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Transfer slabs: the next frontier of the building safety crisis?
Transfer slabs that have not been safely designed or installed are an emerging new risk to resident safety in high rises. What can be done? Peter Apps reports
Transfer slabs that have not been safely designed or installed are an emerging new risk to resident safety in high rises. What can be done? Peter Apps reports #UKhousingOn the face of it, Sapphire Court in Stratford, east London, is a success story for housing policy.
It looks like one of the plush, expensive new build developments that have popped up in this part of east London in the years since the 2012 Olympics. Most of the 131 flats are used for social rented housing, and let out to local people who would otherwise struggle to afford to live in the area.
But look closer, and a different story emerges. The building was constructed in 2011 for Genesis Housing Association, now Notting Hill Genesis (NHG), under a ‘design and build’ contract by Ardmore Group. It was part of a scheme that included the nearby 43-storey Halo Tower – then one of the tallest residential buildings in London.
Since 2017, though, the buildings have had problems. First, combustible high-pressure laminate (HPL) cladding was discovered on several blocks around the site, including Sapphire Court. A long programme of remediation took place – the issue was discovered in 2019 and work completed in summer 2024.
Residents may have finally breathed a sigh of relief, but that would have been short-lived. Their problems were about to get worse.
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Structural engineering advice on the project had been sub-contracted by Ardmore to URS Corporation, a firm that is now part of the consultancy AECOM.
Over the past few years, URS’s work has come under the microscope – structural problems emerged in buildings it had designed for the private house builder Barratt Homes, with a major series of court cases launched to work out the liability. The problems stemmed from reliance on computer models, which produced unreliable calculations about stability.
AECOM then carried out further reviews of URS’s historic work and landed on the Stratford Halo development. In February 2025, AECOM alerted NHG that it should check ‘transfer slabs’ contained in Sapphire Court and some of its neighbouring blocks.
Transfer slabs are a way of transferring the weight of a building between floors when the layout changes from floor to floor. Normally, each column in a building sits directly on top of the one below, and its weight passes straight down. But if the floor layout changes (for an underground car park, for example) the load needs to transfer across the floor of the building to the differently arranged columns below.
A ‘transfer slab’ is the means of achieving this. But the slab carries a lot of weight, and if the calculations are wrong, the weight of the column above can crack it and break through – a failure known as ‘punching shear’, which is a potentially catastrophic event for a building.
Since the Building Safety Regulator (BSR) published an advice note on the issue in December, the Stratford Halo development is the only major development with a transfer slab to hit the public domain.
“I can tell you for sure it’s way more than one building,” says a structural engineer who asked not to be named.
“Everyone is keeping it well under wraps, people are not highlighting it. The ones you’ve heard of are the ones where they have had to make it public. But large developers have been going through a systematic assessment of their buildings and finding where forces are underestimated.
“It could end up being another very expensive problem for the industry, which is obviously already under so much pressure.”
NHG worked with AECOM to investigate and made some worrying discoveries. There was cracking in some locations around the slab edges in Sapphire Court, and the transfer slab incorporated some ‘knock-out panels’ (designed so they could potentially be removed at a later date to install an additional lift).
An issue relating to a lack of rebar to reinforce the strength of the concrete at column heads was also discovered.
At first, a small group of residents were moved out of Sapphire Court, but then in November NHG evacuated not only that building, but three others on the estate that had similar issues. Residents have been in temporary housing in Newham ever since – with occasional visits permitted to retrieve belongings.
It is Sapphire Court that is worst affected – the other buildings may be made safe with propping work – but the complexity of the problem there means intrusive inspections and potentially major remediation work will be needed. NHG has told residents that work will continue through 2027.
In a statement (which you can read in full below) NHG told Inside Housing that its top priority had been resident safety, that it had gone “the extra mile” to support residents to move into temporary housing, and that it had offered “fair and equitable” buy-back deals to leaseholders.
And this could just be the start of the story. There are major concerns about the design of transfer slabs in buildings around the country, and other buildings are already understood to have been found to need remediation work – although details are being kept quiet.
So is this another major chapter in the seemingly ever-growing building safety crisis? How did it happen? And what can be done?
