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

Abstract

Wildland-Urban Interface (WUI) fires, a worldwide problem, are gaining more importance over time due to climate change and increased urbanization in WUI areas. Some jurisdictions have provided standards, codes and guidelines, which may greatly help planning, prevention and protection against wildfires. This work presents a wide systematic review of standards, codes and guidelines for the design and construction of the built environment against WUI fire hazard from North American, European, Oceanic countries, alongside with trans-national codes. The main information reviewed includes: the definition of WUI hazards, risk areas and related severity classes, the influence of land and environmental factors, the requirements for building materials, constructions, utilities, fire protection measures and road access. Some common threads among the documents reviewed have been highlighted. They include similar attempts at: (a) defining WUI risk areas and severity classes, (b) considering land factors including the defensible space (also known as ignition zones), (c) prescribing requirements for buildings and access. The main gaps highlighted in the existing standards/guidelines include lacks of detailed and widespread requirements for resources, fire protection measures, and lacks of taking into account environmental factors in detail. The main design and construction principles contained in the reviewed documents are largely based on previous research and/or good practices. Hence, the main contributions of this paper consist in: (a) systematically disseminate these guidance concepts, (b) setting a potential basis for the development of standards/guidelines in other jurisdictions lacking dedicated WUI fire design guidance, (c) highlighting gaps in existing standards/guidelines to be addressed by current and future research.

Introduction

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.

Methods for the Review

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.

Table 1 Summary of the Documents Reviewed Classified According to the Country/Area of Reference, and Information Concerning Their Type, Main Content and Jurisdiction Level

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:

  • A macro-category (from A to G);

  • A second-level sub-category;

  • A third-level sub-category.

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.

Figure 1
figure1

Summary of the categories taken into account for the consistent review of the information contained in the selected documents. Macrocategories are on the left (named from “A” to “G”). Second level sub-categories within each macro-categories are connected on the right (e.g. Land has sub-categories of Vegetation and Topography). The third-level sub-categories are reported below each relevant second-level category (in a lighter colour than the second-level categories). For instance, Vegetation includes Defensible Space, amongst other examples

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.

General WUI-Relevant Information for the Areas Considered

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.

Figure 2
figure2

Summary of the information relevant to WUI fires for the geographical areas considered. Specific data sources: Climate Change Knowledge Portal (1901–2015) for temperatures and rainfall, http://www.fao.org/forestry/fra/80298/en for forest types, Köppen scale for climate. Notes *Annual mean daily (1997–2014), **average monthly rain volume (1961–1990) (US Census/US Geological Survey)

  • The extent of the Wildland-Urban Interface in the areas considered, together with additional information about fires and other relevant issues, are reported as well. Since, in this case, a common world database with data of equivalent and refinement does not exist, the data used derived from local sources. In fact, although general definitions of WUI areas may be provided, different buffers between structures/infrastructures and the vegetation, or different percentages of built areas may be used for precise definitions of the WUI area. For example, Modugno et al. (2016) highlight that the definition of WUI area is standardized in the USA (built areas with < 50% vegetation, lying within 2.4 km of an area with at least 75% wildland vegetation, and at least 5 km2 wide, as reported by [39]), based on [20]. In contrast, in some European countries the definition of WUI areas are not standardized (or they may vary between countries). Thus, comparisons between countries may be difficult without taking into account the local legal frameworks. For instance:

  • Canada WUI area: 32.3 million ha (5.8% of the territory), largest WUI areas in Quebec (about 6.9 million ha), highest WUI percentages among the total State in Nova Scotia (45.1%) [26]. 5780 fires/year and > 1 million ha burnt by wildfires (10-years average, 2017); with most of the fires/year (811) and largest area burnt/year (> 163 k ha) in Alberta.

  • USA 46 million homes in the WUI area (2012), estimated conversion rate from wildlands to WUI: about 810,000 ha/year since 1990, 8 million projected new homes in next 10 years [23]. 89,000 ha designated by State foresters as at WUI fire high-risk, with about 100,000 wildfires burning 2.8 million ha/year, and 2970 homes/year lost on average since 2000 [23];

    California: WUI area estimated in 746,037 ha, estimated number of houses in the WUI area: > 5 million [47]. Average area burned/year by wildfires (period: 2011–2015): 1272 km2 (US Department of Agriculture, Forest Service).

