Guidance on Design and Construction of the Built Environment Against Wildland Urban Interface Fire Hazard: A Review

Fires in the Wildland-Urban Interface (WUI) have become a global issue, with recent disasters taking place in a number of places, including Europe, North/South America and Oceania [29]. A wildland fire is defined as an: “unplanned and uncontrolled fire spreading through vegetative fuels, at times involving structures” [32]. If it develops in a wildfire-prone boundary between structures and vegetation, than it can be considered as a Wildland Urbane Interface (WUI) fire [30].

The WUI fire issue may get worse in the future, due to climate change [27] and population growth in the WUI areas [36]. In fact, the current situation may evolve towards more dangerous scenarios in areas which have already experienced a long history of fires, such as the USA, Canada, Australia, Southern Europe, etc. [43]. However, other regions which have been not traditionally subject to wildfires may become more vulnerable in future years, due to climate change and modifications in the location and wildfire intensity, such as South America, Africa and Northern Europe [27]. Thus, the phenomenon of fires in WUI areas should be considered from a broad international perspective.

For example, in Canada, the number of wildfire evacuations per year has increased by 1.5% from 1980 to 2014, with more than 20 evacuations per year after 2010. The 2016 Fort McMurray fire alone had the costliest impact in the Canadian history in terms of insured losses [43]. Due to these urgent needs, research has started to address the consequences of these incidents [8, 30], provide measures to aid evacuation planning [9], and coupling fire, traffic and pedestrian models to aid response to such incidents [25, 42, 43].

The development (and continuous revision) of standards and guidelines can be crucial in reducing the negative impacts of WUI fires on communities involved through appropriate dedicated measures and design guidance. Some countries have already developed standards and codes concerning measures for response planning, prevention, protection, fighting, etc. of WUI fires. Other jurisdictions can rely on guidelines and local provisions, or on provisions which cover the issue of WUI fires, but are included in other general codes (e.g., Building Codes, Environmental Codes, Fire Codes, etc.). An International Code for WUI fires is also currently available [24]. On the other hand, there are other countries which do not have similar regulatory tools (and which do not adopt international provisions) and design guidance, even if they are already affected (or they may be affected in the future) by WUI fires.

The development (or revision) of standard/guidelines for design considering WUI fire-related issues may be eased by a broad knowledge of the current regional, national and trans-national design concepts, mainly based on previous research in this field and/or good practices. For this reason, the main objective of this work was to review provisions and guidelines available worldwide concerning WUI fires. This included both documents specifically focused on WUI fires as well as selected documents which are relevant to WUI fires, but are not explicitly dedicated to them. The document analysis has been done considering mainly countries with Subarctic, Mediterranean and Oceanic climate.

The final objectives of this paper involve: (1) highlighting the main commonalities in the documents reviewed, and (2) highlighting the main deficiencies. In fact, the identification of the common threads through the different provisions and guidance reviewed may be useful for researchers and practitioners who intend to develop or revise WUI-based standards and guidelines. In addition, highlighting deficiencies in standards and guidelines could shed some light on which part of research should be transferred into regulations or, eventually, if future research is needed on some specific topics.

In this section, the main documents reviewed and the methods used for conducting the review are explained in detail, by focusing on the structure and type of information which have been retrieved from the considered documents.

The documents reviewed relate to areas in North America, Europe and Oceania. They were selected given that they had proneness to fires in the WUI similar to the Canadian environment, considering similarities in industrial development, environmental and social conditions. Clearly, the European Mediterranean area could have been specifically targeted for the review of documents given it is prone to WUI fires (i.e., the Iberian peninsula, France, Italy, the Balkan peninsula, see Modugno et al. [31]). Two examples of countries among those most vulnerable were then considered for the review; namely, Italy and France. The language knowledge (English, French, Italian) of the authors was also taken into account in the selection of the reviewed documents. Two trans-national regulations were also considered: International WUI Code and EU Regulations.

