Transfer slabs and safety risks 

Towards the end of last year, the Building Safety Regulator (BSR) issued a letter highlighting risks identified in reinforced concrete multi-storey buildings that incorporate transfer slabs. The concerns centre on the lack of a consistent design methodology and the potential for collapse due to punching shear.

The risks were first raised in November 2024 with the publication of IStructE Guidance on transfer slabs, followed by commentary in the CROSS-UK Newsletter in December last year. RICS subsequently issued a Practice Alert. There have already been at least two instances of emergency building evacuations, and building owners have been advised to determine whether transfer slabs are present and to seek professional advice where there are visible signs of distress or concerns regarding condition or designs.

A transfer slab is a thick reinforced concrete slab designed to support columns that do not continue directly down to the foundations. Instead, the slab transfers loads horizontally to other supporting columns. 

They are typical in residential developments, student accommodation, hotels and mixed-use buildings of five storeys. Common uses include where the column spacing on the ground floor of a building (possibly commercial space) is greater than the spacing on the first floor and above, or where successive floor plates step back from one another to form terraces. 

Typically, transfer slabs are flat without downstand beams, making their formwork simpler and more economical. However, they are thicker than standard slabs – as a rule of thumb, 75mm thickness per storey height – and this can have repercussions in terms of temporary support, the need for large volume continuous pours and the effects of heat and drying shrinkage following placement.

Columns transfer loads to the foundations or other loadbearing elements of a building, but this can be complex and influenced by construction sequencing and temporary works. Over time, concrete shrinkage can lead to potentially significant changes in the way that the loads are transferred. Because a transfer slab is taking point loads, it must be designed to accommodate the risk of punching shear – the potential for the column to punch through the slab. 

Doubts have been expressed over the design methodology applied to the design of slabs in this scenario. Engineers have found that the shear (slicing) forces (and live loads) encountered can be much higher than originally envisaged, particularly where there are short distances between offset columns. 

Detecting punching shear is challenging because failure can occur with little visible warning. An early precursor can be the discovery of cracking in the structure, but this cannot be expected to occur in all cases or evidence might be misleading. Clearly, evidence of deflection in a flat slab beneath a column might trigger concern, but in an occupied building, particularly with suspended ceiling finishes, it might be very difficult to identify such movement without very detailed measurement.

Historically, transfer slabs have not been designed using established principles, and no standard approach exists. Prior to the IStructE guidance issued in November 2024, engineers used a variety of design and modelling techniques. Since the introduction of the Structural Eurocodes in 2005, design standards are more performance-based and less informative. 

While BS8110 has been withdrawn, compliance with building regulations is not dependent upon a specific code as long as the functional requirements are met. Thus, there is scope for various design methodologies including software-based techniques such as finite element analysis and 3D modelling. This is all well and good, but the uncertainty lies in the lack of consistency, the complexity of the different forces at play and the risk that a “one size fits all” approach to design can fail to provide adequate capacity.

The Institution of Structural Engineers has published guidance on this issue. Although primarily intended for engineers, the guide also provides advice in the verification of existing structures.

In its Practice Alert of 14 January 2026, RICS draws mainly upon the BSR’s advice: “The government is currently recommending 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 be instructed to carry out condition surveys of such tall buildings to identify buildings with transfer slabs. If a building is identified, then a suitable expert structural engineer will be required to carry out more in-depth analysis. 

“RICS recommends that IStructE are contacted for suitable competent engineers.”

The risks are such that a warning should be issued, but the mere existence of a transfer slab does not mean that failure is inevitable or that immediate steps need be taken to evacuate the building or take additional precautions. The BSR is currently working with the Building Advisory Committee in co-operation with the Ministry of Housing, Communities and Local Government and so further guidance is likely to be issued in due course.

