The Fall of the House of Usher refers to the actual physical house crumbling into the abyss. It also refers to the fall of the "House" or rather the family of Usher's falling because Roderick and Madeline were the last of the Usher's and they died so the family died as well. In other words the last two Usher's died so the family or the house (which are used synonamously) fell as well. Hope this helps.