The documents reviewed are listed and classified according to the geographic area to which they belong. Moreover, other reference sources and relevant documents considered during the review process are also identified. This information is summarized in Table 1. In this table, groups of documents are defined (e.g., US Standards, Italian Standards/Guidelines). These definitions are henceforth used throughout the remainder of the paper.

The following methodology was used to review the standards and guidelines regarding construction in areas deemed vulnerable to WUI fires. Relevant provisions and/or guidelines for WUI fires were collected from the reviewed documents according to a common template. The information collected is structured in three categorical levels:

This approach was adopted to ensure a consistent representation of the provisions reviewed. The macro-categories and the second- and third-level sub-categories considered are reported and explained as follows. Moreover, they are graphically summarized in Fig. 1.

The macro-category “Hazard” relates to how hazard and risk zones are identified and classified. In particular the definition of WUI (Wildland-Urban Interface) and risk zones, alongside with the procedures used for identifying hazards and circumscribing zones, are reviewed. The procedures to define the different risk severity classes, and the thresholds delimiting the different risk categories are reviewed as well.

The macro-category “Land” relates to the measures adopted for the vegetation surrounding structures in terms of: (1) creating a defensible space (i.e. fuel modification: removal, reduction, substitution) and undertaking specific operations at given distances from the structures, (2) respecting given clearances between vegetation and other vegetation types/structures (or other objects); (3) undertaking maintenance procedures for the defensible space or the vegetation around structures. Moreover, the procedures that influence the terrain (i.e., slopes) and topography on the risk classification are reviewed as well.

The macro-category “Building construction” includes several elements. It includes the review of the general requirements for buildings and building materials with respect to fire safety: classes of ignition resistance, characteristics required for ignition resistance of building materials or part of structures, and clearance between buildings. It also includes the review of requirements for the specific elements: roofs (and the different parts composing them), walls (and other external coverings), openings (such as exterior glazing, external doors, vents and other openings), other structures (decks and other attachments, detachments), interiors (floors, under-floors and other structures).

The macro-category “Resources” includes several elements. It includes the review of the requirements for: (1) water supply (concerning the minimum supply level, the standby power, the location and size of hydrants in case of fire suppression), and other fire-safety related utilities; (2) firefighters, including procedures and activities before and during the emergency; (3) plans to be adopted for fire protection purposes (fire protection plans) and other plans relevant to WUI fire safety (e.g., emergency or development plans). Moreover, it describes the outcomes and general public (outreach) to which documents are dedicated.

The macro-category “Fire protection measures” includes the review of the requirements for water sources to use in fire protections and for buildings. Among protection measures for buildings, those adopted for sprinklers, manual protection systems in/near structures (e.g., fire extinguishers), warning systems (e.g., fire alarms), spark arresters (such as for chimneys), combustible materials and their distance from structures if allowed (e.g., debris, tanks, gases), private shelters against wildfires, are reviewed.

The macro-category “Environment” relates to how weather and fire history influence the measures or procedures to be adopted.

Finally, the macro-category “Access” relates to the fire safety requirements of the road access to structures. It includes the type and/or number of access roads required in the risk areas, the requirements concerning road standards (geometry, materials, parking lots, dedicated lanes) for main access roads and driveways/dead-end roads, markings (on access roads/streets and dwellings markers), and vegetation clearing operations to be conducted within the road limits.

Information relevant to WUI fires for the geographical areas considered are reported below in Fig. 2. This information concerns temperature and rainfall, climate, population, vegetation (forest types). They are important for considering the context in which different standards and guidelines are developed and used. Information relevant for WUI fires are not reported for standards corresponding to transnational areas, such as International or EU, since this information is not location specific. Information concerning temperature, rainfall, climate, population and vegetation may be useful while developing novel standards/guidelines, since the latter should be defined according to the local characteristics of the area under consideration.

