1. Question: Why does Rebbe Akiva Aiger hold you would have to say a blessing on a mezuzah that is already on the doorpost when you move into a house you are newly renting but would not have to say a blessing on the fence that was put around the roof?
Background: A. Rabbi Akiva Aiger holds you have to say a blessing on the mezuzah when moving into the house because it is a new mitzvah for him in this house.
B. There is a mitzvah to put up a fence around the roof or porch if you are able to walk on it so that people won’t fall off.
C. The Birkay Yosef and others who argue on Rabbi Akiva Aiger say that you only make a blessing upon putting on the mezuzah just like upon building the fence.
D. According to Rabbi Akiva Aiger why don’t you say a blessing on the fence since it is a new mitzvah for him in his new house?
E. Hint: What is the motor that sparks the obligation for each mitzvah?
Answer: By mezuzah what sparks the obligation is the person living in the house so when he moves to a new house he has a new mitzvah which prompts him to make another blessing but by the fence what prompts the obligation is the danger and the previous people living there took care of the obligation so there is no mitzvah taking place right now when he moves in.