Regulatory alerts from the Building Safety Regulator, IStructE and RICS have placed transfer slabs under renewed scrutiny – but what are they, why do they fail, and what should surveyors and building owners do next? Trevor Rushton FRICS, Chairman of Watts Group, explains further.
Back in December, the Building Safety Regulator issued a letter to principal accountable persons and building owners highlighting risks identified in reinforced concrete multi-storey buildings containing transfer slabs. The concern centres on a lack of clear, consistent design methodology and the potential for collapse due to punching shear. These risks were first raised in November 2024 with guidance from the Institution of Structural Engineers (IStructE), followed by a CROSS newsletter in December (i). RICS has since issued a Practice Alert. There have already been at least two cases of emergency building evacuations.
What is a transfer slab and where might one be found?
A transfer slab is a thick reinforced concrete floor slab that supports columns above without a corresponding column directly below. It effectively redistributes loads between structural elements.
They are commonly found in residential buildings, student accommodation, hotels and mixed-use developments, typically between five and 20 storeys. They are often used where ground floor layouts (e.g. commercial space) require wider column spacing than upper floors, or where upper storeys step back to form terraces.
Transfer slabs have become more common since around 2000, although there is no definitive data on how widespread their use is. They are typically flat slabs without downstand beams, making temporary formwork easier and cheaper to build. However, slabs are significantly thicker than normal floors (as a rule of thumb, 75mm thickness per storey height), which introduces challenges such as temporary support, the need for large volume continuous pours and the effects of heat and drying shrinkage following placement.
What is the concern?
Transfer slabs carry concentrated loads from columns above, making them vulnerable to punching shear, a failure mode where a column effectively “punches” through the slab.
The complexity of load paths, the influence of construction methods and long-term material behaviour (such as shrinkage) can all affect performance. Engineers have identified that shear forces and live loads may be significantly higher than originally anticipated, particularly where columns are closely offset.
Punching shear is a serious and potentially sudden (brittle) failure. While not all historic collapses relate directly to transfer slabs, similar mechanisms have been implicated in well-known failures such as the Champlain Towers collapse (2021), where punching shear in a slab is believed to have been a contributing factor.
What are the symptoms of punching shear?
One of the difficulties with punching shear is that it is what is termed a brittle failure mode. This means that failures can be sudden, with little or no plastic (permanent) deformation. Cracking may appear, but not always, and can sometimes be misleading.
Transfer slabs (being thicker than normal) can be prone to internal stresses due to curing heat and shrinkage, which can also cause tension cracking, particularly where floors and core walls abut. Visible deflection beneath columns may indicate a problem however, it can be difficult to identify such movement without very detailed measurement, particularly in occupied buildings, where this is often concealed by suspended ceiling finishes.
Are there any reports of failures in transfer slabs?
There are currently no confirmed collapses that have been attributed to transfer slabs, but several high-profile evacuations have occurred. At Wotton Court, cracking around load-bearing columns suggested punching shear, leading to its emergency evacuation and mandated remediation under the Building Safety Act 2022. Whereas, at Stratford Halo in London, multiple blocks were evacuated amid structural concerns, with punching shear suspected as a possible factor.
What guidance is now available?
IStructE published its “Design of Transfer Slabs” guidance in November 2024 (ii), providing recommendations for both new designs and the assessment of existing structures.
RICS followed with a Practice Alert in January 2026, advising that “where there are visible signs of distress or specific concerns regarding a building’s condition or design, building owners should seek immediate professional advice”. RICS members who are chartered building surveyors may identify potential transfer slabs, but detailed analysis must be undertaken by suitably qualified structural engineers.
Should building owners be concerned?
Yes and no. The presence of a transfer slab does not mean a building is unsafe or requires evacuation. However, it does warrant awareness and, where appropriate, further investigation.
A sensible first step is to establish whether a transfer slab exists, typically through a review of drawings and, where necessary, site inspection. And where concerns arise, structural engineering input will be required. However, pay very careful attention to visible areas to see if there is any evidence of cracking, deflection in floors, lack of verticality in columns or other defects that warrant further investigation.
How does the matter affect surveyors?
Surveyors are now expected to be aware of transfer slabs and their potential risks. Identifying their presence, or the possibility of it, and recommending further investigation will increasingly form part of standard practice.
Detailed structural assessment remains outside the typical surveyor’s remit, but failure to flag potential issues may become harder to defend following the RICS Practice Alert.
For high-rise buildings, Building Safety Case Reports may also require review and amendment. While detailed structural analysis falls outside the remit of most chartered building surveyors, there is now a clear expectation that they will recognise, flag and escalate potential transfer slab risks appropriately.
Conclusion
Recent alerts from the Building Safety Regulator, IStructE and RICS highlight legitimate concerns around transfer slabs, but they do not suggest that such structures are inherently unsafe. Instead, they act as a prompt for building owners, surveyors, and engineers to review existing buildings and engage appropriate expertise where needed.
Read more in the latest Tall Buildings Magazine here