A comparative analysis of the information collected from the different sources is conducted here, by highlighting variability in key aspects of standards/guidelines under consideration. The comparative analysis is conducted separately for each macro category, by focusing on the various approaches adopted by different guidance/regulatory provisions. Moreover, for each macro-category (see Fig. 1), a general comment is provided at the beginning of the sub-section, and more detailed comparisons provided for each second-level sub-category, where present (i.e., for land, building construction, resources, fire protection measures, environment).

In this section, we identify the common approaches found and the deficiencies in the reviewed standards/guidelines. A graphical representation of the variables which can be consistently/partially found or which are neglected in the standards/guidelines reviewed (see Table 1) is presented in Fig. 3. This figure is simplified given the need to represent several levels of information in a single summary. Generally, provisions/guidelines vary between different jurisdictions in the quantitative assessments of the various variables considered, and in the methods used for defining measures and indicators. However, some common approaches have been found.

At a general level, the standards reviewed seem to put a great emphasis on the assessment of both risk and hazard as well as the use of risk mitigation measures concerning vegetation. It should be noted that fire risk mitigation measures (e.g. actions on vegetation) are considered in this context within the requirements on buffer zone and type of plans required. In contrast fire protection measures are intended as those measures limiting the impact or reducing the fire (e.g. sprinklers, etc.). The definitions of wildfire risk and hazard levels in the standards/guidelines reviewed are mainly based on topographic, vegetation and environmental factors (except for New Zealand, in which buildings are classified based on risk to their occupants in the event of fires). Methods for identifying and classifying severity in WUI areas are present in the standards/guidelines reviewed, with different level of refinement. This may include a sum of partial ratings based on different factors (e.g., vegetation types, slopes, distance between structures and vegetation) or they can be based on fire history, such as the case of the EU Guidelines. In this context, weather is often considered for defining risk areas or severity zones. The consideration of fire history for defining risk areas or severity zones was retrieved in some groups of standards/guidelines as well. The concept of defensible space is present in the standards/guidelines reviewed except in the EU regulations. The same is valid for prescriptions/guidelines for vegetation clearance and distance between vegetation and other vegetation/structures; and for the influence of topographic and terrain factors (except for EU and New Zealand). Those factors generally affect the definition of risk areas (and/or defensible space), or the position between houses and slopes (Canada). Overall, it seems evident that risk/hazard assessment and mitigation measures appear as key requirements to be included in a WUI fire code and they were addressed (although with slightly different approaches) in all standards reviewed.

Requirements concerning building construction seem to be less consistent among standards and provisions. General requirements for construction materials and ignition resistance have been found in most of the standards/guidelines reviewed (except for EU, France, Canada), although in some instances they are only broadly outlined (e.g., in the Californian and Italian standards) rather than treated in detail. Both the type of construction materials and elements taken into consideration as well as the provisions/requirements provided greatly vary among standards. This seems to indicate a lower level of maturity in the understanding of the solutions needed to address this issue and highlight the need for further research in addressing standardized methods to provide guidance. It should also be noted that construction materials and processes may vary across countries (e.g., some countries may have long-standing tradition towards a specific type of material rather than another), thus any international guidelines should be reviewed and made applicable to the local building materials. For example, explicit references to fire-retardant treated woods for constructions mentioned in the IWUIC code were also found only in US and Canadian standards/guidelines.

At a general level, the requirements concerning resources also greatly vary among standards. Great emphasis is placed on requirements concerning water supply (supply level, water sources, access, hydrants) as those have been found in all standards/guidelines reviewed (except for California, EU, Italy). This is also linked to the actual definitions of water supply provided. Emphasis is placed also on fire protection plans, although with varying level of details. Commonly discussed variables in fire protection plans are weather and fire history. Similarly, the provisions concerning outreach are variable, depending on the type of document reviewed. Generally, guidelines for creating defensible space, clear and maintain vegetation are dedicated to the general public (homeowners, communities and/or planners). Some regulations can be implemented at a local level with modifications allowed. Most standards also include requirements concerning measures for buildings (sprinklers, protection and warning systems), although those are presented with varying levels of refinement.

