step ladder vs step-ladder

step ladder

noun
  • A ladder with steps or treads instead of rungs that is hinged in the middle to form an inverted V, with stays to keep the two halves at a fixed angle. 

  • The player over whom another player marks to take a spectacular mark. 

step-ladder

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