Mansard Roof,

Chambers's Encyclopaedia, Volume 7: Maltebrun to Pearson, p. 25

Mansard Roof, a form of roof invented by François Mansart (1598-1666), a distinguished French architect. It is constructed with a break in the slope of the roof, so that each side has two planes, the lower being steeper than the upper. The framework ought to be arranged so that its parts are in equilibrium. This kind of roof has the advantage over the common form of giving more space in the roof for living-rooms.

A simple line drawing of a Mansard roof, showing a trapezoidal shape with a break in the slope, creating two planes on each side.
Mansard Roof.
Source scan(s): p. 0034