Early in the story , what flaw in the front of the house does the narrator observe ?

The narrator perceived a fizzure, or crack, from the roof to the base in the front of Usher's house.

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.