Habitable rooms in existing buildings shall have a clear ceiling height over the minimum area required by this code at not less than seven and one-third feet, except that in attics or top half-stories, the ceiling height shall be not less than seven feet over not less than one-third of the minimum area required by this code when used for sleeping, study or similar activity. In calculating the floor area of such rooms, only those portions of the floor area of the room having a clear ceiling height of five feet or more may be included. (Prior code § 17-1-34)