purlin vs rafter

purlin

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

rafter

noun
  • One of a series of sloped beams that extend from the ridge or hip to the downslope perimeter or eave, designed to support the roof deck and its associated loads. 

  • A raftsman. 

  • A flock of turkeys. 

verb
  • To furnish (a building) with rafters. 

  • To make (timber, etc.) into rafters. 

  • To plough so as to turn the grass side of each furrow upon an unploughed ridge; to ridge. 

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