A BRE report on Grenfell Tower has found that the fire would not have spread if the building had not been refurbished between 2014 and 2016.
The 210-page interim analysis, which was prepared for the Metropolitan Police to assist with the investigation into the fire on 14 June last year, was leaked to the Evening Standard.
The overall refurbishment was found to fall short of building standards and aided the spread of the fire that killed 71 people.
BRE’s analysis said that if the tower had not been re-clad, the fire would have probably been contained in one flat.
The report said: “Grenfell Tower, as originally built, appears to have been designed on the premise of providing very high levels of passive fire protection.”
It added: “The original facade of Grenfell Tower, comprising exposed concrete and, given its age, likely timber or metal frame windows, would not have provided a medium for fire spread up the external surface.
“In BRE’s opinion… there would have been little opportunity for a fire in a flat of Grenfell Tower to spread to any neighbouring flats.”
Five significant building failings were identified by the report.
Within the flats, the materials used in the PVC window frames were unable to provide 30 minutes of fire resistance, while also being narrower than the concrete surrounds, leaving gaps of up to 15 cm.
These factors combined to allow the fire to spread not only from its source – a faulty fridge – to the cladding, but also “from the facade back into the flats”.
BRE said instead of acting as containment for the fire, the frames acted as a “fuel”.
Once outside, the fire was able to catch the “combustible” facade and spread “up, across and within” it.
The plastic core of the aluminium composite material used for the rainscreen cladding was described as “highly combustible”.
Some cavity barriers, which were designed to seal the gap between the concrete exterior and the cladding in the event of a fire, were found installed upside down and back to front.
It also found some cavity barriers were installed with a 50 mm gap when they were designed to seal a 25 mm gap.
These failings provided a route for the fire to spread rapidly up the exterior of the building.
Inside the building, BRE found that many front doors didn’t have automatic closers, which meant that when residents fled the doors stayed open, allowing more smoke to enter the stairwell and the fire to spread.
BRE said a tower of Grenfell’s height should also have had a wet rising main, which has water at all times, for firefighters to use at the top of the building.
Instead, the building had an extended dry rising main, which needed water fed from a fire engine, that BRE said was “unlikely to provide sufficient pressure and flow of water for firefighting at the top of the tower”.
BRE’s interim report provided its first conclusions after analysing flats on the fourth, fifth, sixth and 23rd floors.
The remaining flats are still being examined, as is the fire resistance of the doors and lifts and the gas installation.
Responding to the leak, a Met Police spokesperson said: “Our aim is to carry out an investigation that has integrity and if it uncovers evidence that any individual or organisation is criminally culpable we want that evidence to be tested through the judicial system.
“As such we are disappointed that an interim draft report appears to have been leaked and published.
“To protect the integrity of the investigation, the MPS will not confirm specifics of the ongoing investigation.”
A spokesperson for BRE declined to comment on the report.
The Grenfell Tower Inquiry is due to hear its first evidence on 4 June.
Last month the inquiry named a number of subcontractors on the core participants list, which allows them advanced disclosure of certain documents and the option to make opening and closing statements.
Among those named is Rydon, which won the £10m refurbishment contract for the tower in May 2014.
In its results for 2017 released last month, Rydon did not make any provision for costs related to the fire.