The finding about Grenfell’s cladding not being in line with building regulations should not be misunderstood as a statement that the government’s regulations (and deregulation) were not to blame for the fire (another thread) (sorry)
The crucial thing to understand is this: in the UK we have ‘performance based’ building standards instead of ‘prescriptive’ rules. That’s a political choice which was largely made in the 1980s.
That means the standard for external walls is just that they must “adequately resist the spread of flame”. It is essentially left up to builders and designers to figure out how best to do this
To use an analogy: if we went to a ‘performance-based’ system for driving, all the speed limits would be scrapped and a regulation would be passed saying “drivers should select an appropriate speed”
When there were loads of car crashes we wouldn’t say “why aren’t people selecting safer speeds” we’d say “why did we scrap all the speed limits”
Well, we did it to ‘free industry’. Here’s what Michael Heseltine said he introduced the modern standards in 1985: 
What's crucial to understand is that alongside the headline standards, the government also publishes guidance to industry on how to comply. In the current context, that means Approved Document B.
What Sir Martin found was that the tower breached this headline standard. He hasn’t made any findings about the guidance yet.
This is fair enough. Too many builders just see their job as hitting the standards in the guidance, dusting their hands and saying the building is compliant. That’s not really how ‘performance-based’ standards should work.
But it does mean the standards in AD-B are really, really crucial in terms of answering the question of why this cladding ended up on Grenfell and 400+ other towers around the country
That is (probably) going to come down to the fact that AD-B set a standard for external surfaces of walls called ‘Class 0’
Arconic’s Reynobond PE had a certificate saying it met 'Class 0, even though there is some pretty strong evidence that actually, it did not. And lots of other ACM products also had Class 0 certificates.
So the questions are: why did such dangerous products have Class 0 certificates? And why was Class 0 the standard in the guidance at all?
The second is especially important given that ministers were specifically told to change it because of the dangers of cladding fires in 2000 and 2014. They didn’t.
The government is very, very resistant to this argument and has been claiming since the fire that Class 0 was never the standard for cladding. More on which here:
Phase Two needs to get to this question and hold the people responsible to account if it wants to have any claim to really be getting to the truth of why this whole mess happened.
Addendum: we have now dropped the performance-based standard for external walls in favour of a simple, prescriptive rule which bans combustibles
This is, as far as I’m aware, the first time we’ve had prescriptive building regulations of any kind since the 1980s and is entirely the result of the incredible lobbying of @GrenfellUnited
