Roof Structure - Sawman
In addition to my comments on this not being a landing place of choice, I do have some experience of the construction to share.
Being around Construction in Glasgow for 25 years, I wouldn't expect to find any steel in that roof, indeed all I can see is timber in the photo's. (in fact the open lattice type purlin beams described in earlier posts are a raretity in the UK, used only in steel framed industrial buildings)
There is a massive stone wall in the middle of the pub, this is intact and its this that the aircraft 'slid' down as it went through the rest of the roof structure to the North, it also explains why the South end of the pub was relatively unscathed.
The original first floor timber floor is likely to have been in place, these joists being up to 12 inches deep and at centres of 18 inches, they would not be on hangers or bolted, but set into pockets in the masonry. On top of this would have been further counter joists to support the (1970's?) original flat roof, and there appears to have been a further batten and board layer to support a later over-roofing.
The AC condensers on the roof, I cant see having caused extensive damage to the aircraft if it had made a semi controlled landing and 'bumped' one with its tail. These units are not fixed down (as to do so breaches the roofing membrane) so they are usually sitting on plastic feet or bolted to a loose paving slab - you could easily push one over.
You might also notice a Flue to the South which is about a 1.5 metres or so above the roof, its fragile and completely intact, so the aircraft must have came in nose down at a steep angle to have avoided it and the first AC condenser, only taking out the second one.
From a building perspective, that wasn't a progressive collapse due to overloading, it was impacted hard, I suspect the nose went straight through the timber structure as the dividing wall crushed the port side of the underbody, it may simply have got wedged here momentarily until one further joist broke or slipped from its pocket, allowing further forward and downward movement.
This is just taken from the photo's I have seen and works on similar buildings.
Sorry for my laymans terms descriptions of the aircraft.