Why did Sal's father start chipping away at the plaster wall in their house in Bybanks, Kentucky?

Sal tells us this story at the end of the first chapter, called “A Face at the Window.” She says that when they lived in the old farmhouse, her parents were taking the time to restore it. This explains why her father was “chipping away” at the plaster wall in the living room, at first. When he got the news that his wife (Sal’s mother) was not coming back to Kentucky, “he pounded and pounded on that wall with a chisel and a hammer,” Sal says. This time, he was probably taking out his anger, frustration, and grief on the wall. He uncovered a brick fireplace hiding behind it, too. This quick vignette foreshadows the truth unspoken throughout most of the book—the mystery of what happened to Sal’s mother. Here, Sal also uses it as a metaphor in the telling of her own story:

The reason that Phoebe’s story reminds me of that plaster wall and the hidden fireplace is that beneath Phoebe’s story was another one. Mine.