He prohibited us from leaving obstacles and stumbling blocks in our land and in our homes, so that people not die from them. And that is His saying, "and do not place blood in your house" (Deuteronomy 22:8). And the language of the Sifrei (Sifrei Devarim 229:6) is, "'You shall make a railing for your roof' - that is a positive commandment. 'And do not place blood in your house' - that is a negative commandment." And the regulations of this commandment have already been explained in Chapter 1 of Shekalim and in the Order of Nezikin. (See Parashat Ki Teitzeh; Mishneh Torah, Murderer and the Preservation of Life 11.)