In all factories, mercantile establishments, mills or workshops, sufficient and reasonable means of escape in case of fire shall be provided, and such means of escape shall at all times be kept free from any obstruction and shall be kept in good repair and ready for use, and shall be plainly marked as such. The commissioner of labor or factory inspector may order fire escapes erected on the outside of buildings used as factories, mercantile establishments, mills or workshops which are two or more stories in height, whenever deemed by the commissioner of labor or factory inspector to be necessary.