purlin vs ridgepole

purlin

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

ridgepole

noun
  • A beam along the ridge of a roof to which the rafters are attached. 

  • A horizontal pole that supports the roof of a ridge tent 

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