NIST Technical Note 1748

Framework for Addressing the 

National Wildland Urban Interface 

Fire Problem – Determining Fire and 

Ember Exposure Zones using a WUI 

Hazard Scale

Alexander Maranghides

William Mell

http://dx.doi.org/10.6028/NIST.TN.1748
NIST Technical Note 1748

Framework for Addressing the 

National Wildland Urban Interface 

Fire Problem – Determining Fire and 

Ember Exposure Zones using a WUI 

Hazard Scale

Alexander Maranghides

Engineering Laboratory

Fire Research Division

William Mell

U.S. Forest Service

Fire and Environmental Research Applications Team

Pacific Northwest Research Station

Seattle, WA 98103

http://dx.doi.org/10.6028/NIST.TN.1748

January 2013

U.S. Department of Commerce

Rebecca Blank, Acting Secretary

National Institute of Standards and Technology

Patrick D. Gallagher, Under Secretary of Commerce for Standards and Technology and Director
i

Certain commercial entities, equipment, or materials may be identified in this

document in order to describe an experimental procedure or concept adequately. Such

identification is not intended to imply recommendation or endorsement by the

National Institute of Standards and Technology, nor is it intended to imply that the

entities, materials, or equipment are necessarily the best available for the purpose. 

National Institute of Standards and Technology Technical Note 1748

Natl. Inst. Stand. Technol. Technical Note 1748, 25 pages (January 2013) 

This publication was a joint effort between the National Institute of Standards and 

Technology and the United States Forest Service (USFS).
ii

Table of Contents

Abstract................................................................................................................................v

1.0 Background....................................................................................................................1

2.0 Overview........................................................................................................................1

3.0 Current WUI Building Codes and Standards Practices .................................................2

4.0 Existing Hazard Severity Assessment Systems.............................................................3

5.0 The Proposed WUI-scale ...............................................................................................4

5.1. The Fire and Ember Exposure Scales.......................................................................6

5.2. Technical Assumptions.............................................................................................7

5.3. Fire and ember exposure from fire in wildland fuels................................................7

5.3.1. Wildland Ember Exposure Zones......................................................................9

5.3.2. Wildland Fire Exposure Zones ........................................................................12

6.0 WUI-Hazard Scale Implementation.............................................................................14

6.1. Building Construction Attributes and Landscaping for Protection against Embers

........................................................................................................................................14

6.2. Building Construction and Landscaping Attributes for Protection against Fire.....14

6.3. Improving WUI Building Codes and Standards.....................................................17

6.4. Special Implementation Considerations .................................................................17

6.5. Designing a New Community Using the WUI-Hazard Scale.................................17

6.6. Evaluating an Existing Community Using the WUI-hazard scale..........................18

6.7. Post Fire Community Reconstruction/Rebuilding..................................................19

7.0 Future Work.................................................................................................................19

8.0 Conclusions..................................................................................................................23

9.0 Acknowledgments........................................................................................................24

10.0 References..................................................................................................................25
iii

List of Tables

Table 1: Building Element Vulnerability to Ember and Fire Exposure............................... 5

Table 2: Fuels and Fire/Ember Exposures to Structures...................................................... 6

Table 3: Representative Ember Zone (E-zone) Fuel, Terrain and Local Weather............. 10

Table 4: E-Scale Building Construction Classes and Attributes ....................................... 15

Table 5: Fire and Ember Exposures and Threat to Homeowners and Fire Fighters.......... 16

Table 6: WUI-scale Framework Summary........................................................................ 21
iv

List of Figures

Figure 1: Primary Fuels Responsible for Fire and Ember Exposure at the WUI ................ 5

Figure 2: Capturing Exposure from Wildland Fuels............................................................ 8

Figure 3: Ember Exposure from Wildland Fuels Matrix..................................................... 9

Figure 4: Illustration of wildland ember exposure on community .................................... 11

Figure 5: Community illustration of wildland fire exposure ............................................. 13

Figure 6: Reducing WUI building Losses; Coupling Ember Exposure to Building 

Construction and Landscaping Classes............................................................. 14

Figure 7: Defensive Actions - reducing exposure and/or reducing ignition vulnerability 16

Figure 8: Illustration of wildland ember exposure on community surrounded fuel 

treatment............................................................................................................ 18

Figure 9: Technical framework for developing the WUI Hazard Scale ............................ 23

Figure 10: Snapshot from a computer simulation, using the Wildland-urban interface Fire 

Dynamics Simulator (WFDS),18 of a WUI fire in Worley, Idaho. Tree locations

and heights were obtained from LiDAR data. .................................................. 24
v

Framework for Addressing the National Wildland Urban Interface 

Fire Problem – Determining Fire and Ember Exposure Zones using a

WUI Hazard Scale

By

Alexander Maranghides and William Mell

Abstract

Destruction of homes and businesses from Wildland Urban Interface (WUI) fires has

been steadily escalating as have the fire suppression costs associated with them. Since 

2000, in the United States, over 3,000 homes per year are lost to WUI fires. This is 

compared to about 900 homes in the 1990s, and 400 homes in the 1970s. In 2011, in 

Texas alone, over 2,000 homes were destroyed during WUI fires. The WUI fire problem

affects both existing communities and new construction. In the U.S, the problem is most 

acute in the western and southern states; however, WUI fires have also recently 

destroyed homes in the Mid-Atlantic States and the Pacific Northwest.

