When one builds a new home, he is obligated to make a fence around its roof.
The reason to do this is so that no one falls from the roof and dies, which would bring “blood” upon his home when the faller falls. The Torah calls the person a “faller.” Rashi explains that this person was pre-ordained to fall off a roof and die.
By putting a fence around his roof, the homeowner is showing that he does not want his property to be the cause of someone else’s downfall, even if it was destined to happen.
Rabbeinu Bachya elaborates and says that each person, when he is created, is shown the entirety of his life: How long he will live and when he will die, what events will happen to him, and even what his livelihood will be like, and the person gladly accepts his life.
Thus, the person was destined to fall from the time of the Six Days of Creation, and it was not the fault of the homeowner. However, if one is remiss, and provides an opportunity for another to suffer at his hand, then HaShem visits the misfortune upon him and it is attributed to him.
The person is directed to make a fence, to show that he is concerned about others, but not to build a wall, because it will not protect anyone from falling if it is so ordained.
This reminds us that our free will is limited to making the effort, but what will ultimately happen is the will of Hashem.