  • Australia More than 11,000 houses lost in Australia in the period 1939–2007; with most of the houses lost in the Victoria State (more than 60% of losses) [4].

  • New Zealand National average annual total area burned in the period 1991–2007: 5865 ha, average number of wildfires increased from 1200 to 4000 annually in the same period, with the north islands accounting for two-thirds of all the wildfires, and grass fires accounting for the majority of area burned (54%) [2].

  • France WUI area particularly present in some areas (e.g., in Bouches-du-Rhône: > 15% of the Department, 47% of fires starting in the WUI). About 4000 fires/year, 5.5 million ha of forests potentially exposed to fire risk, but a limited State portion severely attacked by fires (e.g., in Provence-Alpes-Cote d´Azur: 7000 burnt ha/year) [19]; MeteoFrance).

  • Italy In the first seven months of 2017, 74,965 ha burned. The highest amount of ha. burnt were in the Regions: Sicily, Calabria and Campania [28].

Comparative Analysis of the Results from the Review

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).

Hazard

A definition of risk and hazard zones and the hazard classification are provided in all the standards/guidelines reviewed. Specific details concerning the hazard classification in the documents reviewed are provided in Table 2.

Table 2 Comparative Summary Between the Standards/Guidelines Reviewed, Classified According to the Relative Jurisdictions

Definitions of wildfire risk and hazard levels are present in all groups of documents reviewed, except for the New Zealand group. The definition of specific risk areas is essentially based on some topographic, vegetation and environmental factors and should be made by the relevant authority. For example, the US standards base the definition of wildfire risk areas on characteristics such as: fuels, fire weather, defensible space, terrain, building construction and water supply, while the Australian standards define the wildfire risk for structures based on surrounding vegetation, heat flux exposure, predicted bushfire attack/exposure. The definition itself of WUI area is slightly different between different standards. In Australia, those areas are referred to as Bushfire-prone areas, and the wildfire risk level is defined as Bushfire Attack Level (BAL). Methods for identifying and classifying severity in the interface areas have been found in all the standards/guidelines reviewed, with different levels of detail. In the more detailed provisions/guidelines, the severity class is assigned based on a rating obtained as a sum of partial ratings related to different factors (e.g., in Canadian, US, Australian, and International documents reviewed). The main factors on which the total severity rating is obtained are: type of building construction and materials (i.e., roofs, exterior structures), vegetation (including distance from vegetation), fuels, slopes, critical fire weather frequency (the latter in the IWUIC, 2015). In the EU Guidelines, it is essentially based on geographic considerations based on fire history. Specific high-risk zones are identified in this way: Portugal, Spain, Greece, some regions of France and Italy. Risk assessments should be constantly conducted for these.

Land

Provisions/recommendations about the influence of land characteristics on the measures and procedures to be adopted in wildfire-prone areas are provided in all the groups of documents reviewed, except for the European Standards.

Vegetation

The feature “vegetation” includes the quantitative and qualitative provisions for creating the defensible space, the clearance between different vegetation types (i.e., between tree crowns and between tree crowns and ground vegetation) and between vegetation and structures/infrastructures (i.e., buildings, power lines), and for maintaining vegetation around structures. Even with different quantitative provisions, the concept of defensible space is present in all the standards/guidelines reviewed, except for EU Standards. The same is valid for the prescriptions/guidelines about clearance between vegetation and structures and/or between different types of vegetation; and the prescriptions/guidelines about maintenance (pruning, removal of dead vegetation, etc.). Specific quantitative details concerning the defensible space are provided in Table 2. Local French provisions [37] on clearance are worthy of note due to the high level of details. For example, clearance between vegetation (trees and bushes) and other plants (e.g., vertical distance between the lower boundary of a crown tree and the top of a bush should be high ≥ 2 times the height of the bush, but anyway > 2 m); structures (e.g., trees should be at least 3 m from structures); power lines (e.g., high voltage lines ≥ 1000 V should be at least three meters from the vegetation) are mentioned.

Topography/Terrain

Topographic and terrain factors are considered in all the groups of documents reviewed, except for EU and New Zealand standards/guidelines. Those factors can affect the definition of risk areas and/or the defensible space for standards/guidelines belonging to USA, California, Australia, France, Italy, and the IWUIC. In Canadian guidelines, the relative position between houses and slopes is considered. Specific quantitative details concerning the influence of topography/terrain are provided in Table 2.