One of the fundamental issues driving the destruction of homes at the interface is the 

very limited coupling between building codes and standards and potential fire and 

ember exposure. The limited exposure 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 to building assemblies testing. 

A WUI fire and ember exposure scale (WUI-scale) needs to be created to help 

consistently quantify the expected severity of WUI fire events based on measures, or 

scales, of expected ember and fire exposure. Once established, these technically based 

ember and fire exposures for the WUI can form the technical foundation for the 

development of a set of performance based building codes aimed at providing a level of 

structure ignition protection commensurate with the expected fire and/or ember 

exposure. 

The concept is based on quantifying expected fire and ember exposure throughout an 

existing WUI community. The proposed WUI-scale can be used to explicitly identify 

WUI areas that have a fire problem, as opposed to areas that meet housing density or 

wildland vegetation requirements as is frequently done. The scale can therefore be used 

to provide the boundaries where specific land use and/or building construction 

regulations would apply. Finally, the exposure scale can be used for both new and 

existing WUI communities. 

KEY WORDS: Wildland Urban Interface, WUI, fire behavior, fire exposure, ember exposure, WUI 

exposure scale
1

1.0 Background

Destruction of homes and businesses from Wildland Urban Interface (WUI) fires have 

been steadily escalating as have the fire suppression costs associated with them.

1 Since 

2000, in the U.S., over 3,000 homes per year are lost to WUI fires. This is compared to 

about 900 homes in the 1990s, and 400 homes in the 1970s.i In 2011, in Texas alone, 

over 2,000 homes were destroyed during WUI fires.

ii The WUI fire problem affects both 

existing communities and new construction. In the U.S, the problem is most acute in the 

western and southern states; however, WUI fires recently have destroyed homes in the 

Mid-Atlantic States and the Pacific Northwest.

WUI fires present a unique challenge to the fire fighting and fire protection engineering 

communities. The scale of the events can be vast, spanning in many cases over 40,000 

ha (100,000 acres). The moving fire perimeter can be tens of kilometers long with 

potentially thousands of structures at risk. The severity of the fire is dependent 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.

There are many WUI definitions based on different attributes including vegetative fuels 

or population density. In the Federal Registrar “the urban wildland interface community 

exists where humans and their development meet or intermix with wildland fuel.” iii

Further WUI sub-characterizations include interface, intermix and occluded type 

communities. Attempts have been made to characterize the hazard as well as the risk; 

however, results have been mixed due primarily to the diverse nature of the objectives 

driving the definitions. 

2.0 Overview

One of the fundamental issues driving the destruction of homes at the interface is the 

very limited coupling between building codes and standards and potential fire and ember 

exposure. The limited exposure 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 not taken place. The resulting 

gap between exposure and structure ignition therefore has resulted in a lack of tested and 

implementable hazard mitigation solutions.

A WUI fire and ember exposure scale (WUI-scale) needs to be created to help 

consistently quantify the expected severity of WUI fire events based on measures, or 

scales, of expected ember and fire exposure. Once established, these technically based 

ember and fire exposures for the WUI can form the technical foundation for a set of 

performance based building codes aimed at providing a level of structure ignition 

protection commensurate with the expected fire and/or ember exposure. 
2

The concept is based on quantifying expected fire and ember exposure throughout an 

existing WUI community. The proposed WUI-scale can be used to explicitly identify 

WUI areas that have a fire 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. Lastly, the exposure scale can be used for both new and existing WUI 

communities. 

3.0 Current WUI Building Codes and Standards Practices

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

iv,v have failed to address three 

critical components:

1. Impact of Defensive Actions

The influences of defensive actions are typically not considered. This critical 

information can be collected post-event through discussions with the first responders and 

homeowners, albeit with significant effort. Certain defensive actions taken by fire 

fighters, police, home owners or others are readily apparent to the trained first responder

conducting a post-fire study. Many times, however, information on defensive actions is 

lost and, as a result, structure survivability can be misinterpreted. For example, the initial 

information collected within The Trails community in San Diego, after the 2007 

Witch/Guejito fires, suggested that very limited defensive actions occurred within the 

community. Subsequently, more comprehensive data collection and discussions with 

homeowners and first responders determined that one out of every three structures in 

that community had been defended. Without this information on defensive actions, a 

complete assessment of the relevant factors underlying the survivability of structures 

could not be assessed.vi

2. Systematic Documentation of Structure Response to WUI Fires

Second, until recently, a systematic method for documenting structure response and 

exposure, throughout the area exposed to a WUI fire, has not been developed or applied. 

