The Collaborative International Dictionary of English v.0.48:

Mansard roof \Man"sard roof"\ [So called from its inventor,
   Fran[,c]ois Mansard, or Mansart, a distinguished French
   architect, who died in 1666.] (Arch.)
   A hipped curb roof; that is, a roof having on all sides two
   slopes, the lower one being steeper than the upper one.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mansard roof
    n 1: a hip roof having two slopes on each side [syn: mansard,
         mansard roof]