The tip of the iceberg
The broader issue with transfer slabs has emerged slowly. In November 2024, the Institution of Structural Engineers (ISE) released new guidance on the design of slabs. This was the first guidance released on how to properly design the features – despite their use being relatively widespread in the industry since at least the early 2000s.
While it didn’t cover the question of assessing existing buildings, it did raise some broad concerns.
“Despite being a widespread typology, the document suggests that there is currently a lack of understanding, robustness and consistency in the design approaches adopted for transfer slabs,” the ISE guidance said.
Conversations behind the scenes then pushed the BSR to seek assurances that existing buildings were safe. In December, it made a public announcement.
“The Building Safety Regulator is making you aware of a potential structural safety issue affecting reinforced concrete buildings constructed with ‘transfer slabs’,” it said, advising building owners to identify if such slabs were present and seek independent advice from a competent structural engineer.
“We are working with industry experts and the Ministry of Housing, Communities and Local Government (MHCLG) to better understand the extent of the risk, and how the risk can be identified and managed proportionately in existing buildings,” the BSR added.
Since then, there has been no public communication from the regulator, the ISE or the government – although more detailed advice is understood to be under development.
So what should housing providers be doing?
“I completely support [the BSR advice] and would add that owners should also seek to obtain as much structural design and construction information about their buildings as they reasonably can,” says Andrew Lawrence, a fellow at the engineering consultancy Arup.
He adds that the design has been increasingly common in the past 25 years, particularly in “mixed-use developments, buildings with podium levels and schemes incorporating retail or parking at lower levels”.
This should allow housing providers to quickly narrow their searches to buildings with these features, at least in the first instance.
“The first thing to do is work out if you’ve got a [transfer slab],” adds the anonymous structural engineer. “If you have existing drawings, you can start to do it through that. And then you need targeted, measured surveys to the building – a defect survey to the underside, checking the column above and the column below.
“There would almost certainly be early signs of stress cracks if there is a problem. A good thing about concrete is you will get signs of failure before it all comes crashing down.”
“I’d like to see the Institution of Structural Engineers and other industry bodies leading on clear advice, rather than leaving social landlords to go out to 25 different bodies”
There is a danger, however, from over-reliance on visual checks – an engineer needs to assess the design and state of the building. “Visual checks alone are unlikely to identify transfer slab risks,” Mr Lawrence says.
“Cracking in concrete slabs is common and is usually not problematic, and so distinguishing between benign cracks and cracks that would indicate a concern requires specialist expertise from a competent structural engineer.”
John Carpenter, a senior structural engineer who recently quit the Institution of Civil Engineers over its response to Grenfell, says building owners face a challenge in measuring their response.
“This could impact three or four-storey buildings as well as [high-rise buildings], so there is a potentially a lot of checking to do,” he says.
“I’d like to see the Institution of Structural Engineers and other industry bodies leading on clear advice, rather than leaving social landlords to go out to 25 different bodies and getting 26 different answers. There is a danger from going over the top, and a danger from not doing enough.”
One building safety manager at a housing association says that their discussions with the BSR indicate that this issue is “a real, key priority” currently. The regulator is understood to be working towards robust guidance to responsible persons, but is still investigating the issue.
“They don’t know the scale of it at this stage,” the building safety manager says. “They are working to understand how widespread the risk is.”
Mr Lawrence says that there are various options available if issues do emerge with a building.
“One is to provide local strengthening around the top of the column – a so-called ‘drop head’. Another is to install new columns immediately below the affected areas and thereby reduce the load carried by the slab,” he says.
“However, remediation strategies are highly building-specific and depend on factors such as the degree of understrength, structural layout and accessibility as well as occupancy or operational constraints.”
A problem is that this will take time. For high-rise buildings, emergency work will be done immediately, but longer-term fixes will require sign-off from the BSR. And depending on the state of the building, it may need to be evacuated while the work is carried out.
Key terms defined
Transfer slab: A reinforced concrete floor that transfers the weight from columns above to those below, when the layout of columns changes from floor to floor. They are most commonly found in mixed-use buildings, for example when there is an underground car park beneath flats
Punching shear: When a concentrated load, such as a column, pushes directly through a thinner structural element below it – like a pen pushing through a sheet of paper
Brittle and elastic failure: Brittle failure occurs when something snaps suddenly without prior signs of stress, whereas elastic failure occurs when a element shows clear signs of stress, such as cracking, before it breaks
Reinforced concrete: Concrete slabs that have had steel bars or mesh embedded before the concrete sets to increase their ability to handle heavy loads without cracking or breaking
The idea of a building collapse caused by a transfer slab failure is terrifying. It has only been five years since the Surfside condominium collapse in Miami killed 98 people.