A systematic approach of documenting all the structures within the fire perimeter, or a 

community of interest, will support a significantly more reliable assessment of structure 

ignition vulnerabilities. Documenting only destroyed and damaged structures can result 

in erroneous data interpretation with respect to structural ignitions. Here is an example: 

at The Trails community in San Diego, CA, the Witch/Guejito fires in 2007 destroyed 

74 homes and damaged 16. Out of the 74 destroyed homes, 12 had wood shake roofs (of 

varying ages and treatments), while 37 had Spanish tile roofs (with and without bird 

stops), 24 had composite roofs, and there was one metal roof. The wood shake roofs 

accounted for 16% of the destroyed structures, while the Spanish tile roofs were 50% of 

the destroyed structures. There were a total of 245 structures within the fire at The 
3

Trails. Considering the performance of all roofs within the fireline, 100% of the wood 

shake roofs exposed were destroyed while only 24% of Spanish tile roofs were 

destroyed. By documenting all structures within the fire line, the relatively high (all 

other factors being equal) vulnerability of wood shake roofs stands out. This baseline 

comparison is critical to understanding the true impact of different construction and 

landscaping attributes to structure survivability. While quantifying structure 

survivability is a complex process that involves construction particulars and measures of 

fire and ember exposure, the above example illustrates how misleading partial 

information can be. 

3. Quantification of Fire and Ember Exposure

There is a lack of consistently applied measures to characterize fire and ember exposures

during WUI fire events. This is one of the key constraints to improving the understanding 

of structure survivability and is essential to the development and assessment of 

implementable national WUI hazard reduction methodologies. A common yardstick for 

exposure will enable reliable comparison of the relative performance of different hazard 

reduction solutions. The response of a structure exposed to a severe ember assault cannot 

be compared to the response of a structure that experienced only a minor ember attack. 

Only by using a common yardstick of exposure can comparisons be made within and 

across different WUI fire incidents.

4.0 Existing Hazard Severity Assessment Systems

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 

Zones

4 is used to determine fire hazard on a 9 m (30 ft) grid. This information is applied 

in areas under State jurisdiction. The assessment was done for all of California and was 

used by the State Responsibility Areas (SRA) for stricter requirements and provide an 

avenue for Local Responsibility Areas (LRA) to use stricter construction for new 

construction) to identify where the stricter Californiavii WUI construction building code 

are required for new construction. The FRAP Fire Hazard Severity Zones are 

geographical areas designated pursuant to California Public Resources Codes Sections 

4201 through 4204 and classified as Very High, High, or Moderate in State 

Responsibility Areas, or as Local Agency Very High Fire Hazard Severity Zones 

designated pursuant to California Government Code, Sections 51175 through 51189.”viii 

The FRAP program is one of the few programs in the U.S. that links fire severity 

(exposure) and building codes (construction attributes). While the FRAP program 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 fire and ember exposures such as from an 

adjacent burning structure. The FRAP system, with respect to building construction is 

two tiered: a structure is either in the WUI or it is not. This exposure classification 

system looks at wildland exposure as bimodal and does not address the likelihood that 

buildings could be destroyed from exposure sources other than from the wildlands. 

Other similar programs, but less complex WUI hazard rating systems exist and are 

implemented across the U.S. 
4

The Home Ignition Zone (HIZ)

ix

concept represents another WUI hazard severity 

assessment framework designed to be implemented at a parcel or structure level. The 

home ignition zone includes the home and an area surrounding the home within 100 to

200 feet (30 to 60 m). The method has been successfully used to educate homeowners 

on the different parameters affecting structure survivability. The primary limitation of

the HIZ methodology in the context of this paper 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 different 

exposure scenarios. A framework similar to the HIZ is also used by the International 

Wildland-Urban Interface Codex as well as many other national and state hazard 

mitigation programs.

As mentioned earlier, it should be pointed out that, to date, WUI hazard assessment tools 

have generally not been developed using the results from a systematic and standardized 

field data collection methodology. This uncoupled approach limits the reliability of these 

tools as there has been no framework for coupling field exposure data to structure 

survivability. Additionally, there has been no coupling between ember/fire exposure and 

first responder and homeowner safety.

