"a new house": Rebbi says: From the time of its "newness" (i.e., even before it is inhabited), make a railing for it. "a house": to exclude an ulam (an entrance hall). How (high a railing is required for) a place (where one keeps) his rolling machine? Three hand-breadths. For the part used for walking about? Ten hand-breadths.
"you shall make a railing for your roof": a positive commandment. "and you shall not place blood in your house": a negative commandment.
(Ibid.) "when the faller falls from it": He was destined to fall from the six days of creation, but "merit is channeled (via the Heavenly ordinance) through the meritorious, and guilt through the guilty" (so that he who is guilty of not building the railing bears the guilt of the fall).