MINER ON ROOF WEARING SAFETY HARNESS BUT NOT TIED OFF TO EXISTING CABLE WHERE EXISTING RETRACTABLE WAS AVAILABLE. MINER WAS STEPPING BACKWARD WHILE PULLING HOSE IN PREPARATION FOR COATING ROOF WHEN HE STEPPED ONTO & THROUGH SKYLIGHT SHEETING. MINER FELL APPROX 17 FT ONTO CONCRETE FLOOR BELOW & SUFFERED BROKEN FEMUR.