That failure started with a problem with slabs and columns on the pool deck and resulted in the total loss of the building. Could something similar happen with a transfer slab in the UK?
Such a catastrophic failure is not guaranteed.
“There is an inherent strength in reinforced concrete structures which makes [a total loss of a building due to a transfer slab failure] unlikely,” says the anonymous structural engineer. “The structure would react to remove the loads away from the area that’s collapsed; that’s a design condition in a lot of buildings.”
Nonetheless, the danger exists. Structural engineers talk about the difference between elastic failure (where you see warning signs before a collapse) and brittle failure (where the collapse happens suddenly, with little or no warning).
The Pipers Row car park collapse in 1997, for example, occurred through punching shear – with a 120-tonne section of the top floor collapsing suddenly. The car park was unoccupied at the time, which meant the incident passed without injury.
A report found that it had occurred due to poor maintenance: a localised breakdown of the waterproof membrane had allowed water ingress around the column and frost had weakened the concrete over time.
“Brittle failure is something that you have to avoid at all costs. Punching shear gets pretty close to that,” Mr Carpenter says.
“You would get cracking, but that’s no good as a precursor if you have a suspended ceiling. In a reinforced concrete frame building, you do have an inherent ability for loads to go elsewhere. That’s a design condition for the larger structures, but nonetheless, [punching shear through a transfer slab] is closer to a brittle failure situation than anyone would be comfortable with.”
Lessons to learn
Are there lessons for the construction and housing sectors in the way this latest chapter of the building safety saga has emerged? Once more, we are scrabbling to deal with the legacy of poor building work in the first two decades of the 21st century, when a generation of blocks of residential flats were built with limited oversight and a light-touch regulatory regime.
“This seems to me to go back to some really fundamental flaws in the industry. The first is checking, and the second is supervision on site,” Mr Carpenter says.
He explains that in the first decades of his 40-year career in the industry, designs would be thoroughly checked, either in their totality or with a selective focus on the safety-critical elements, and then given an overall assessment by an individual in the firm known as a ‘passer’.
“A passer was a trusted individual in the practice with grey hair, who had seen it all before,” he says. “They would be given the drawings and put in a room for a few hours, and then they would grill you on all your decisions – how have you designed this, what actions have you taken to mitigate this risk, and so on.”
Similarly, structural engineers would attend the building site to ensure the reinforcement was placed correctly, that the correct depth of concrete was poured and that other safety-critical elements were installed correctly. Now, that rarely happens.
“In today’s world of competitive pricing, often everybody involved has been appointed on a lowest-bid basis, and independent supervision has largely disappeared,” he says.
“The designer is reluctant to say, ‘we want to spend three days on site’, and to price that into their quote, because they know they will lose out on the job to someone cheaper if they do.”
He explains that the designs may be worked out by different professionals – a client designer may begin the process, which is then completed by a designer appointed by the contractor.
“By that point, the client designer may have been let go,” he says. “So they may never meet.”
Mr Lawrence agrees that additional checking is critical: “Reducing the risk of structural safety issues of this nature requires a combination of stronger oversight, clearer accountability and more robust checking processes.”
He says that appointing a separate firm of engineers to independently check plans would add less than 1% to overall construction costs. “Given the potential cost and disruption associated with remediation, this represents a proportionate and effective risk reduction measure,” he says.
There is also, potentially, a lesson about over-reliance on computer models, which has a wider resonance as the industry adopts AI.
“The introduction of these ever more powerful software systems has created the opportunity for engineers, maybe under commercial pressure, to very quickly create large and complex structures,” says the anonymous engineer. “Over-reliance on these systems creates the risk of this sort of thing happening.”
For now though, it is another safety risk that building owners must attempt to address, which may lead to more disruption and uncertainty for residents.
“You wonder what’s next,” the building safety manager says. “We have had cladding, and then RAAC concrete, and now this. I don’t think this is the tip of the iceberg anymore, but it certainly isn’t the bottom of it either.”
