AN UNINTENTIONAL ROOF FALL OCCURRED ON THE 4A BELT LINE AT 21 + 42 X-CUT IN ENTRY #4 ON THE 1ST NORTH PANEL OFF OF 3RD EAST MAIN. THE FALL MEASURED 35' LONG X 20' WIDE X 7' HIGH. NO ONE WAS IN JURED & PASSAGE OF MEN OR VENTILATION WAS NOT AFFECTED. FALL WAS DUE TO HIDDEN SLIPS ABOVE