When you build a new house, you must place a guard-rail around your roof. Do not allow a dangerous situation to remain in your house, since someone can fall from [an unenclosed roof].
Ki tivneh bayit chadash ve'asita ma'akeh legagecha velo-tasim damim beveytecha ki-yipol hanofel mimenu.
The same is true if one buys a house (Sifri), or rents one (Yad, Sekhiruth 6:3). However, some maintain that the latter cases are obligations only by rabbinical legislation (Hagahoth Maimonioth, on Yad, Rotze'ach 11:1; Tzafenath Paaneach).
At least 10 handbreadths (30') high (Bava Bathra 61a; Yad, Rotze'ach 11:1,3).