INJURED EMPLOYEE WAS ON AN EXTENSION LADDER LEANING AGAINST THE OUTSIDE METAL WALL OF THE ELECTRIC SHOP ASSISTING WITH REPLACEMENT OF A PIECE OFSTRIPPING ON THE ROOF EDGE. HE RE-POSITIONED THE LADDER UNDER THE ROOF'S EDGE BUT DID NOT SHORTEN ITS LENGTH SO IT WAS AT AN ANGLE. LADDER SLID OUT FROM UNDER HIM. LANDED ON HIS FEET BREAKING A BONE.