The Question and Answer section for The Fall of the House of Usher is a great resource to ask questions, find answers, and discuss the novel.
I'm sorry, what details are you referring to?
Usher has been feeling ill and distressed lately. He asks the narrator, an old friend, to visit for support.
The narrator observes that there are small cracks(fissures) in the very foundation of the walls and house. This of course foreshadows both Roderick's demise as well as the house itself.