nigelh - thoughts are plausable at that particular point in the flight, others have commented on how relativley undamaged the aircraft appeared, and others have commented primarily from witness accounts who were under the impact point, how there was a noise first, the band commented on it and began to play again, then the roof came in. This is significant in terms of considering the impact speed/force and its immediate effect. I would hope that the AAIB considers reconstructing this part of the roof at a test facility and drop test some weight onto it. They would however have to salvage some of the remaining roof to do so to replicate aged structures etc. There was a considerable amount of roof undamaged and I would assume that much of the various main roof layers were laid at the same time and would provide a benchmark, a better physical fit benchmark than a theory. I only mention this as a avenue for consideration, the AAIB may well have this in mind.