hammer beam vs summertree

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. 

summertree

noun
  • A horizontal beam brought even with the face (breast) of a wall, to support a wall above a gap or opening; a beam bearing a load above. 

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