A roof fall occured 4 Main 20 X # 7 Hdg intersection. Approx 12' high, 18' wide, 18' long above anchorage. 6' super twist & 12' post tension cable bolts were in use. No men or machines injured or trapped. Ventilation was impeeded in face of 7 x 8 @ 20x. Fall was 180' outby face and even with the loading point.