“We have had cladding, and then RAAC concrete, and now this. I don’t think this is the tip of the iceberg anymore, but it certainly isn’t the bottom of it either”
Back in Stratford, the residents certainly are being required to endure the difficulty that comes with living in a building someone failed to construct safely.
“My clients are only able to access their homes through pre-booked appointments with a chaperone,” says Eleanor Solomon, a partner in housing and property litigation at law firm Anthony Gold, who is acting for a number of the residents.
“But that access gets turned off quite frequently, because NHG is monitoring the building for movement, and if they detect anything unusual then no one is allowed in at all.”
NHG says it is doing “everything we reasonably can to facilitate access safely and fairly”, and has recently extended the length of the bookable slots from 30 minutes to an hour. It says that precautions around movement in the building are necessary to protect residents.
The residents are in serviced apartments, not far from the building – and while the quality of the accommodation is reasonably high, there is still a real hardship in being away from home for such a long and uncertain period of time.
“The actual quality of the accommodation, objectively, is good,” says Anthony Gold’s Ms Solomon. “In fact, they’re incredibly expensive, which NHG is at pains to tell my clients at every opportunity.
“But there is still quite a lot of pressure on my clients, especially because lots of them have young children. Quite a lot of them have autistic or otherwise disabled children.
“And being in this block with other people who are paying quite a lot of money to be there, where all the furnishings are completely new, is actually really stressful for a lot of my clients – they are constantly on edge trying to keep everything under control.”
NHG says it “does not accept the suggestion” that it has emphasised cost inappropriately. It says that it needed to move residents quickly, and keep them in the Stratford area – which meant it had to make use of available housing options, including high-value serviced accommodation.
Ms Solomon says that residents dissatisfied with the accommodation have been told to make homelessness applications to Newham Council. NHG responds that there had been just one instance where someone was told to contact the council “as a matter of last resort” after rejecting offers of accommodation.
Ms Solomon adds that some families who were reluctant to leave were threatened with anti-social behaviour injunctions – which warned of the distress that may be caused to emergency workers if the building collapsed and people were killed.
“[NHG] have been very heavy-handed,” Ms Solomon says. “The majority of people who didn’t leave, that I know of, and there may be others whom I don’t act for, just wanted more information. They didn’t need to be threatened with an injunction with arrest powers attached to it.”
NHG says it is not accurate to characterise its role as heavy-handed, and adds that it had provided email updates, a dedicated microsite, resident meetings and a dedicated officer for every household, a 24-hour phone and email hotline and an in-person walk-in hub. It says the injunctions were sought as a “last resort” for a “small number of residents who declined to vacate the building”.
Regardless of how it was handled, the major cause of the disruption in Stratford is familiar: the failure to build people’s homes with proper care. Before lessons can be learned, the sector must now do what it has to do to ensure this latest chapter in the building safety crisis does not end in a greater disaster.
Statements from Notting Hill Genesis and AECOM
Matthew Cornwall-Jones, chief homes officer at Notting Hill Genesis, said in a statement: “Since the day we were informed of this issue our top priority has been on ensuring the safety of our residents. We exhausted every avenue to ensure people who needed to remain in Stratford were able to do so, while working with households with more complex needs to ensure we supported them however we could. In addition, we have offered our leaseholders a fair and equitable buy-back offer.
“Relocating several hundred families has been an immense undertaking. As with any project of this size and scale, it’s impossible to get everything right, but I am incredibly proud of the care and commitment our teams have shown every day to go the extra mile in supporting our residents.
“With all residents having now moved from the affected buildings, our attention continues to be around the structural issues found at Stratford Halo. We know this has been a distressing time for residents and that they want certainty about the future. This is why we are keeping them fully informed of our plans and progress, and continue to have a dedicated team in place to meet them, answer questions and hear their concerns.”
An AECOM spokesperson said: “The original structural engineer of the Stratford Halo development was URS. URS was subsequently acquired by AECOM in 2014. As part of AECOM’s ongoing commitment to health and safety and, in light of the Building Safety Act, following a health and safety review of the original URS design we notified NHG as building owner and principal accountable person of potential structural issues in certain lower-rise buildings at the development.
“Our engineers have worked and continue to work proactively and diligently alongside NHG and its appointed engineer on these specific issues. Health and safety of residents during this process is our top priority.”
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