A recent alert from the Building Safety Regulator (BSR) has highlighted potential structural safety issues affecting buildings constructed with reinforced concrete transfer slabs.
A transfer slab is a thick reinforced concrete floor used to support columns when there is no column directly beneath. It spans between supporting columns and is commonly used where lower floors have wider column spacing, for example, a commercial space on the ground floor of a residential block. They are commonly found in residential and mixed-use buildings, purpose-built student accommodation and high rise developments.
Concerns raised regarding historic transfer slab design
Guidance published by the Institution of Structural Engineers (IStructE) in November 2024 – detailing how transfer slabs should be designed – raised some questions on the adequacy of historic transfer slab design in existing buildings. Until December 2024, there has been no universally accepted methodology for the design of transfer slabs, meaning there have been inconsistencies in how they were designed.
Concerns relate to the risk of punching shear failure, where concentrated loads from columns above cause the column to “punch” through the slab. This failure mode can occur suddenly and may not always be preceded by visible cracking. While no building collapses have been reported in the UK, the risk has prompted precautionary action. For example, London landlord Notting Hill Genesis recently evacuated residents from a development in Stratford due to cracking concrete.
An intrusive inspection revealed cracks in three of the lower blocks. The landlord is reported to have considered that the buildings could remain occupied provided temporary propping was installed. However, these works were not carried out, and the landlord relocated all households from three blocks into temporary accommodation and is not expecting them to be able to return for “several months”. Parts of the same development had also been reclad after post-Grenfell checks found they were wrapped in flammable panels.
Although many transfer slabs may have been designed in line with accepted industry practice at the time of construction, updated technical understanding may now identify vulnerabilities.
A prompt to review existing structures and seek professional advice
While the recent alert from the BSR highlights legitimate concerns around the design and performance of transfer slabs, it does not necessarily follow that the construction is defective. The alerts serve as a prompt to review existing structures and engage professional advice as necessary.
The BSR continues to work with industry bodies to develop guidance on identifying and managing these risks. Further updates are expected, and stakeholders are encouraged to stay informed by subscribing to BSR alerts on the gov.uk website.