A prudent first step would be to review any potential cases to see if a transfer slab is likely to exist and start to narrow down the issue to more manageable proportions. Such an exercise ought to be within the remit of suitably experienced chartered building surveyors in the first instance, but detailed analysis is something that will demand structural engineering advice. 

A desktop study of construction drawings ought to indicate whether there is potential for a transfer slab, but this will probably need to be followed up by site inspection. Examination of the structural philosophy, if it is within the health and safety file, may also show whether transfer slabs have been employed.

The difficulty arises in ascertaining whether there are visible signs of distress or specific concerns regarding a building’s condition or design, simply because in a completed building most of the affected areas will be concealed from view either by finishes and furnishings or by suspended ceilings, insulation and so on. There will be an onus on surveyors to identify whether a transfer slab exists and 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 raise suspicion.

Deflection is probably the most common indicator of a problem; with the structure concealed from view, indicators may include ill-fitting partitions, windows, doors or furniture elements such as cupboards or desks no longer level or distorted. Deflection can also induce significant bending moments in columns or shear walls, hence the need to be vigilant for signs of unusual cracking.

Regulated firms need to have regard to the nature of the concerns and would be expected to have regard to the guidance available at the time of any survey. This would probably include the identification of a transfer slab, as opposed to a slab designed with downstand beams, and usually a recommendation for further enquiry.

In addition to the above, and specifically in relation to high-rise buildings, it is possible that building safety case reports may need to be revisited and amended. 

A detailed analysis of a transfer slab is something that is outside the remit of the average chartered building surveyor, but recognition of the form of structure is not and there will now be a reasonable expectation that a surveyor will have flagged the existence or possible existence of this form of construction and made appropriate recommendations for further advice. 

More detailed analysis is certainly the remit of a suitably qualified engineer, but they may also be faced with similar difficulties – lack of access, concealed surfaces and so on. There are various measures that should be possible to check, for example strength of concrete and thickness of slab, top and bottom reinforcement etc. Non-destructive techniques may help up to a point, but it’s more difficult and potentially unreliable to determine the nature of the critical shear reinforcement near column bearing and head positions.

The project monitor may not be involved with quality monitoring, ie making a distinction between bank monitoring roles and monitoring for a purchaser or tenant, but the adoption of a transfer slab is probably something that could have consequences later in the life of a building. The monitor may not have the ability to influence design in the same way as a project manager, but it would be appropriate to flag at least that this form of construction is being used along with an explanation of the potential risks that may be involved.

There are several aspects to consider when analysing the legal position around transfer slabs. First, in terms of negligence claims for failing to identify a transfer slab say during a technical due diligence exercise, it may be that a reasonable defence is that until January 2026 (or arguably late 2024) the issue was not well known or understood. A reasonably qualified and experienced surveyor would not necessarily have been aware of the problem and so would not routinely report on it. 

Post-January 2026, and given the RICS Practice Alert, the position may be less clear cut. Of course, if there were reasonable grounds to identify cracking, deflection or even visible signs of punching shear, then the position might be different.

Non-compliance with the current guide does not necessarily mean that the earlier design was negligent, building standards and practices change all the time and it is unreasonable to judge performance by comparison with documents that did not exist at the time of construction. 

Regarding the implications this will all have on high-rise residential buildings, the building safety case will need to consider the existence of and the design of the transfer slab so that risks can be identified and dealt with. Such an exercise may dictate the need for a certain amount of back calculation by an engineer as well as physical inspection and potentially monitoring. 

If a problem is identified, there may be mitigation measures such as the introduction of strengthening, propping or load reduction. The building safety course ought to be reviewed every five years unless there are indications of deterioration in the meantime.

By adopting a proportionate, evidence-based approach, beginning with the identification of potential transfer slabs and escalating to specialist engineering appraisal where justified, the industry can manage risk sensibly while awaiting further clarity from the regulator. 

Understanding, careful documentation and early engagement with competent structural engineers will be key to ensuring building safety, regulatory compliance and continued confidence in the built environment.