Building
Business took me away for a couple days, but I would like to publicly thank "CJ Romeo" and "Electric John" for their input, as well as "andrewruk" who sent me a PM.
I will defer to these folks as to the construction of the Clutha, as they know more of it than me, and f they say the roof was all wood then it is. As I said, all I had to go on were pics which weren't hi-res enough to be absolutely sure of what I saw and part my interpretation of them appears to have been wrong, which is shown in newer pics. My apologies, my experience is in the US and apparently it doesn't cross over here well where things are done differently.
Still, I think we all agree that this was a much stronger roof than usual which would have withstood a much harder impact without immediate failure than one would suppose, especially with the masonry wall they refer to inside, of which I was unaware prior to their posts. That changes a lot of things as far as strength and support issues go.
CJ Romeo is right about rooftop AC units, gravity does most of the work holding them in place and little other fastening is needed or done, so if any part of the A/C struck one it would not have presented much of an obstacle.
One last point as to noise of impact. Being large and flat, a roof when struck acts much like a drumhead, emitting a low-ish frequency 'boom' when something strikes it. I've heard that 3 times when crane rigging failed and the load dropped. Once I was on the roof when an AC unit twice the size of the one on the Clutha dropped from about 20 ft- luckily I was at the other end, but it felt like an earthquake under my feet! Also luckily it didn't go through but it made one heck of a dent. And then once I was 50 ft away from a full concrete truck that overturned on a concrete bridge and I heard absolutely nothing- very strange indeed as others say they looked only when thy heard it impact the barrier and then flip, and that it was a loud event. I can't explain that at all but I got the driver out safely.... Life is indeed strange!
I will leave the thread to others from this point on and comment only via PM to those who contact me to keep the clutter down. May we learn what can be learned and not ever need another thread like this again.