5.0 The Proposed WUI-scale

WUI fire events, unlike other natural disasters such as hurricanes, tornadoes and 

earthquakes, do not weaken with distance away from a well defined epicenter. 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 local and transient variations in fuel, topography 

and local weather. 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. This paper outlines the technical foundation for the creation of a WUI fire and 

ember exposure scale as well as a framework for linking the proposed scale to national 

WUI building codes and standards. 

The WUI-scale is designed to quantify fire and ember exposure in the WUI over 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: wildland fuels, ornamental 

vegetation and burning of structures (including homes, auxiliary buildings such as sheds 

and garages) and vehicles (figure 1). The WUI-scale is designed considering these 

sources as well as the local weather. These combined parameters are referred to as 

FTLW, short for fuels, topography, and local weather1. In the proposed framework, an 

exposure rating is uncoupled from ignition, and the exposure rating is independent of 

the response of a particular structural element or landscaping attribute. 

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.

During a WUI fire, a given structure can be exposed to fire and/or embers. Both threats 

 1 Local weather here refers to local wind speed, wind direction, temperature and relative humidity.
5

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.

Table 1: Building Element Vulnerability to Ember and Fire Exposure

Building Elements Potential Ignition Vulnerability

Embers Direct Fire

Metal Frame Closed Window No Yes 2

Untreated Wooden Deck Yes Yes

Attic Insulation Yes 3 No

The approach outlined in the text below offers a qualitative framework for establishing a 

WUI-scale. Two issues must be dealt with 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. An exposure and structure response 

database is required to successfully develop the WUI-scale approach. An explicit 

technical plan to meet these needs has been outlined in the paper by Mell et al.

xi

The 

technical plan to be followed is summarized and presented here in the last section of the 

current paper.

Figure 1: Primary Fuels Responsible for Fire and Ember Exposure at the WUI

 2

Window may break under direct flame exposure. 3 Combustible insulation may ignite from embers inside attic, away from exterior attic vents

Burning 

Structure

Structure

Burning Ornamental 

Vegetation

Burning Wildland 

Vegetation

Burning Vehicle

Ornamental Vegetation

Vehicle

Arrows indicate fire and/or ember exposures
6

Unlike a hurricane that gets an overall rating based on maximum sustained winds, a WUI 

fire, being so dynamic and variable in space and time, cannot be characterized by a single 

rating. The proposed scale uses the local conditions that create the fire and embers to 

generate a local exposure or rating. In this context, the proposed scale may be used for a 

particular community to rank the threat from the wildlands. A wildland fire may present a 

higher threat to one community compared to another based on the wildland fuels 

throughout the interface, local winds, and topographic features. The scale is designed to 

work along the interface and nominally 1.2 km (0.75 mile) into the community from the 

perimeter.

4

Another distinction between a hurricane (or other natural hazard) and a WUI fire is that 

the hazard level of a fire may be mitigated ahead of time through fuel removal. The 

proposed methodology can provide the science to enable benefit-cost analysis on the 

extent of implementation of fuel removal or treatments for specific fuel, topographical 

and weather conditions. The ecological and environmental impacts of such treatments are 

beyond the scope of this document. 

5.1. The Fire and Ember Exposure Scales

In the WUI-scale approach, the fire and ember exposure threats each can be binned in 

different categories. The intensity of the exposure increases from category 1 to 4 and 

the exposure levels decrease with distance away from the burning source. When 

applied in practice, both ember and fire exposure levels need to be used to capture the 

total threat to a structure. Additionally, both need to be accounted for when hardening a 

structure against ignition. 

This two component (fire and ember) exposure scale is necessary as these two 

exposures threats have different origins, each with a different “reach.” The radiative

heat flux generated by a fire decays with the square of distance away from the flame

front and is mainly affected by the fuels in the immediate vicinity of the fire.

5,6 The 

embers, on the other hand, can travel hundreds of meters or more.

Embers from far afield may pose a threat to a particular structure even if the same fire 

creating these embers is not exposing the structure to the fire heat. Table 2 illustrates 

how different fuels and their proximity to a target structure contribute to both the fire 

and ember exposure scale.

Table 2: Fuels and Fire/Ember Exposures to Structures 

Burning Fuel Fire Exposure Ember Exposure

Vegetation near structure Yes Yes

Nearby wildland vegetation Yes Yes

 4 This value may be refined as additional post-fire data collection and analyses are conducted using a systematic 

method for documenting exposure, defensive actions and structural behavior.

5 Convective heating is also included in the determination of the fire exposure.

6 Well irrigated, low flammability vegetation near a structure can block radiation from an approaching fire.
7

Distant wildland vegetation No Yes

Nearby structure Yes Yes

Distant structure No Yes

Extensive quantification is necessary both for the fire and ember exposures. The 

proposed exposure scales offer a framework to initiate the implementation of this 

concept. Quantification of the fire and ember exposures could be achieved through a 

combination of post fire studies, laboratory and field experiments, and computer 

modeling. 11

A community may include one or more areas or zones of a given exposure level. Any 

one location in the community will have both a fire and an ember exposure rating. As an 

example, a location could have no fire exposure, yet have an intermediate ember 

exposure.

