§ 241-a. Protection of workmen in or at elevator shaftways, hatchways and stairwells. Any men working in or at elevator shaftways, hatchways and stairwells of buildings in course of construction or demolition shall be protected by sound planking at least two inches thick laid across the opening at levels not more than two stories above and not more than one story below such men, or by other means specified in the rules of the board.