Requirements concerning access have been found in most of the standards/guidelines reviewed (except for California, retrievable in the general Fire Code, and EU). Those generally discuss the type/number of access roads, and road standards (geometry, pavements, turnarounds, clearance, loads).

A set of deficiencies can be highlighted in groups of standards/guidelines. In fact, the requirements concerning resources (firefighters and other types of utilities), fire protection measures (especially for water sources) are often missing or limited in many of the standards/guidelines reviewed. In particular, requirements concerning resources (firefighters) have been found only in standards/guidelines from Canada, USA, New Zealand, France and Italy. Note that other types of utilities, such as evacuation routes, have only been found in New Zealand standards. Requirements concerning measures for water sources (clearing, defensible space, materials, protection, connections for water sources) have only been found in the USA, Australian and International standards/guidelines reviewed.

For other variables, the level of detail of some groups of standards/guidelines is limited to high-level provisions (i.e., only for some sub-categories considered in Fig. 1). This is particularly the case for requirements for building construction and road access. While all the standards/guidelines reviewed make provisions for specific building elements, some categories of parameters reviewed are neglected in some cases. For example, standards from Italy and EU do not consider walls, windows and other openings, decks/detachments, floors/under-floors. Those from New Zealand do not consider windows and other openings and decks/detachments. Moreover, among the third-level sub-variables listed in the methods section of this paper, wall coverings are considered only in the French and Californian standards/guidelines reviewed. Provisions about floors (by defining materials to be used and their ignition resistance) have been found only in Australian and New Zealand standards (including provisions about other interior structures). Moreover, while most of standards/guidelines include general requirements for roads (i.e., road standards), some detailed variables have not been considered, such as road markings. The latter provisions (considering visibility, location, text of markings) have only been found in the USA, International and New Zealand standards.

Another limitation was found in relation to how environmental factors (weather and fire history) are taken into account for defining risk areas or plans. In particular, weather is only considered in the USA, Australian, French and International standards/guidelines reviewed. In contrast, fire history is only considered in the USA, Canadian, French, EU and International standards/guidelines reviewed. This seems to indicate that there is still not an international consensus on the variables to be considered during the definition of risk areas. Further research should explore if these differences in the approaches are mostly driven by the choice of the regulators or are the result of the local/regional conditions.

This paper presents a review a selection of international standards and guidelines concerning fires in the Wildland-Urban Interface areas. A template was developed and used to consistently present the information regarding hazard, land, building construction, utilities, fire protection measures, environment and access. The main objective was to document the provisions and enable a comparison between the regulations/guidance examined, in order to identify the underlying common threads and the main deficiencies. This comparison is summarized in Table 2 and then discussed.

In some locations, provisions for WUI fires are included in general Fire Codes (e.g., California); elsewhere, requirements are implemented at a local level (e.g., France and Italy). Some countries have standards specifically dedicated to the issue of WUI fires (e.g., USA and Australia).

Based on the review conducted, the following main common themes can be highlighted: the definition of WUI risk areas and severity classes for these areas, the influence of land factors including the definition of defensible space, recommendations/requirements for building construction components/materials and road access. Whereas, the following main deficiencies can be highlighted among others: the requirements/recommendations about resources, fire protection measures (for water sources), and the consideration of environmental factors are scarce or absent in several documents reviewed. Moreover, differences in the methods for defining severity classes and in the requirements for buildings have been particularly highlighted.

The recommendations and requirements identified from the standards and guidelines considered may be useful for the development or revision of future standards/guidelines concerning WUI fires prevention, protection, planning and management. This might become increasingly important as the problem evolves, especially in those jurisdictions where WUI was previously not a concern, and therefore, there is no history of adopting regulatory guidance in this domain. Moreover, the identification of common deficiencies in the existing standards and guidelines reviewed may help in setting goals for future research and/or to transfer existing research into practice. In this sense, results from the presented review may be of use for both researchers and practitioners.