These lines could imply that Rose's obsession with the physical fence she wants to build exposes her lack of faith in the spiritual fence, leaving her and her family essentially defenseless before the impending breakdown of the family structure.
See that roof you got over your head at night Let me tell you something about that roof. It's been over ten years since that roof was last tarred. See now . . . the snow come this winter and sit up there on that roof like it is . . . and it's gonna seep inside. It's just gonna be a little bit . . . ain't gonna hardly notice it. Then the next thing you know, it's gonna be leaking all over the house. Then the wood rot from all that water and you gonna need a whole new roof. (222)
Knowing this, knowing that it is the roof that needs to be made stronger, Troy works on the fence his wife wants, perhaps because of his guilt at breaking his marital vows (he has scaled the marital fence).
As events turn out, Rose would have perhaps been happier trusting with a full heart in a spiritual fence to protect her and hers, rather than in a physical one which only seems to hasten the breakdown of the family.