An architectural feature to make Grenfell Tower "look nice" was instrumental in the fire's spread around the building, an inquiry has heard.
The so-called crown - made up of cladding panels containing combustible polyethylene (PE) - at the top of the block of flats was part of the refurbishment carried out between 2012 and 2016.
Expert witness Dr Barbara Lane told a public inquiry into last year's June 14 blaze, which killed 72 people: "It's an architectural feature ... it's just to make the top of the building look nice, I suppose."
Dr Lane, a chartered fire safety engineer and director at specialist design group Arup, said helicopter footage shows the fire spread around the building, causing burning PE to flow down.
"Once the flame got up to level 23 in the first place above Flat 16, it appears then to have been able to travel horizontally in both directions through the crown," she explained.
She said it was "significant" in how it affected the flats on the top floor of the 24-storey building, directly below the crown, where a large number of people died.
Asked whether any fire safety measures could have been installed to stop the fire's spread around the top of the building, Dr Lane said: "The only way you could stop the crown from being a flame front on its own is to not clad it in a combustible material."
In a report prepared for the inquiry, Dr Lane has highlighted a litany of fire safety flaws introduced over more than a decade.
She said safety measures within the building were not suitable and many external materials were flammable
She concluded that, once there was a fire in a flat anywhere near a window, there was a "very high likelihood" that it would break out into the cladding.
Outlining how the blaze spread, Dr Lane said: "At every turn there is something there that can participate in the combustion process, so all the time the flame front has something that will allow it to carry on."