EE AND HIS PARTNER WERE WORKING ON UNIT 4 ROOF MONITORS. THEY HAD TO REMOVE SOME SHEETING TO GET AT THE LOUVERS. WHEN USING AN IMPACT TO REMOVE THE SHEETING SCREWS, ALL THE BOTTOM SCREWS WERE OUT AND WHEN HE STARTED TO REMOVE THE CENTER SCREWS THE SHEETING BROKE OFF STRIKING EMPLOYEE'S LEGS RESULTING IN A CONTUSION TO HIS RIGHT LEG.