CJ Romeo's post above agrees with what I posted earlier about the probable strength of the roof, being the old first floor of a tenement. But if the impact was nearly directly over an internal wall as well, which would originally been the base of one several stories high and massively strong, the building may have been able to almost resist even an impact severe enough to crush the helicopter cabin, shear off the tail, and the helicopter might not even have gone right into the pub initially, until the nose tipped through the now weakened section under the front of it.
That would fit with the band having time to make a joke about the ceiling after the initial crash and before the helicopter then fell through into the pub.
Also, the initial impact was described as sounding like a gas explosion
We were in the pub having a good night. The band was on, it was fine: no drama, really good atmosphere. People say it was 22:27 but I have no view if it was or it wasn't.
There was a huge bang
Suddenly I heard this bang. I wasn't sure what had happened. At first I thought the speakers had blown but then I looked around and this cloud of dust suddenly took over the whole pub.
Nancy Primrose and her sister, Ann Faulds, talked about the moment of impact.
Ann Faulds: Nancy got blown straight up into the air and onto the ground. She was just a couple of feet away from me. Within seconds of the blast, there were people getting pulled out
I had absolutely no idea it was a helicopter. I thought it was a gas explosion.
This was a very heavy impact, whether a complete free fall I will leave to the experts to decide. The pub effectively had three ceiling, this seems to explain the delay in the roof collapsing after the initial impact.