an extra story for the house with the additional space known as ’the garret’. This type of roof is not recommended for areas that have a great deal of snowfall. Heavy snow build up could occur on the low slope portion of the roof placing undo strain on the bracing.
Buildings with Mansard roofs (sometimes referred to as Second Empire) enjoyed a popularity in North America in the mid to late 1800's as a part of Victorian style architecture.