This article is an excerpt from NIST Technical Note 1748.1

Destruction of homes and businesses from Wildland Urban Interface (WUI) fires has been steadily escalating, as has the fire suppression costs associated with them. Since 2000, more than 3,000 homes per year have been lost to WUI fires in the United States. The WUI fire problem affects both existing communities and new construction.

One of the fundamental issues driving the destruction of homes at the interface is the very limited consideration of potential wildland fire and ember exposures in building codes and standards. The limited information currently available does not address the full range of realistic WUI exposures and offers little context for the design of ignition resistant landscapes and buildings. While the principles of ignition and fire spread at the WUI have been known, actual exposure quantification has been very limited. The resulting gap between exposure and structure ignition has therefore resulted in a lack of tested and implementable hazard mitigation solutions.

As an example, there is currently little quantifiable information that links the ember generation from wildland fuels (treated or untreated) to building assemblies testing. Additionally, there has been no consideration of first responder and homeowner safety to ember/ fire exposure.

WUI fires present a unique challenge to the firefighting and fire protection engineering communities. The scale of the events can be vast, spanning in many cases more than 40,000 ha (100,000 acres), and the moving fire perimeter can be tens of kilometers long with potentially thousands of structures at risk.

The severity of the fire depends on vegetative (wildland and ornamental) and structural fuels, topography, and weather. Compared to hurricanes and earthquakes, fire intensity can vary significantly over relatively short distances (fractions of a kilometer) requiring complex fire suppression and evacuation operations.

The WUI exposure scale concept is based on quantifying expected fire and ember exposure throughout an existing or proposed new WUI community. The proposed WUI scale can be used to explicitly identify WUI areas that have a fire and ember exposure problem, as opposed to areas that meet housing density or wildland vegetation requirements. The scale therefore can be used to provide the boundaries where specific land use and/or building construction regulations would apply.


WUI building construction is influenced by codes and standards developed from the cumulative expertise and experience of the participating committee members. This includes the evaluation of structural performance during past WUI fires, limited laboratory work, and very limited WUI fire modeling.

WUI post-fire assessments consider structural performance, and if conducted systematically, should be used as part of a comprehensive approach that includes laboratory and full-scale experiments as well as computer modeling to guide and confirm the effectiveness of changes to buildings codes, standards, and best practices.

To date, post-fire WUI field data collections have failed to address three critical components: impact of defensive actions on structure survivability, systematic documentation of structure response to WUI fires, and quantification of fire and ember exposures.


An example of an existing community-scale hazard severity assessment program is the one developed by California Department of Forestry and Fire Protection (CAL FIRE). The CAL FIRE and Resource Assessment Program (FRAP) Fire Hazard Severity Zone is used to determine fire hazard on a 9 m (30 ft) grid. This information is applied in areas under state jurisdiction. FRAP is one of the few programs in the United States that links fire severity (exposure) and building codes (construction attributes). The FRAP system, with respect to building construction, is two-tiered: a structure is either in the WUI or it is not.

While FRAP links expected exposure to specific building code requirements, its classification system focuses primarily on proximity to wildland fuels and does not address the likelihood that buildings could be destroyed due to other sources of fire and ember exposures, such as from an adjacent burning structure. Other similar programs with less complex WUI hazard rating systems exist and are implemented across the United States.

The Home Ignition Zone (HIZ) concept represents another WUI hazard severity assessment framework designed to be implemented at a parcel or structure level. HIZ includes the home and surrounding area within 30 to 60 m (100 to 200 feet). The method has been successfully used to educate homeowners on the different parameters that affect structure survivability.

The primary limitation of the HIZ methodology in the context of this article is that it does not offer a framework to link the fire and ember exposure threat to building codes and standards. An additional limitation of the HIZ system is that it does not account for WUI scenarios with higher housing densities. A framework similar to the HIZ is also used by the International Wildland-Urban Interface Code2 as well as many other national and state hazard mitigation programs.


Fire behavior in the wildlands and the WUI is a function of fuel (vegetative and structural), topography and local weather during the event. A fire and ember exposure driven WUI scale, therefore, needs to account for these local environmental conditions. Using such a rating, an overall WUI area may receive a range of ratings. The ratings will reflect the potential severity of a WUI fire event at specific locations. Additionally, the framework links fire and ember exposure and resident and firefighter safety.

The WUI scale is designed to the range of fire and ember exposure conditions experienced by structures at the WUI. Fire and ember exposure can be traced to four primary sources of WUI fuel: wildland fuels, ornamental vegetation, structures (including homes, auxiliary buildings such as sheds and garages), and vehicles (Figure 1). The WUI-scale is designed by considering these sources as well as the local weather. These combined parameters are referred to as FTLW, which is short for fuels, topography, and local weather.

