I toured the House of the Seven Gables in Salem, MA, many years ago and the story you see below is the same story they told us in Salem about the “dead as a doornail” phrase and 17th century American building practices. The phrase is also used in Nathaniel Hawthorne’s novel, “House of the Seven Gables”.
Dead As A Doornail:
While this expression may be even older, its first known use can be dated to 1350. In those days only nails (no screws) were available to be used by carpenters to hold wood together, especially for doors. It was common practice, especially on large things such as medieval doors, that after a nail was hammered through the wood it would be flattened on the other side to stop it from loosening over time. Once this was done, the nails (which were often reused because they were expensive) would be considered ‘dead’ or unusable.
Source: Strange Phrases from the English Language, Ellen Castelow for Historic UK
I’ll add more information about the House itself when I find my old photos.