5.2. Technical Assumptions

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 are the linearly combined exposures of the external and 

internally generated exposures. As an example, structures within a zone will 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 below specifically as to exposure from the wildlands.

5.3. Fire and ember exposure from fire in wildland fuels

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 wildland fire and ember exposure 

from the wildlands. Fire and ember exposure from burning structures, ornamental 

vegetation or vehicles will be considered at a latter date following the same framework. 

This exposure framework, together with the 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 
8

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 and high wind. Four fuel categories will be used 

to provide initial matrix 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. 

A similar type of matrix will provide, in the future, the fire and ember exposure from

burning structures, ornamental vegetation, and vehicles in different local weather and 

topographical conditions. The approach is designed to quantify the fire and ember 

exposure (Figure 2) and, together with improved building codes and standards, support the 

design and maintenance guidelines of ignition resistant structures. Figure 3 illustrates the 

ember exposure from various wildland fuels. Certain wildland fuels, under certain 

conditions may generate severe ember exposures to a target, while under different 

conditions they may generate significantly lower exposures.

Figure 2: Capturing Exposure from Wildland Fuels

Wildland Fuels Embers

Topography

Local Weather Flat

Slope

Ravine

No wind

Low wind

High wind

1 2 3 4

1: homogeneous surface fuels 

2: inhomogeneous surface fuels 

3: inhomogeneous shrubs and 

low vegetation 

4: canopied forest 
9

Figure 3: Ember Exposure from Wildland Fuels Matrix

5.3.1. Wildland Ember Exposure Zones

Ember flux data are needed to characterize the ignition of both structural elements and 

vegetation and to specify the four proposed ember zones as follows:

E1: The first ember zone, E1 is defined as having no significant flux of embers (in 

grams/m2

-sec).

7

E2: The second ember zone, E2, is defined as one where there is a low ember assault. 

This area or band typically in the interior of the community will offer improved 

protection from a direct wildfire assault. Ember flux data is not currently available. 

E3: This ember zone is typically found on the perimeter of the community and close to 

or adjacent to wildlands. This zone will experience a significant ember assault. Ember 

flux data is not currently available; experiments are needed to characterize the ignition of 

structures and structural elements as a function ember mass flux.

E4: This ember zone is found on the perimeter of the community and will be subject to 

extreme ember exposure. As an example, the area around terrain features such as a 

chimney or ravine may be classified as E-4 if the wildland fuel load is significant and low 

humidity and high local winds are present or expected. E-4 is typically found at the 

interface between wildland and residential fuels. E-4 may extend into the community 

based on topographical and fuel features. 

Guidelines for the WUI-scale determination are summarized in Table 3 for embers. In 

conducting a WUI-scale evaluation, the fuel is evaluated followed by terrain and local 

weather. The fuel drives the overall evaluation, since if fuel is not present, the hazard 

is dramatically reduced. Fuel is a critical but not sufficient part of characterizing the 

exposure. After the fuel had been characterized, terrain and local weather must be 

 7 The mass and type (e.g. size) of embers contribute to its potential to contribute to the ignition of combustibles.

The mass flux is used as a simplified way to quantify exposure. Additional research is needed on the attributes of 

embers being generated by burning wildland and ornamental vegetation as well as by burning structures.

E4 – Most Severe

E3

E2

E1 – No Wildland Ember 

Exposure

E3 E4 E4 E4

E3 E4 E3 E3

E2 E4 E3 E2

 Wildland Fuels Matrix Ember Exposure Scale

Topography

Local Weather
10

assessed before the WUI-scale can be determined for a particular locale. While the 

presence of fuel is a key component to determining the ember exposure, it is the 

combination of fuel, topography and local weather that is responsible for the final 

ember exposure. Table 4 contains representative fuel, terrain and local weather

conditions that are expected to yield different ember fluxes and E-scale ratings. 

The WUI-scale concept is multi-threat and exposure driven and therefore other 

combinations may result in similar exposures. All three columns represent maximum 

attribute characteristics for each zone. For instance, the local weather can be 

identified as the worst case scenario during the local fire season. While Table 3

represents in general terms four different ember exposures and their associated fuel, 

terrain and local weather characteristics, modeling and field experiments will be used 

to populate the table with variations of the above characteristics. As an example, light 

fuel loading in steep terrain with low humidity and strong winds may generate a 

significant ember exposure for structure ignition. 

