What early details foreshadow the final crumbling of the house in "The Fall of the House of Usher"?
One of the most interesting is this one: the narrator notices a crack that runs a very long length in the estate's structure. This is symbolic as well as being an instance of foreshadowing. The long crack is symbolic of the fractured nature of Roderick Usher's mental capacities. He is in a deep depression and it only gets worse as the story goes on.
The crack is also symbolic of Roderick's relationship with his sister, Madeline. There is some evidence that their family had a history of incest, so it is not a stretch to say that perhaps Roderick and Madeline has this type of relationship. Neither was married and they lived with each other. They clearly had issues within their relationship and it was fragile at best.
The relationship between Roderick and Madeline crumbles, just as the house does, when Madeline is buried alive. She is able to make her way out of her tomb and falls upon a completely shocked Roderick, who dies instantly. The narrator is able to escape and turns to watch the house fall in on itself.