Ee was preparing to install a roof bolt in an out-by area of the 6 mains section. When the ee raised the canopy, a rock (3 ft x 1 ft x 3 inches thick) fell from the top & struck the edge of the canopy. The rock rolled around the edge of the canopy & struck the ee's lower back. The x-ray showed hair line fractures to 4 of the lumbar vertebraes. Dr stated that surgery isn't needed.