Table 3: Representative Ember Zone (E-zone) Fuel, Terrain and Local Weather

Characteristics

E-Zone Ember

Exposure

Fuel (necessary) Terrain Local Weather

E1 no ember

exposure

light fuels- wet 

environment

flat terrain - low or moderate

humidity 

- light winds

E2 minimal

ember exposure

light fuel loading moderate

slope

- low humidity

- intermediate winds

E3 significant ember 

exposure

intermediate fuel

loading

moderate

slope

- low humidity

- strong winds

E4 extensive ember 

exposure

- dry flash8 fuels

- high fuel loading

- steep slope

- chimneys

- low humidity

- strong winds

Figure 4 illustrates ember exposure zones within a community from wildland fuel 

generated embers. The ember generation is a function of fuel, topography and local 

weather (lining up of topographical features with the prevailing local wind). The 

different ember exposure intensities are illustrated by differing colors in Figure 4. In the 

case illustrated, the presence of large quantities of volatile wildland fuels around the 

community together with steep terrain and the likely presence of high winds resulted in 

severe ember exposure (E4-dark red) at several locations on the perimeter of the 

community. At one location where a ravine is lined up with the prevailing wind, E4 

 8 Flash Fuels: Fuels such as grass, leaves, draped pine needles, fern, tree moss and some kinds of slash, that ignite 

readily and are consumed rapidly when dry. Also called fine fuels. Source: Glossary of Wildland Fire Terminology, 

National Wildfire Coordinating Group, PMS 205, October 2006 
11

reaches further into the community. In this case, combinations of fuel and topography, 

or fuel and prevailing wind are present, resulting in significant ember exposure (E3-

orange). Further away from the wildlands, the E-2 areas (yellow) represent low ember 

exposure from the wildlands. Finally, E-1 (white) illustrates the area in the community 

when no embers from the wildlands are expected. The represented community is on a 

knoll with an E3 area almost encircling the community. The actual extent of the different 

exposure areas is provided for illustration only. Specifics of the different ember and fire 

exposure areas or zones follow. It should be noted that the ember exposure zones are 

intended to capture or represent the majority of the embers generated. Embers have been 

known to travel for many kilometers and these far field embers can cause ignitions. In the 

context of the WUI communities discussed in this paper these limited far field embers are 

not considered to pose a significant exposure issue when compared to the near field 

embers from the burning adjacent wildlands.

Figure 4: Illustration of wildland ember exposure on community

Prevailing 

Local Winds

E4

E3

E2

E1

1.75 Km

Zone depths are for illustration purposes

Zone Depth

Ravine
12

5.3.2. Wildland Fire Exposure Zones

The following Fire Exposure Zones (F-zone) are proposed to indicate the increasing 

susceptibility of structures within the community to radiative ignition from the fire 

flames (as opposed to the embers):

F1: The first F-zone is defined as having no significant heat flux from burning 

wildland vegetation. No special WUI fire exposure building construction is 

recommended in this area. There is no expected external heat flux above ambient level.

F2: The second fire zone, F2, is defined as one where there is low heat flux exposure. 

This area or band typically in the interior of the community will offer improved 

protection from a direct wildfire assault. Intermediate construction requirements are 

recommended in the F2 zone. Exposed combustibles are permitted on the structure 

itself. Combustibles may also be attached to the structure. The incident heat flux can 

cause harm to humans. No window breakage or ignition of exterior combustibles is 

likely to occur in Zone 2. Expected heat fluxes above ambient level: up to 0.8 W/cm

2

.

xii

F3: The third fire zone, F3, is defined as one where there is moderate heat flux. 

Hardened construction requirements are recommended in the F3 zone with no exposed 

combustibles on the structure itself. Combustibles may be on the property, a significant 

distance away from the structure. The incident heat flux can cause harm to humans. 

Window breakage of unhardened glass and ignition of exterior combustibles will occur 

in Zone 3. Heat flux levels are expected in the range of 0.8 to 2 W/cm

2

.

xiii

F4: the fire zone F4 is defined as the area where there is severe heat flux and 

prolonged direct flame contact. Heat flux level is expected to be greater than 2 

W/cm

2

.

Determining the extent of the E and F-zones for the different FTLW will be accomplished 

through the use of computer models, engineering calculations and field experiments. The 

methodology generates a matrix of community exposure areas or zones (Figures 4 & 5) 

based on the most likely worst case scenario for the each of the factors: wildland fuels, 

weather (specifically local wind and relative humidity) and topography, specifically:

• Wildland fuel build-up is addressed by selecting the steady state fuel loading that 

can be supported in that particular ecosystem. If fuel treatments are implemented and 

are part of the long term maintenance of the community surroundings then the loading 

of the treated fuels should be used in the WUI-Zone determination.9

• Local weather conditions are determined from the local weather history.