Building Construction

Provisions/recommendations about the building materials and structures to be adopted in wildfire-prone areas are provided in all the standards/guidelines reviewed, except for the EU Standards. The level of detail of the provisions/guidelines is extremely variable across the different locations/jurisdictions considered, as reported in Table 2.

General Requirements

General requirements about construction materials to be used in hazard zones and ignition resistance of building materials have been found in all standards/guidelines except for the EU, French and Canadian guidelines. Some generic requirements have been found in the Californian and Italian standards. A detailed definition of ignition resistance classes can be found in the standards reviewed from Australia (Fire Resistance Level -FRL- determined through three ratings obtained from a standard test representing: structural adequacy, integrity and insulation) and International standards (three classes of ignition resistance -IR- obtained as a combination of three fire hazard severity classes, and of classes of conformity of water supply and defensible space to standards). A detailed requirements of ignition resistance for materials can be found in standards/guidelines reviewed from USA, Australia and the IWUIC code, with reference to results of standard tests. The prescribed minimum separation distance between buildings varies between 4.5 m and 15.2 m (US standard, depending on building height, type, presence of sprinklers), 9 m (Canadian guidelines, for high-density dwellings), and 10 m [5].

Roof

Requirements concerning roofs have been found in all standards/guidelines reviewed except for New Zealand, EU and Italy. Provisions about roof covering, eaves and gutters are given in all those standards/guidelines (except for gutters in Canadian guidelines). Provisions about valley flashing (i.e., material to be used, non-combustibility, thickness, etc.) are given only in the Australian, Californian and International standards/guidelines. Generally, provisions concerning roof covering and eaves consist in defining materials to be used and their ignition resistance.

Walls and Other External Coverings

Requirements concerning walls and other external coverings have been found in all standards/guidelines reviewed except for EU and Italy. Provisions about exterior walls have been given in all those standards/guidelines. Provisions about wall coverings have only been found in the French and Californian standards/guidelines reviewed. Generally, provisions concerning exterior walls consist in defining materials to be used and their ignition resistance.

Windows, External Doors and Vents

Requirements concerning windows, external doors and vents have been found in all standards/guidelines reviewed except for New Zealand, EU and Italy. Provisions about exterior glazing, external doors and vents have been given in all those standards/guidelines (except for exterior glazing and doors in French documents reviewed, and Canada for doors). Generally, provisions concerning glazing and doors consist of defining materials to be used and their ignition resistance. Prescriptions/guidelines concerning vents and other openings are related to their corrosion-resistance and to the presence of openings in the metal grid.

Decks and Detachments

Requirements concerning decks and detachments have been found in all standards/guidelines reviewed except for New Zealand, EU and Italy. Provisions about decks and other attachments have been found in all those standards/guidelines (except for France). Provisions about detached structures have been given in the documents reviewed for USA, Australia, International, France. Generally, provisions concerning decks and other attachments consist in defining materials to be used and their ignition resistance. Provisions/guidelines concerning detachments include also other information such as the distance from the main structures (i.e., minimum 9 m in US standards, 6 m in Australian standards, except if accessory structures comply with requirements for main structures or they are separated through appropriate walls, 50 feet/15.24 m in the IWUIC, otherwise accessory structures should mostly comply with same requirements for exterior walls).

Floor and Interior Structures

Requirements concerning floors and interior structures have been found in all standards/guidelines reviewed except for EU, France and Italy. Provisions regarding under-floors have been found in all those documents, except for New Zealand. Provisions about floors have been found only in Australian and New Zealand standards, while provisions about other interior structures only in New Zealand standards (where fire-cells are defined: inside spaces, enclosed by a combination of fire separations, external walls, roofs and floors). Generally, provisions concerning floors consist in defining materials to be used and their ignition resistance.

Resources

Provisions/recommendations about resources in wildfire-prone areas have been provided in all the standards/guidelines reviewed, except for the EU, Californian and Italian standards. The level of detail of the provisions/guidelines varies between the different jurisdictions/areas considered, as reported in the summarizing Table 2. For the United States, Australia and in the IWUIC, the provisions are more quantitative and detailed.

