What flaw in the front of the house does the narrator see early in the story?

The narrator sees a "fissure" or crack that compromises the structural integrity of the 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.

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