• Topography should be considered in conjunction with weather, specifically local 

wind. For example, in Southern California, the Santa Ana wind direction is predictable 

 9 It should be noted that if a community is built using an exposure rating based on treated wildland fuel 

loadings, the anticipated hazard reduction will only be achieved if these wildland fuel treatments are 

maintained. If the fuels treatment is not maintained, the community’s ratings will change and building 

construction and landscaping attributes may no longer provide the resiliency they originally did.
13

Figure 5: Community illustration of wildland fire exposure

Prevailing 

Local Winds

1.75 Km

0.15 Km

F1

F3

F4

F2

Zone depths* are for illustration purposes

Zone Depth

Parcel Boundary

* Zone depths around the community depend on fuels, topography and local weather. 
14

and well established. Topographical features that line up with the wind flow can result in 

increased flame lengths and ember transport, increasing the severity of downwind 

structure exposure conditions.

6.0 WUI-Hazard Scale Implementation 

In the framework presented here, there are four different WUI building construction and 

landscaping classes, WUI 1 through WUI 4. The following sections of the paper couple 

fire and ember exposure ratings to building construction and landscaping attributes. 

6.1. Building Construction Attributes and Landscaping for 

Protection against Embers

A WUI community can have up to four different types of structure and landscaping 

categories based on ember exposure levels as identified by the WUI-hazard scale. Figure 6

outlines the coupling between ember exposure and building construction and landscaping 

classes. The WUI building construction and landscaping classes and attributes are 

summarized in Table 4. 

 Ember Exposure Scale (E-scale) Building Construction and 

Landscaping Classes

E4 - Most Severe WUI4 – Most Severe

E3 WUI3

E2 WUI2

E1 WUI1- Unrestricted Construction*

 * Unrestricted only with respect to WUI exposure

Figure 6: Reducing WUI building Losses; Coupling Ember Exposure to Building 

Construction and Landscaping Classes

6.2. Building Construction and Landscaping Attributes for 

Protection against Fire

Defensive actions can be used to reduce ignitions by one of two mechanisms: reduce 

exposure or reduce vulnerability. Two defensive action examples are provided to illustrate 

this: pre-wetting a structure reduces its ignition vulnerability, while suppression efforts on 

the surrounding burning vegetative fuels reduce the structure’s exposure to fire and 

embers (figure 7). The proposed exposure framework, with the associated ember and fire 

exposure scales can also be used to describe the potential threat to homeowners and 

firefighters. 
15

Table 4: E-Scale Building Construction Classes and Attributes

WUI 

scale

Building 

Construction 

Class

Ignition 

Vulnerabilities 

from Embers

and Fire

Building Construction and 

Landscaping Attributes for 

Protection against Embers

E1 or F1 WUI 1 None Normal Construction Requirements:

- Maintained Landscaping

- Local AHJ-Approved Access for 

firefighting equipment

E2 or F2 WUI 2 In this area, highly 

volatile fuels could be 

ignited by embers. 

Weathered, dry 

combustibles with 

large surface areas can 

become targets for 

ignition fro m embers.

Low Construction Hardening Requirements:

- Treated combustibles allowed on structure

- Attached treated combustibles allowed

- Treated combustibles allowed around 

structure

- Low flammability plants

- Irrigated and well maintained Landscaping

- Local AHJ-Approved Access for 

firefighting equipment

E3 or F3 WUI 3 Exposed combustibles 

are likely to ignite in 

this area from high 

ember flux or high 

heat flux

Intermediate Construction Hardening 

Requirements:

- No exposed combustibles on structure

- Combustibles placed well away from

structure

- Low flammability plants

- Irrigated and well maintained landscaping

- Local AHJ-Approved Access for 

firefighting equipment

E4 or F4 WUI 4 Ignition of 

combustibles from 

direct flame contact is 

likely.

High Construction Hardening Requirements:

- No exposed combustibles

- All vents, opening must be closed

- Windows and doors must be covered 

with insulated non-combustible

coverings.

- Irrigated and well maintained low 

flammability landscaping

- Local AHJ-Approved Access for 

firefighting equipment
16

Figure 7: Defensive Actions - reducing exposure and/or reducing ignition 

vulnerability

Table 7 summarizes the fire and ember threats presented to homeowners and fire 

fighters. It should be noted that fire fighters are explicitly differentiated here from other 

first responders because of their fire resistive personnel protective equipment.

