Why does Rose want the fence built? Why does Troy choose hard wood for the fence?

Rose's desire for a fence symbolizes the way in which she seeks to protect her family. She knows that Troy's checkered past is always there and that he is, perhaps, only moments away from making decisions that forever affect her and her child. Rose's fence seeks to keep the family in and the dangerous world out. It is a symbol of protection.

Troy uses good outside wood because he says it might last forever.