Utilities

The category “utilities” includes requirements mainly concerning water supply, but also other means such as evacuation routes (for New Zealand). Requirements concerning water supply have been found in all standards/guidelines reviewed except for California, EU and Italian Standards/Guidelines. Generally, provisions/guidelines concerning supply level, water sources, access for water sources and hydrants are given for water supply. A very detailed definition of water supply was found in the US standards, as a function of the total volume of the structure, the occupancy hazard and construction classification, and the hazard exposure. The minimum level of water delivery rate to the fire scene is determined as well (to be provided at the same level for at least 1–2 h at 138 kPa), based on the water supply level. According to US standards, water sources should be maintained, accessible and they should guarantee the same capacity and delivery on a 1-year basis. Specific standards for fire hydrants are provided concerning markings, location, spacing, pipe diameters, dead-end pipes. Standards are provided for dry hydrants for non-pressurized water supply as well. Other detailed definitions have been found in the IWUIC and the Australian guidelines. In the IWUIC, the minimum water supply for new buildings, not equipped with sprinkler systems in the WUI area, is set to 63–95 l/s for 30 min-2 h, depending on floor area and the number of families. The water source should be ≤ 305 m far from the building and the natural water sources should guarantee a minimum water supply, being equipped with hydrant/draft site.

Firefighters

Requirements concerning firefighters have been found in standards/guidelines from Canada, USA, New Zealand, France and Italy (in the Italian case, some information was found in regional guidelines). In this case, the provisions/guidelines found are heterogenous and scarce. For example, in Canadian guidelines, the cooperation between partner agencies of fire protection, training and exercises are encouraged. In the US standards, the required number of firefighters are assessed, and the safety requirements and procedures to be followed in case of incident are described.

Planning

Requirements concerning planning have been found in all standards/guidelines reviewed. In particular, provisions about fire protection plans have been found in all the other documents reviewed, except for New Zealand. Some requirements concerning other types of plans different than fire protection plans, which may be of interest for WUI fire safety (such as emergency or development plans), are mentioned in all the other documents, except for California and France. More detailed provisions about required plans have been found in the US standards/guidelines and the Canadian guidelines reviewed. Further details concerning plans provided in all the documents reviewed are reported in Table 2. The Canadian guidelines include plans for high wildfire hazard and risk zones: the Wildfire Preparedness Guide, the Wildfire Mitigation Strategy. Other plans suggested are the sprinkler development and the municipal development plans, the municipal emergency management program guide and the land use bylaws. The US standards require a mitigation plan including prevention activities, fuel modifications, hazard mitigation for structures, public information, infrastructures. Other plans required by the US Standards are: a fire lane, wintertime access maintenance, multi-agency operational, minimum water supply, dry hydrant, fire response, preparedness, incident action, containment and control, construction plans.

Outreach

The outreach in standards/guidelines is variable, depending on the type of document reviewed. Generally, guidelines for creating defensible space, clear and maintain vegetation or similar activities are included in documents intended for use by the general public (e.g., Canadian or Californian guidelines). Some regulations can be implemented at a local level with modifications allowed (such as the IWUIC Code which can be adopted by other jurisdictions).

Fire Protection Measures

Provisions/recommendations about fire protection measures to be adopted in wildfire-prone areas are provided in all the standards/guidelines reviewed, concerning measures for water sources and buildings, except for the EU and French Standards. The level of detail of the provisions/guidelines varies between the different areas considered. In fact, the level of detail of Canadian, Californian and Italian standards is limited. The most significant measures are reported in the sections below and summarized in Table 2.

Measures for Water Sources

Requirements concerning measures for water sources have been found only in US, Australian and International standards/guidelines reviewed. Generally, provisions consisted of defining clearing, defensible space, materials, protection and connections for water sources. The American provisions/guidelines reviewed are the most detailed. The provisions require that: the fire department connections for sprinkler systems should be < 100 feet (30.5 m) far from fire hydrant; the space around fire hydrants protected by barriers should be cleared. Protections are required for dry hydrants as well.

