Every employer shall provide, and cause to be used, suitable roof ladders or duck-boards or crawling-boards for persons required to work on any roof that has an unsafe pitch or surface, or that is covered or is to be covered with material through which a person could fall: Provided that suitable safety belts attached to the structure or any similar effective equipment may be used in place of roof-ladders, duck-boards or crawling boards on pitched roofs covered with non-fragile material.