1. Except as otherwise provided in subdivision two of this section, every required stair, fire-stair and fire-tower in dwellings three stories or more in height shall have a bulkhead constructed as provided for fireproof dwellings in section one hundred four, except that the bulkhead of any required stair which is not required to be fireproof may be built of wood covered on the outside with twenty-six gauge metal and on the inside with wire or metal lath covered with two coats of cement plaster or other fire-retarding material approved by the department.
2. Such a dwelling which is two stories or less in height, or is three stories in height and erected pursuant to plans filed with the department on or after May first, nineteen hundred fifty-nine and occupied by not more than one family on each story, shall be provided at each required stair, fire-stair or fire-tower either with such a bulkhead or with a scuttle at least two feet by three feet in size, located in the ceiling of the public hall on the top story and so arranged as to provide direct and uninterrupted access to the roof. Every such scuttle shall be arranged to be readily opened, shall be covered on the outside with metal and shall be provided with stairs or a stationary iron ladder or ladders leading thereto and easily accessible to all the tenants of the dwelling.
3. No bulkhead or other superstructure on the roof shall be used for human occupancy.
4. When a dwelling has a pitched or sloping roof with a pitch or slope of more than fifteen degrees, no bulkhead or scuttle or stair or ladder leading thereto shall be required.