joist vs purlin

joist

noun
  • A piece of timber or steel laid horizontally, or nearly so, to which the planks of the floor, or the laths or furring strips of a ceiling, are nailed. 

verb
  • To fit or furnish with joists. 

purlin

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

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