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CROSS Safety Report

Fire spread through balconies

Report ID: 1052 Published: 17 November 2022 Region: CROSS-UK


This report highlights that the extensive use of combustible materials in the construction of balconies can result in fire spread over the external wall to an extent which is inadequate for building safety, even buildings less than 18m.

Key Learning Outcomes

For designers:

  • Consult with a competent fire safety professional to ensure that the building resists adequately the spread of fire

For fire and rescue service:

  • Be aware that some balconies can pose an additional fire spread risk, and that appropriate procedures are in place

For building managers:

  • Ensure that occupants are informed about the excessive storage of materials, including gas cylinders, on their balconies that could pose a hazard in a fire event

Full Report

Find out more about the Full Report

The Full Report below has been submitted to CROSS and describes the reporter’s experience. The text has been edited for clarity and to ensure anonymity and confidentiality by removing any identifiable details. If you would like to know more about our secure reporting process or submit a report yourself, please visit the reporting to CROSS-UK page.


A report has been received which touches upon the presence of combustible materials in balconies. The concern is two-fold; one is the extent in the use of combustible materials in the construction of the balconies, while the other is the inability of the balcony construction to sufficiently limit the spread of fire.

The reporter drew the conclusion from a (recent) event that the fire spread over the external wall was inadequately resisted. This is judged, by the reporter, as deficient performance, even for such a building which is below of 18m in height, because the compartment boundary was by-passed at an early stage, undermining the concept of compartmentation upon which a stay-put approach was in place.

Two phenomena are presented that indicated the breach of compartmentation, and two possible causes:

  • One is the lateral spread of fire, horizontally over the elevation, between balconies of different flats, involving an external volume beyond the compartment of origin. The potential role of dividing screens in preventing or contributing to fire spread, and the continuity of balconies in enabling fire spread should both be explored. The presence of combustible materials in the construction of the balcony is referred to by the reporter as the cause of this type of spread; specifically, the presence of timber, and plastic netting below decking.
  • The second phenomenon is the vertical spread of fire, through involvement of the balcony in the flat above the compartment of origin. This is attributed to the use of perforate decking in the soffit, which allowed the exposing to flames and subsequent involvement of combustible materials in the balcony above, leading to rapid vertical fire spread.

In both cases, potential solutions that involve any kind of treatments for the timber elements should be considered carefully, accounting for the effects of weathering and aging, along with their ability to resist large fire exposures at any point in their service life.

Additionally, the form and orientation of any timber used on the building should be considered as these are aspects that can affect its burning behaviour. Reaction to fire classifications should be checked to ensure that they are based on a relevant test, and are implemented while considering the form of the element and any surrounding materials.

Fire strategy considerations

The reporter considers that the extent of using combustible materials in balconies should take into account the possibility of fire spread and the potential undermining of a stay-put strategy. The building’s fire strategy is provided to address the case of a fire in a single flat unit, which also usually forms a compartment. If there is external fire spread at a rate so rapid that can compromise the effectiveness of active fire safety systems, such as smoke control or suppression systems, even in buildings less than 18m in height, then it is the opinion of the reporter that it could be questioned whether requirement B4 of the Building Regulations 2010 is still satisfied.

Issues related to this report are expected to be included in ongoing government issued research on “Fire Safety: Balconies, Spandrels, and Glazing” (CPD 004/0120/205). This is an indication that the state of knowledge in fire engineering is still evolving to cater for these new developments in construction practises.

Expert Panel Comments

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Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.

The panel agrees that this issue requires attention while appreciating the fact that government has commissioned research, as part of the review of Approved Document B, yet it is key that this research is ongoing. Issues regarding the fire safety of balconies has previously been the topic of a Building Research Establishment (BRE) report.

As the reporter mentioned, it is generally assumed that the risk of fire spread is a combination of the combustibility of the materials that the balcony is made from and any combustible materials that are placed on them, and the panel is aware of cases where surveys recognised both issues.

competent designer to choose a solution they feel comfortable satisfies the functional requirements of the Building Regulations

Different professionals and organisations have been following different approaches to this issue, in anticipation of the research findings, which is an indication of no definitive solutions. The form and orientation of timber panelling should be carefully considered on a case-by-case basis, with cognisance of how the reaction to fire classification has been obtained by fire testing, and whether that classification is considered appropriate for the end use. For existing buildings, PAS 9980 includes the assessment of risk of balconies, and eventually it is up to the competent designer to choose a solution that they feel comfortable satisfies the functional requirements of the Building Regulations.

CROSS has published a Safety Alert regarding issues associated with balconies, fire safety included, and a Safety Report about deck board in common access balconies, both of which touch upon and address some of the issues recognised in this report. CROSS has also drawn attention in the past to the dangers of storing gas cylinders on balconies. 

It should be noted that, in England, from 1 December 2022, the guidance will be updated to apply to balconies on residential buildings with a floor above 11m. This change will cater for the nature of the balcony’s construction.

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