EE PRESSURIZED ROOF BOLTING MAHCINE ASTRS AGAINST ROOF. THE OTHER SIDE OPERATOR NEEDED TO REPOSTIION THE MACHINE SO THE INJURED EE LET DOWN HIS ATRS. WHEN HE DID THIS A ROCK FELL OUT CAUSING A LACERATION TO LEFT ARM REQUIRING STITCHES & A CONTUSION & ABRASION OF LEFT CALF. THE ROOF SHOWED NO SIGNS OF DETERIORATION