It is also the case that the twins Madeline and Roderick are also symbolic in their nature. For Poe, the story itself is about the duality of human nature and the struggle between the dualism of emotion/body and rationality/mind within the individual. Roderick represents the human innate mental capacity as he is a character who is all mind in a weak and deteriorating body. Roderick is constantly engaged in the intellectual; he engages almost entirely in reading, making music, and creating art. Conversely, his doppleganger Madeline, is entirely a description of her body without reference to her mind. Madeline is "the gradual wasting away of a person" and most notably a body/corpse in the end of the story. The two are unable to survive without one another as demonstrated by Madeline's return from the tomb to reclaim her twin. This symbolism suggests that the human body and mind struggle to exist within the same individual but are ultimately necessary to complete each person. Moreover, denial of the body or the mind results in an untenable existence and the only way to achieve true harmony is to accept the necessity of both the mind and the body.
You can actually start with the title of the story, which alludes to the absolute chaos that lurks within the mansion. This "broken" house is "falling" in every way. Genetically speaking, the Ushers clearly sound like they are the product of generations of inbreeding, or else generations of careless mating. They are a sickly clan, vulnerable, and weak.
He suffered much from a morbid acuteness of the senses; the most insipid food was alone endurable; he could wear only garments of certain texture; the odours of all flowers were oppressive; his eyes were tortured by even a faint light; and there were but peculiar sounds, and these from stringed instruments, which did not inspire him with horror.
The Ushers' penchant for isolation also places them in a somewhat supernatural realm where they exist with the world, but not "in" the same world as everyone. It is as if "the house of Usher" is the only niche where they can actually exist...and now, it can no longer resist it: it is falling apart.
The house, as well as its resident, is described:
with an utter depression of soul which I can compare to no earthly sensation more properly than to the after-dream of the reveller upon opium--the bitter lapse into everyday life--the hideous dropping off of the veil
All of this is symbolic of the mental, spiritual, and physical state of this rare family, which (as it is debated) could even represent a fragment of the narrator's own state of mind. As "a house", the "house of Usher" really encompasses the entire generation of Ushers. They are a dying race; they are disappearing and the house is falling from its foundation.
There are quite a few symbols used in this short story -- some are more easily noticable than others. The first symbol, and probably the most important, is the Usher's Mansion which symbolizes the deterioration or decline of the Usher family; later on in the story, the collapsing mansion becomes a symbol of the actual fall of the Usher family. Another symbol here is the lake that surrounds and reflects the image of the mansion. This lake symbolizes how the Ushers try to isolate themselves from society and its reflection symbolizes Madeline as Roderick's twin. The bridge that goes over the lake symbolizes the narrator as the only link that Roderick has with the outside world. Finally, the storm represents the mood and the violent and unstable emotions that the characters experience throughout the story.
The fissure (split) in the Usher mansion is another symbol. This represents the split of the twin personalities of the surviving Ushers, predicting the destruction of the family and the house.
Roderick's lyric, "The Haunted Palace", is another symbol in the book. It represents death and madness. His abstract painting is a "phantasmagoric" idea of the narrator and represents the "fantastic character" of his guitar playing.
The inability of Madeline to have children symbolizes the end of the Ushers. When Roderick dies, there will be no one to carry on the name.
According to Poe, the whole story is symbolic of the Apocalypse, the end of the world.
The decrepit nature of the house itself symbolizes the deconstruction of the Usher family. Also, the storm is symbolic for the events that occur to the characters. The "vacant-eye"-like windows mirror those seen in Roderick and Madeline. All symbolize a lifelessness.