§ 138. Parapets and guard railings. Except as hereinafter provided every open area of a roof, terrace, areaway, outside stair, retaining wall or porch shall be protected by a parapet wall, a guard railing, or both, extending three feet six inches or more in height above the level of such area or, in the case of a stair landing, stair window or window in a public hall, above the level of the floor adjacent thereto. In dwellings two stories in height, it shall be sufficient if the guard railing is erected at least five feet distant from and encloses any required bulkhead or scuttle opening to the roof. This section shall not apply to dwellings one story in height.