CROSS Safety Report
Risks associated with historic stone balconies
A reporter highlights the structural issues of a stone balcony they encountered when working on a Grade II listed terrace house.
Key Learning Outcomes
For owner and tenants:
- Structural problems with balconies in historic buildings can cause concern
- Sometimes these are due to inherent weaknesses and sometimes due to deterioration
- Balconies may fail without warning and there have been many cases around the world of fatalities occurring from such events
- Secondary problems occur with parts of balcony structures becoming detached and falling
- Regular inspections and timely maintenance can reduce the risks
For architects, designers and structural engineers:
- The importance of balconies as life-safety structures should not be under-estimated
- Design and construction should be such as to resist the ingress of water
- Note that cantilever balconies need structurally adequate and suitably protected fixings to the main structure of the building
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Stability problems and structural issues in relation to balconies are generally common, both for historic and more recently constructed buildings and structures.
Regarding historic buildings, balconies are often very delicate structural elements. The one, in this case, is part of a Grade II listed terrace house (dating to the early Victorian period - 1835-1844) comprising three storeys and a basement. The balcony is made of stone with a cast-iron balustrade around its perimeter and it is evident that the bent corner baluster is precariously supporting a cornerstone fragment (Figure 1).
Common issues and risks with balconies
There are various reasons why a balcony’s stability and condition may be compromised: balconies, similarly with other sensitive elements of existing/ historic buildings (for example cantilevered staircases) are very prone to movement and settlement. Sometimes inherent weaknesses associated with the original design and construction are revealing themselves later in the life of the building, as for example poor structural properties of the materials, thin stone sections, or inadequate embedment of the stone slab into the masonry wall. Corrosion of the metal balustrades is also often associated with cracking issues in the surrounding stone.
Sometimes inherent weaknesses associated with the original design and construction are revealing themselves later in the life of the building.
Another common culprit in such cases is the non-symmetrical arrangement of openings to the front elevation between the ground floor and the levels above. The side edge of balconies is often directly above the brickwork arches above the main entrance, or the ground floor windows. Even the slightest movement of the arch can cause the balcony to crack or rotate. Here, the cause of the problem is not clear. Cast iron has excellent resistance to corrosion and there are no obvious signs of movement. However, metal T-sections have been introduced to support the soffit of other balconies of the terrace demonstrating that problems do exist.
Regular inspections and maintenance can reduce risks
In this case, the risk is somewhat reduced as there is only a small lightwell below. However, for a different size of balcony or arrangement, the risk to pedestrians could be far greater. This example is an opportunity to remind homeowners, landlords and stakeholders of their duty to ensure their properties are well-maintained and do not pose a risk to the safety and wellbeing of the public.
In some cases even removing or mitigating the risks could be adequate until proper repairs are undertaken; in this case the balcony could be easily inspected, loose stone fragments removed or a net installed underneath. Remedial works to stone balconies often involve ‘pinning’ stone pieces together with discreet stainless steel dowels, stone replacement, strapping or masonry rebuilding to mitigate rotation.
Expert Panel Comments
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As the reporter says, this highlights the need for owners to inspect properties regularly, in particular with higher risk elements. How many owners fully understand their responsibilities, or have the wherewithal to actually carry out an inspection? It is probable that most residential owners would not.
In engineering terms balcony slabs are generally cantilevers and hence of particular importance as they have no redundancy and can fail, and fall, without warning. Indeed there may be double cantilevers when the railings are supported only by the edge of a balcony.
Most catastrophic failures of balconies, and there are many around the world, occur during an event such as a party when there is a sudden influx of people onto the structure and hence an increase in live load. An Alert on balconies will be published by CROSS in 2021 covering these and other points.
Most catastrophic failures of balconies, and there are many around the world, occur during an event such as a party when there is a sudden influx of people onto the structure and hence an increase in live load.
Landlords, property owners, clients and building agents should implement a regular programme of appropriate inspections undertaken by a suitably qualified and experienced chartered engineer or building surveyor. Any identified defects should then be included in a programme of remedial works undertaken in an appropriate timescale.
As part of any recommendation following a structural review, advice should be given on whether the balcony should actually be used prior to repairs. They may be dangerous structures and in severe cases protection may be needed for those who might pass underneath.
A secondary issue is that action may be required sooner for listed buildings, as deterioration may continue while approval for remedial work is obtained.
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