One of the rescue and recovery officers has reported that there were
three roofs on the pub - the original ceiling of the ground floor building, the floor of the previously demolished first floor of the tenement block and the timber and felt roof installed to provide weather protection.
The space(s) between the roof structures contained sound-absorbing materials (which explains the considerable debris that fell into the bar).
The strength of the 'roof'' was greater than previously anticipated, and the severity of the impact that resulted in the aircraft penetrating the structure was accordingly greater than a simple autorotating landing.
BBC News - Glasgow helicopter crash: Aerial views show pub damage