The roof fall was discovered by EE on 8-13-07 at 9:40 AM as he was firebossing area. Fall was in the intersection & measured 40' long x 20' wide x 0-12' thick. Area was developed 12-2002. No injury resulted, no ventilation interruption occurred, no equipment involved. Fall will not be cleaned up. Fall occurred on Southwest Mains bleeder panel in #3 heading @ survey station #10.