No inflammable structure, other than a frame to sustain pulleys or sheaves, shall be erected over the entrance of any opening connecting the surface with the underground workings of any mine, and no preparation plant, tipple, breaker or other inflammable structure for the preparation or storage of coal shall be erected nearer than two hundred feet to any such opening, but this act shall not be construed to prohibit the erection of a trestle for the transportation of cars from any slope to such breaker or structure; neither shall it apply to any shaft or slope until the work of development and shipment of coal has commenced: Provided, That this section shall not apply to breakers that are now erected.