Historic Buildings and Bushfires Protection: a Verification Method

The devastation of historic buildings and assets that occurred in January 2025 in the Los Angeles area is one of the most serious cases of damage to buildings of historical and cultural interest following a vegetation fire. This important case should be studied to deepen the techniques of risk assessment to which buildings of historical or cultural value are exposed that are located in areas with a risk of vegetation fire. In this regard, the Bushfire Verification Methods Handbook of the Australian National Construction Code (NCC), which addresses this aspect of fire risk to buildings, may be useful for historic buildings, although it does not explicitly provide for application to this type of asset.

Main buildings vulnerabilities against bushfires

According the NCC, the main vulnerabilities of a building to wildland fires, according to sources, include:

• Entrance of embers through cracks and openings. This includes cracks from where the fire-exposed side to the unexposed side can penetrate a 3 mm diameter probe.
• Prolonged burning on the external façade. Burning on the fire-exposed side after 60 minutes of exposure carries the risk of continued spread into the building.
• Ignition within the building. This includes burning sustained for more than 10 seconds on the unexposed side.
• Internal spread of fire due to radiant heat. A radiant heat flux of 15 kW/m2 at 365 mm from the unexposed face can cause ignition.
• Heat conduction through walls (insulation failure). Mean and maximum temperature rises above 140K and 180K respectively on the unexposed side.
• Escape path obstruction due to heat flux. A radiant heat flux from the fire face exceeding 3 kW/m2 at 250 mm from the fire face.
• Temperatures within the external façade that trigger initial fire spread. Average and maximum temperatures exceeding 250°C and 300°C, respectively.
• Debris accumulation. Windblown debris that collects on predominantly horizontal surfaces adjacent to building elements.
• Adjacent structures. Fire in adjacent houses and outbuildings.
• Stored materials. Materials stored adjacent to buildings.
• Inadequate vegetation. Inadequate vegetation adjacent to buildings, including combustible mulches.
• Building element interface vulnerability. The performance of building element interfaces is the most vulnerable part of the building envelope.
• Wind exposure. The wind exposure of a building is transient and can change rapidly during a wildfire. Wind exposure will be modified as the wind interacts with the building and other landscape elements.
• Window failures. High winds can potentially “open buildings” before the fire front passes, dislodging roof tiles and breaking windows, increasing susceptibility to attack from flying embers/brands.

It is important to note that the NCC specifies a maximum probability of a fire starting within the building of 10%. This assessment must be based on the compliant construction of the critical aspects of the approved design and the ongoing maintenance of those aspects to ensure that the performance of the design is maintained. G5V1 and H7V2 also require consideration of the likelihood of non-compliant construction and the likelihood that the critical aspects of an approved design will be fully functional during the life of the building.

It is necessary to premise that the document provides criteria for assessing the risk for individual classes of activity. This assessment, in the case of historic buildings, for the loss of material value must be carried out by the owner of the asset, while for the risk to human life the document from the Australian Building Codes Board is certainly applicable.

Performance Requirements and Compliance Options

The document outlines the Performance Requirements (G5P1 and G5P2 in Volume One and H7P5 in Volume Two) related to bushfire resistance for different classifications of buildings located in areas designated as bushfire risk zones.
In our opinion, the G5P1 requirements are closer to those that should guide the risk assessment of a historic building as they “must be designed and constructed to … maintain the structural integrity of the building for the duration of the design bushfire“.

The Performance Requirements aim to reduce the risk of ignition, take into account the intensity and duration of wildfires and maintain the structural integrity of buildings. The document also describes the corresponding Objectives and Functional Statements, which recognise that the risk cannot be completely eliminated, but aim to reduce the danger to life and the risk of building loss. Various options are presented to demonstrate compliance with the relevant Performance Requirements, including Performance Solutions, Deemed-to-Satisfy Solutions (DTS) and Verification Methods.

It’s important to understand that, while these requirements aim to reduce the risk of ignition, take into account the intensity and duration of wildfires and maintain the structural integrity of buildings, the solutions comply with them are presented in various options.

Another important aspect for those who are not familiar with risk assessment and the choice of safety measures is that the document, describing the objectives and corresponding functional statements recognises that the risk cannot be completely eliminated.

So safety measures are aimed to reduce the danger to life and the risk of loss of the building. In particular (G501) to: … safeguard occupants from injury from the effects of a bushfire; and protect buildings from the effects of a bushfire; and facilitate temporary shelter for building occupants who may be unable to readily evacuate the building prior to a bushfire“.

Moreover, the GF1 chapter “Construction in bushfire prone areas” specifies that : ” A building constructed in a designated bushfire prone area to provide a resistance to bushfires in order to reduce the danger to life and minimise the risk of the loss of the building; and if occupied by people who may be unable to readily evacuate the building prior to a bushfire, is to be constructed so as to provide its occupants shelter from the direct and indirect actions of a bushfire.”

