In order to qualify as habitable, rooms shall have a clear ceiling height of not less than seven feet, six inches, except that in attics or top-half-stories used for sleeping, study or similar activities, the ceiling height shall be not less than six feet, six inches over at least one-half of the floor area. In calculating the floor area of such rooms in attics or top-half-stories, only those portions of the floor area of the room having a clear ceiling height of five feet or more may be included.
(`89 Code, § 27.270)