Roof fall discovered 10 belt 77 block, 6' high, 25' wide and 60' long (approximate). Fell on top of old, unused, overcast, knocking it down on top of belt. Re-supported with 8 and 12' bolts, screen and floor to roof supports. Fall occurred due to obscured deterioration of slicken sided clay material. No problems with training, ppe, mining methods or compliance issues.