What has happened? Barton House, a 65-year-old 15-storey tower block in Bristol, was built in the late 1950s using reinforced concrete cross walls, pre-cast concrete floors and reinforced concrete external walls. On Monday, a structural engineer commissioned by the council to investigate three of the flats delivered a report that led the council to conclude that an explosion could pose a risk to the structure. On Tuesday the building was evacuated, leaving hundreds of residents temporarily homeless. Marvin Rees, the city mayor, said on Wednesday that the building “may not have been built to the [design] specification”, that there was a “lack of structural ties between the floors and the load-bearing external walls” and that less concrete was used than set out in the original plans for the floors. The council said that “in the event of a fire, explosion or large impact, there is a risk to the structure of the block”. It now says it is carrying out further surveys. It says the design and age of Barton House make it unique within the council’s housing estate and there is “currently no evidence to suggest the issues identified … are present elsewhere”. When was the danger first spotted? Strengthening works and concrete repairs were carried out on the block around 1970, but there were no records of any structural surveys between then and at least 2018, according to a freedom of information response from the council five years ago. That response confirmed that Barton House was one of five housing blocks the council owned that were built with what is known as large panel system (LPS) construction. This system has long been known as potentially dangerous after a gas explosion at the LPS-built Ronan Point tower in east London in 1968 led to its partial collapse, killing four people. The other blocks named in the response as being built with LPS in Bristol were Charleton House, John Cozens House, Haviland House and Langton House. Three other LPS blocks, Glendare House, Rockingham House and Wintour House, had already been demolished at the time of the 2018 FoI response. The council said: “We have no records of any structural surveys carried out on these blocks, although the 1970 records suggest that structural surveys were carried out at that time.” This is likely to raise fears among residents that the problems at Barton House might not be as isolated as the council’s current evidence suggests. Is Barton House an isolated case nationally? LPS systems were used widely in the postwar housing boom and there have been problems on many different council estates, leading to residents being moved out and buildings demolished. There are about 575 social housing blocks that used LPS systems in England, according to Tower Blocks UK, an information and resource hub for people concerned about towers. The Ledbury Estate in Peckham was one of those built using the flat-packed concrete panels, which were delivered on site by lorries. In 2017 engineers were called in to investigate 3cm-wide cracks between the panels that were opening and closing depending on the weather. It found “the buildings were at risk of a disproportionate collapse if there was a gas explosion from the piped gas supply”. Last year demolition and rebuilding was approved by Southwark council. Other affected estates include Broadwater Farm in Tottenham and Aintree in Hammersmith and Fulham. Residents in LPS blocks have been moved out in Portsmouth, where Horatia and Leamington House were demolished in 2022, and at Biart Place in Rugby, where residents were moved out of buildings with defective LPS walls and the blocks were demolished in 2021. Can the problem be fixed? Engineers say strengthening works on LPS blocks are possible, but they often require the buildings to be stripped of all their fixtures, which is a major cost and upheaval. So in many cases demolition is the preferred option. A key risk in LPS blocks is the existence of a piped gas supply. Switching piped gas off is one way to reduce the risk of explosions, and portable gas canisters are also banned in many cases. What is the government doing about it? The government surveyed councils about LPS in housing in 1984, and in 2017 the government wrote again to owners of LPS blocks to urge them to investigate their buildings. The director of the government’s building safety programme told council chief executives about the emerging risks of cracking, gas explosions and fire compartmentation. She said: “The structural design of large panel system buildings may vary from building to building, even where they are ostensibly of the same original design. This means that each building needs to be assessed on its own merits.” However, concerns have been growing inside Whitehall that not enough action has been taken. Last December a senior civil servant made a formal submission to Lee Rowley, then the building safety minister and now the housing minister, urging him to approve a nationwide programme to investigate the safety of ageing social housing blocks built with LPS concrete. The Guardian revealed last month that, frustrated that potentially catastrophic building safety risks were not being properly addressed, the civil servant resigned. In their resignation letter, seen by the Guardian, they said that in 2021 the government’s structural safety working group raised concerns including “the safety of existing large panel system buildings and Arup’s concerns that many LPS towers have not been strengthened following the Ronan Point disaster in 1968”. Did the Grenfell Tower disaster change the government’s approach to building safety? Last year the Building Safety Act became law. This requires building owners to assess the structural and fire safety of their buildings. From next April they will need to apply for a safety certificate from the new Building Safety Regulator, according to the Institute of Structural Engineers. It said these assessments concentrate on how the structure performs in a fire or after an accidental event. About 12,500 buildings need to be assessed over five years. What happens to the Barton House residents? Council officers have advised tenants to “stay with friends and relatives for a short period of time whilst further survey work and analysis is undertaken”. The council said that for those who could not find somewhere to stay, a temporary rest centre had been put in place at the Tawfiq Masjid & Centre with beds, food and drink. It said “further rest centres are in the process of being established”. The council said the length of this temporary arrangement was dependent on a further survey of the building.