An unplanned Roof Fall occurred - 1st West Pillar Section between XC 17 & 18 - #9 Entry, inby the loading point. Approximately 17 by 20 and 4.5 high. No one was injured or entrapped and no ventilation was interrupted. Preliminary diagnoses; Cable Bolts were being installed to replace Floor to Roof Supports. When 2 timbers were knocked out with the ATRS the fall occurred.