A roof fall was found in the return aircourse of South Mains ""B"". Fall is approx. 40' long by 18' wide and 8' high. Area was bolted with 8' Install III bolts and resin with bacon straps and wire screen. Air not affected. Location of fall is between 25-A xcut and 26 xcut. Cause of fall was deterioration of beam created by roof bolts due to weathering of the roof.