Early in the story "The Fall of the House of Usher," what flaw in the front of the house does the narrator observe?

The narrator describes the entire setting and his own mood in the first five paragraphs of the story and then mentions, almost as an incidental observation, a flaw in the front of the big building.

Perhaps the eye of a scrutinising observer might have discovered a barely perceptible fissure, which, extending from the roof of the building in front, made its way down the wall in a zigzag direction, until it became lost in the sullen waters of the tarn.

This one long sentence serves at least two purposes. It is a very subtle foreshadowing of the "fall" of the House of Usher which will be described at the very end of the story. It also explains what will cause that catastrophic fall. There has to be a physical reason as well as a symbolic or poetic reason for the great building to collapse into the tarn the way the narrator describes it at the end of this bizarre tale.

The radiance was that of the full, setting, and blood-red moon which now shone vividly through that once barely-discernible fissure of which I have before spoken as extending from the roof of the building, in a zigzag direction, to the base. While I gazed, this fissure rapidly widened—there came a fierce breath of the whirlwind—the entire orb of the satellite burst at once upon my sight—my brain reeled as I saw the mighty walls rushing asunder—there was a long tumultuous shouting sound like the voice of a thousand waters—and the deep and dank tarn at my feet closed sullenly and silently over the fragments of the “HOUSE OF USHER.”

The narrator survives the collapse of the great building because he has "fled aghast" after seeing both Roderick Usher and his twin-sister Madeline die in each other's arms. The narrator has to survive in order to be able to describe what happens to the House of Usher at the end. Notice that he twice refers to the "fissure" which he mentioned only briefly in passing at the very end of the fifth paragraph of the story. He now calls the reader's full attention to that "fissure" as it widens and widens until he can see "the entire orb of the satellite," in other words, the full blood-red moon, through the fissure. The house had obviously been doomed to destruction from the very beginning. It was going to collapse because of that fissure, and it does so, coincidentally but quite appropriately, when Roderick and Madeline die together. The most striking image in Poe's story is that of the blood-red moon in full view behind the widening fissure between the two halves of the doomed building.