This and other safety failures meant the policy of telling residents to stay in their homes had "effectively failed" within half an hour of the blaze's outbreak - yet it remained in place for almost two hours, fire safety engineer Barbara Lane concluded.
Officials say 72 people died on June 14 past year at Grenfell Tower in the west London district of Kensington. It started with a faulty refrigerator in one apartment before tearing through the block.
Her damning assessment was one of five expert reports released on Monday by the public probe detailing how the inferno started, why it spread so quickly and the effectiveness of the fire protection measures in the 24-storey building. But, she said, residents weren't told to evacuate for more than an hour after that.
Another expert, Jose L Torero, blasted the inadequacy of building guidelines and tests which allow for "obvious dangers" to be incorporated into cladding systems routinely.
The report, by fire safety engineer Barbara Lane, found fire safety breaches including fire doors that only resisted the blaze for 20 minutes rather than 60 and said the aluminum-and polyethylene cladding, which emitted toxic smoke that slowed down firefighters, was "substantially to blame for the tragedy".
The inquiry, chaired by retired judge Sir Martin Moore-Bick, is tasked with establishing what happened at Grenfell and making recommendations to prevent a similar fire from causing such devastation again.
It will examine how the blaze started and spread, the response of emergency services and local authorities, and the country's high-rise building regulations.
The link between the kitchen window and the rainscreen cladding system on both the column and above the windows was the "primary cause of the early stage of the fire spread", she said.
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A 72nd victim died in January.
Making his opening remarks Richard Millett QC, counsel to the inquiry, said: "The fundamental question which lies at the heart of our work is how, in London in 2017, a domestic fire developed so quickly and so catastrophically that an entire high-rise block was engulfed, and how it was that 71 people lost their lives in a matter of hours, leaving family and friends in shock, grief and bewilderment".
The Grenfell Tower public inquiry marks the beginning of a "long road to justice", survivors and bereaved families have said on the first day of evidence hearings.
The inquiry is one of the biggest ever held in Britain, involving dozens of lawyers representing more than 500 "core participants", including residents of the tower. It follows nearly two weeks of personal tributes from friends and family of the victims.
It follows seven days of emotional tributes by the friends and families of those who died.
Grenfell Tower sits in the affluent London borough of Kensington and Chelsea, but many of its residents were low-income earners living in social housing. "What makes it even worse for us is the knowledge that these deaths were completely avoidable".
In parallel with the public inquiry, police have also launched an investigation which they say could result in criminal charges against individuals or organizations involved in the building's history.