hammer beam vs purlin

hammer beam

noun
  • A member of a kind of roof truss (a hammer-beam truss), so framed as not to have a tiebeam at the top of the wall. Each principal has two hammer beams, which occupy the situation, and to some extent serve the purpose, of a tiebeam. 

purlin

noun
  • A longitudinal structural member bridging two or more rafters of a roof. 

How often have the words hammer beam and purlin occurred in a corpus of books? (source: Google Ngram Viewer )