lean-to vs mansard

lean-to

noun
  • A shelter with a sloped roof; also a building with a similar construction attached to the side of a building as an extension. 

mansard

noun
  • The upper storey of a building, surrounded by such a roof 

  • A mansard roof 

adj
  • having two slopes on each side, the lower being steeper than the upper 

How often have the words lean-to and mansard occurred in a corpus of books? (source: Google Ngram Viewer )