Measures for Buildings

Requirements concerning measures for buildings have been found in all standards/guidelines reviewed except for EU and France. Only some provisions/guidelines have been found in Californian standard (for combustible materials), in Australian standard (for private shelters), and in Italian local standards (for combustible materials and private shelters). Generally, provisions/guidelines concerning sprinklers, protection and warning systems consist in setting the conditions where they are needed and the tools needed. For example, the US Standards require that all residential buildings and all the buildings with > 2 stories or > 30 feet (9.1 m) tall (with some exceptions) should be provided with automated sprinkler systems. Standpipe systems with specific fire department connections (at indicated distances) and fire extinguishers should be installed in new buildings and other specific buildings in case of available municipal water systems. Automatic fire warning systems, with different prescriptions for residential and non-residential structures should be installed as well. Combustible materials, unprotected heat and flame sources, inappropriate storage of liquefied petroleum gas should be avoided within 30 feet (about 9 m) of the structure.

Environment

Provisions/indications about the environmental factors related to wildfire-prone areas are provided only in some of the standards/guidelines reviewed: American Standards and Guidelines (only for weather in California), Australian standard (only for weather), IWUIC Code, French provisions and guidelines, EU Regulations (only for fire history), as summarized in Table 2. Environmental factors are consistently used for risk assessments and definition of hazard zones.

Weather

Weather is mentioned in all standards/guidelines reviewed in the procedures for wildfire risk and hazard definition or for developing fire protection plans (France), except for Canada, New Zealand, EU and Italy. The weather factors generally considered are: wind, humidity, temperature, precipitation and fuel-related features (e.g., fine fuel moisture).

Fire History

Requirements concerning fire history have been found in all standards/guidelines reviewed except for California, Australia, New Zealand and Italy. Fire history factors in the standards/guidelines reviewed have been considered mainly for defining risk areas or severity zones; or for developing fire protection plans (International standard).

Access

Provisions/recommendations regarding accessibility and road standards to be adopted in wildfire-prone areas are provided in all the standards/guidelines reviewed, except for the EU Regulations and in the section of the California Fire Code concerning WUI fires. The level of detail of the provisions/guidelines varies between the different areas considered, as summarized in Table 2. Provisions concerning type/number of access roads, road standards, markings (only in US, New Zealand and International standards) and vegetation clearing between roads and vegetation (only in US, Australia and French standard/guidelines). The US standards reviewed are the most detailed with respect to access requirements. Generally, the types of access roads/routes are defined in the appropriate section of standards/guidelines. Access routes for emergency vehicles, fire service, and public are considered. In the US standard, the number of access routes required is set, depending on the number of households or parking spaces (e.g., 3 for > 600 households or > 3000 parking spaces in case of mixed areas). Provisions/guidelines concerning road standards include geometric standards, pavements, turnarounds, clearance, loads. In particular, in the US Standards requirements concerning materials, minimum clear width, minimum vertical clearance, minimum curve radius, intersection control, traffic calming measures, hydraulic calculations for bridges and culverts, grades, emergency pull-offs, maximum angles of approach and departure are considered. Generally, provisions/guidelines about markings concern visibility, location and text on markings.

Standards/guidelines are normally not provided with requirements for vegetation clearing around roads. An example of these requirements is reported in the French standard reviewed: both sides of public roads in hazard zones should be cleared within 20 m, and driveways of structures < 200 m from the forest, should be cleared from vegetation for a distance of 10 m.

Discussion

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.

Figure 3
figure3

Simplified graphical representation of the variables and provisions/recommendations considered in the standards/guidelines reviewed from different countries. Legend: black if all second-level sub-categories are considered in detail for the relevant macro-category listed on the left of the figure, dark grey if all second-level sub-categories are considered but some of them are less detailed, light grey if at least one second-level sub-category is missing, white if only one second-level sub-category is adequately considered, white with bold contour if the first-level macro-category is neglected

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.

Conclusions

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.

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Acknowledgements

Open access funding provided by Lund University. Funding was provided by National Research Council of Canada (Grant No. Design Guidance on wildland urban interface (WUI) fires, Contract No. 892587). The authors wish to thank Chunyun Ma for her comments on the article prior publication.

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Intini, P., Ronchi, E., Gwynne, S. et al. Guidance on Design and Construction of the Built Environment Against Wildland Urban Interface Fire Hazard: A Review. Fire Technol 56, 1853–1883 (2020). https://doi.org/10.1007/s10694-019-00902-z

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Keywords

  • Wildland-Urban Interface
  • WUI
  • Wildfire
  • Design guidance
  • Standard
  • Fire