The NCC Method

The process of assessing the risk of a dwelling that is located in the middle of vegetation and the choice of fire safety measures, according to the Australian Standards, can be summarised as follows:

• Verification of Performance Requirements: First, the performance requirements relevant to buildings in fire risk areas must be verified. Then the level of importance of the building must be identified, which must therefore consider the case of a historic or artistic building.
• Prescriptive solutions (Deemed-to-Satisfy) or alternative solutions (Performance Solutions): You can choose to follow the “Deemed-to-Satisfy Provisions” (DTS), which are prescriptive solutions that, if followed, automatically satisfy the performance requirements. Alternatively, you can develop a “Performance Solution”, which is a specific design solution that demonstrates that it meets the performance requirements through verification methods, expert judgement or comparison with the DTS.
• Verification Methods: Quantitative verification methods can be used to demonstrate the compliance of a Performance Solution. For buildings in fire risk areas, there are two main verification methods: G5V1 (for specific buildings such as healthcare facilities, schools, etc.) and H7V2 (for Class 1a, 1b and 10c buildings).
• Fire Risk Assessment (H7V2): The H7V2 method takes into account the probability of ignition of a building exposed to a fire that does not exceed 10%. This assessment must consider:
• Building Importance Level: Buildings are classified according to the risk they pose in the event of a structural failure (for example, a private fire shelter has a higher importance level than a warehouse). Again, in this case a historic building will need a careful assessment of the characteristics of the structures.
• Annual Probability of Exceedance (APE): Each level of importance corresponds to an annual probability that an event (fire) will exceed a certain intensity. The APE can be determined through a simple or complex analysis.
• Design Actions: Possible actions of a fire on the building should be identified, such as direct flame attack, radiant heat, windblown embers, and accumulation of flammable debris.
• Event Tree Analysis: An event tree analysis is used to assess the probability of ignition of the building based on different fire scenarios.
• Multiple Factors: The assessment should take into account several factors, including weather conditions, topography, surrounding vegetation, building materials, size and height of flames, slope of the land, and duration of exposure.
• Selection of fire safety measures: Based on the risk assessment, the most appropriate safety measures are chosen to reduce the likelihood of ignition of the building. These measures may include:
• Management of vegetation: Maintain adequate distance between the building and vegetation, remove combustible debris and create firebreaks.
• Fire-resistant building materials: Use fire-resistant materials for external walls, roof, windows and doors.
• Protection from embers and flames: Install rainscreens, perimeter seals and gutter protection systems to prevent the entry of embers.
• Active protection systems: Install sprinkler systems on the roof and walls, or provide water reserves for extinguishing.
• Fire safety plan: Develop a safety plan that includes evacuation procedures, building preparation and emergency management.
• Documentation and approval: The entire risk assessment process and the selection of safety measures must be documented in a “bushfire safety plan” or equivalent document and submitted for approval to the competent authorities. The documentation must also include the probability of correct implementation and maintenance of the safety measures.

Probability of fire assessment

Estimating the probability of fire at a given site involves several steps and considerations, which can be addressed through a simple or complex analysis. Here is how the probability of fire can be estimated, based on the information provided by the sources:

• Simple Analysis: This method specifies an annual probability of exceedance (APE) for weather conditions favorable to fires. It is assumed that, when these conditions occur, the fire will penetrate the urban interface. This approach is consistent with what is specified in AS 3959-2018. However, in some circumstances, this method can be overly conservative, especially where the frequency of fires is low.
• Complex Analysis: This method considers the probability of exposure of a building to a wildfire event, taking into account a series of events. It requires consideration of the frequency of ignitions and the probability of fire spread from surrounding areas. The APE, in this case, is expressed in terms of exposure to wildfire attack.

The complex analysis must consider:

• The probability of ignition of a forest fire.
• The spread of fire towards the urban interface adjacent to the building.
• The weather conditions when the fire reaches the interface.
• The local topography and vegetation.

The relationship between these factors can be expressed as: D = I × S × E × G × H, where:

• D is the risk to humans and property.
• I is the probability of ignition in the landscape.
• S is the probability of fire reaching the urban interface.
• E is the probability of fire spreading into the built environment.
• G is the probability of fire spreading within the built environment.
• H is the probability of fire spreading within buildings.

Other factors to consider.

• Vegetation: The presence of trees, shrubs and dry grass in the immediate vicinity of the building increases the risk of fire.
• Topography: The slope of the land can affect the rate of fire spread and the exposure of the building to flames.
• Weather: High temperatures, low humidity and strong winds increase the risk of fire.
• Building materials: The use of fire-resistant materials can reduce the likelihood of ignition of the building.
• Distance from the fire front: The closer the building is to the fire front, the greater the risk of direct exposure to flames and radiant heat.

Estimating the probability of fire using quantitative risk assessment techniques
Techniques such as event trees or fault trees can be used to determine the probability of fire within a building when exposed to wildfire attack. It is important to find a balance between applying practical approaches and maintaining sufficient technical rigor.

An Example of fire risk assessment

• For a brick clad house, built on a concrete slab with a timber frame and steel roof cladding, a review of the proposed construction form, design details and materials can be undertaken.
• Failure probabilities can be estimated for different components (e.g. windows, doors, walls, roof) based on their exposure to fire actions.
• Fault trees can be used to estimate the probability of ignition for each of the critical vulnerabilities when exposed to the appropriate design actions.
• The probabilities should then be summed to provide a total risk of fire ignition within the building when exposed to the design actions.

It is essential to consider the probability of non-compliant construction and the probability that critical features of an approved design will be fully functional during the life of the building. The design should document how these aspects have been addressed and the estimated probabilities for compliant construction and maintenance of the critical features.

Conclusions

The exposure of historic or cultural buildings to fire risk is increasingly important. An adequate assessment is a complex process that, for example, the Australian NCCs outline in a comprehensive way. In any case, a first estimate that can be carried out by any owner, verifying the building’s vulnerabilities. In short, the process is fundamentally similar to that envisaged for ordinary buildings, but taking into account the value attributed to them in the assessment phase.