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Government-approved fire testing regimes which are used to judge the safety of external cladding systems on buildings are based on how fire behaves when indoors, the Grenfell Inquiry has heard.

Expert witness Dr Barbara Lane told Tuesday鈥檚 hearing that criteria used to classify the safety of external cladding systems was derived from 鈥渋nternal room fire phenomena鈥�.

It came amid a forensic assessment of the flaws within the UK鈥檚 fire safety testing systems over the past 70 years delivered by Lane (pictured), a fire safety expert who has spent nearly 19 years at Arup.

She told the inquiry that 鈥榬eaction to fire鈥� test regimes referenced in Approved Document B of the government鈥檚 building regulations as a measure of classifying the safety of building materials are based on how fire is expected to behave within enclosed rooms, not external walls.

She said: 鈥淭he reaction to fire testing regime is associated with internal room fire phenomena, yet they are relied upon in the statutory guidance document to reduce the surfaces鈥� susceptibility to ignition from an external source and to reduce the danger from fire spread up the external face of the building.鈥�

She added that the 鈥楥lass 0鈥� specification for building materials, which supposedly marks a cladding system as safe to use on external walls, is actually based on two UK testing regimes used for internal fires.

The scope of the first regime, BS476 part 6, is for internal walls and ceilings, while the scope of the second regime, BS476 part 7, is for walls and ceilings.

Lane said that just one of the national 鈥榬eaction to fire鈥� tests included in the government鈥檚 statutory guidance tested the spread of flames over surfaces, and this only measured spread in a horizontal direction, not vertically, as would be expected in an external cladding fire as seen on Grenfell Tower.

She went on to detail how the definition of 鈥楥lass 0鈥� for external cladding systems has 鈥渄egraded with time鈥� since its introduction in 1965.

The minimum fire performance for external walls was watered down in 1985 from requiring construction using non-combustible materials to materials of 鈥渓imited combustibility鈥�.

In 1992, the definition of 鈥榟igh rise鈥� was also increased from 15m to 20m 鈥� although this was lowered to 18m in 2000.

She told the hearing: 鈥淓xtensive work seems to have gone into defining these tests, how to derive results from them and how to rely on them to classify construction products and materials.鈥�

She added that it would be 鈥渙f considerable interest鈥� to hear from those that created and monitored the UK鈥檚 fire test regimes and the companies which carried them out 鈥渢heir understanding of how those parameters were derived, how those parameters were relied upon, and how relevant they considered them to be to the fire performance of external walls鈥�.

Lane was speaking as the second module of the hearing into the tragedy, which killed 72 people in June 2017, continued its focus on the role of product manufacturers and suppliers.

The inquiry continues.