Figure 1. Primary Fuels Responsible for Fire and Ember Exposure at the WUI – Wildfire approaching a WUI community (top), with parts of the community ignited (bottom)

In the proposed framework, an exposure rating is uncoupled from ignition, so that the exposure rating is independent of the response to a particular structural element or landscaping attribute. The figure at the beginning of this article illustrates community ember exposure zones from a wildland fire. Figure 2 illustrates the proposed matrix for capturing fire and ember exposures from widland fuels.

Figure 2. Capturing Exposure from Wildland Fuels

The proposed WUI scale is developed with the primary objective of reducing the ignition risk of buildings in the WUI. This will be accomplished by linking the ignition resistance required of structures to anticipated exposures by using the exposure scale. Also, an understanding of exposure can help improve the effectiveness of wildland fuel treatments.

During a WUI fire, a given structure can be exposed to fire and/or embers. Both threats need to be independently quantified and addressed. A structure can be hardened for embers, fire, or both. Table 1 is used to illustrate how three distinct building elements may be vulnerable to exposure from embers and/or fire.

Building Elements Potential Ignition Vulnerability
Embers Direct Fire
Metal Frame Closed Window No Yes*
Untreated Wooden Deck Yes Yes
Attic Insulation Yes† No

Table 1. Building Element Vulnerability to Ember and Fire Exposure
* Window may break under direct flame exposure.
† Combustible insulation may ignite from embers inside attic, away from exterior attic vents.

Two issues must be addressed to make the scale quantitative: the critical lack of quantitative information on the exposure of structures to embers and fire; and the lack of a well-characterized, systematic effort that combines pre- and post-fire observations, laboratory, and field experiments, and fire modeling needed to characterize the ignition regimes of different WUI fuels.


The following assumptions are used in the development of the WUI scale:

  1. The fire and ember exposure conditions at a given location can originate from fire in wildland fuels and fuels within the WUI community. The fire and ember exposure each zone experiences is the linearly combined exposures of the external (wildlands) and internally generated exposures. As an example, structures within a zone may experience a significant ember assault from its proximity to wildland fuels, and from any burning fuels within the zone itself.
  2. During a WUI fire, both the fire exposure and ember assault at a given location will change with time. The fire and ember scales are intended to capture both the peak intensity and maximum duration of the exposure/assault.

The distance from the interface and width of each zone will be a function of fuel, topography and local weather (FTLW). The four zones selected for each of the fire and ember exposures are described next specifically as to exposure from the wildlands.


Wildland fire and ember exposures in very high-risk areas can result in significant structural losses at the perimeter of many communities. Field observations from first responders have identified burning homes as large ember generators, posing a significant threat to surrounding and particularly downwind structures and vegetation. By preventing the ignition of structures in very hazardous locations, significant reductions in further fire spread are achievable within WUI communities.

The proposed approach will therefore initially focus on fire and ember exposure from the fire in wildland fuels. Fire and ember exposure from burning structures, ornamental vegetation, or vehicles will be considered at a later date following the same framework. This exposure framework, together with supporting updates to building codes and standards, will make the WUI scale directly applicable to new construction.

Additionally, the current approach will enable the WUI scale to be used for evaluating existing communities, highlighting weaknesses and identifying retrofit solutions. Figure 2 illustrates the fire and ember exposure matrix for wildland fuels. The proposed exposure matrix is developed using three categories for terrain: flat, steep slope, and ravine; and three categories for wind: no wind, low wind, and high wind. Four fuel categories will be used to provide an initial characterization: homogeneous surface fuels (such as prairie grasses), inhomogeneous surface fuels (such as palmetto), inhomogeneous shrubs and low vegetation (such as chaparral), and canopied forest (such as what is found in the Intermountain West). The selected topographical, weather, and fuel attributes, while not all-encompassing, provide realistic input ranges for the characterization of fire and ember exposures. Modeling and field data collection from prescribed burns will be used to define the specifics of the topography, weather, and fuel attributes.

Figure 3. Fire Exposure from Burning Structure on Ornamental Vegetation – as a function of distance from the burning structure (NIST Photo, Witch/Guejito Fire, CA 2007)

In the future, a similar type of matrix will provide the fire and ember exposure from burning structures, ornamental vegetation, and vehicles in different local weather and topographical conditions.


This work was made possible through technical collaboration with numerous organizations including but not limited to CAL FIRE, San Diego Building Codes Department, the International Code Council, and the National Fire Protection Association

The authors also acknowledge Dr. Shyam Sunder, director, NIST Engineering Laboratory. This work is in part funded by the Joint Fire Science Program Project 11-1-3-29, "Evaluating the Effectiveness of Mitigations Activities in the Wildland Urban Interface.”

Alexander Maranghides is with the National Institute of Standards and Technology. William Mell, Ph.D., is with the U.S. Forest Service.


  1. Maranghides, A. and Mell, W. "Framework for Addressing the National Wildland Urban Interface Fire Problem – Determining Fire and Ember Exposure Zones using a WUI Hazard Scale,” NIST Technical Note 1748, National Institute of Standards and Technology, Gaithersburg, MD, 2013.
  2. International Wildland-Urban Interface Code, International Code Council, Washington, DC, 2012.