Table 5: Fire and Ember Exposures and Threat to Homeowners and Fire Fighters

 10 Fire fighters wearing appropriate Personal Protective Equipment (PPE).

Fire Exposure Scale Threat to Homeowners Threat to Fire Fighters10

F1- None None None

F2 – Low heat flux Severe None

F3 – Moderate heat flux Severe Moderate

F4 – Severe heat flux

(prolonged direct flame 

contact)

Severe Severe

Ember Exposure Scale

E1 – None None None

E2 – Minor Moderate None

E3 – Significant Severe None

E4 – Extreme Severe None

Defensive Actions: reducing exposure and/or 

reducing ignition vulnerability

Exposure [Fire (Radiant heat/Flame), 

Embers] = function of (topography, local 

weather and fuels*)

Fuels* Ignition 

Vulnerabilities

* Fuels = wildland vegetation, residential vegetation, structures, infrastructure and other 

combustibles)
17

6.3. Improving WUI Building Codes and Standards

In order to successfully implement the exposure scale, building codes, standards, and best

practices must be developed to address the fire and ember exposure as estimated by both 

the E and F-scales. Figure 6 illustrates the ember exposure – building construction 

coupling. While construction in the E-4 or F-4 zones is possible, in an attempt to provide 

cost effective solutions, focus is placed here on the zones where fire and ember exposure 

are not as extreme, namely E1 through E3 and F1 through F3.

6.4. Special Implementation Considerations 

The landscaping issues in and around a community fall under the regulatory purview of 

land use planners and zoning officials. In organized subdivisions, these issues may also 

fall under the purview of an architectural review board. The proposed WUI-scale 

framework could be used to provide guidance on fuel treatments in and around existing or 

new communities. As an example, in order to reduce the ember exposure generated in E-4 

zones, an option may exist to conduct fuel treatments around a particular community. To 

date, fuel treatment effectiveness has been evaluated primarily from an ecological 

perspective. There is, therefore, very little technical information on how to effectively 

“treat” wildland fuels in order to generate a known hazard reduction for the adjacent 

community. The Joint Fire Science Program (JFSP), US Department of Interior - Bureau 

of Land Management, 2011 Request for Applicationsxiv was aimed at addressing this 

specific technical gap. There are many implementation challenges associated with fuel 

treatments such as jurisdictional issues, maintenance and environmental considerations. 

These are beyond the scope of this paper. 

6.5. Designing a New Community Using the WUI-Hazard Scale

The first step in using the WUI-scale is to evaluate the exposure of a community from the 

fire in wildland fuels for the local topography and typical fire weather conditions. Once 

the wildland exposure has been determined, the next step may be the creation of fuel 

treatment zones if any are necessary and their implementation and maintenance are 

possible. Fuel treatment zones would be useful to consider any time E4 and/or F4 

exposure levels are identified. This can be accomplished in one of two ways. An example 

is provided for an E-4 zone for illustrative purposes. The E-4 threat can be reduced by fuel 

treatment to decrease the area originally classified as E-4 or a fuel treatment zone can be 

created around the initial E-4 zone (see green area in figure 8). The fuel treated area can 

remain wildland or be adapted for use as common areas such as green spaces. The end 

result in both cases is the introduction of the buffer zone between the threat and the first 

built zone in order to attenuate the local ember exposure. Benefit-cost analysis tools are 

being developed by NIST’s Engineering Laboratory specifically for this purpose.

xv These 

tools will enable community planners and land use administrators to weigh the benefits of 

long term wildland fuel treatments against the increased construction and maintenance 

requirements associated with a community exposed to the higher fire hazards from 

untreated wildland fuels.
18

Figure 8: Illustration of wildland ember exposure on community surrounded fuel 

treatment

6.6. Evaluating an Existing Community Using the WUI-hazard 

scale

Existing communities could be evaluated using the WUI-hazard scale in order to assess 

vulnerabilities, potential losses from a WUI fire, and to focus retrofit efforts. The WUIscale evaluation of the community can also be used to assess the impact of wildland fuel 

treatments and how they can affect the retrofit requirements within the community.

Evaluating an existing community using the WUI-hazard scale is a two step process. The 

first step is to assess the community’s level of fire and ember exposure from the 

wildlands based on the fuels, terrain, and local weather (FTLW). This evaluation will 

determine WUI-zones throughout and around the entire community. The second step is 

to collect construction and landscaping/vegetation attributes for the structures in the 

community and compare them to the recommendations associated with the different 

WUI-zones. This step will highlight vulnerabilities and identify areas where structures, 

given their existing construction and landscaping, will be more at risk. The WUI-zone 

concept is based on quantifying fire and ember exposure throughout an existing WUI 

community. If structure ignition vulnerabilities are not addressed, then the initial hazard 

zoning assessment will not be valid. If structures are at a significant risk of igniting 

Prevailing 

Local Winds

E3

E2

E1

Ravine

1.75 Km

Zone depths are for illustration purposes

Zone Depth

Fuel Treatment
19

during a WUI fire event, then a new assessment of fire and ember exposure should be 

conducted in the community.

6.7. Post Fire Community Reconstruction/Rebuilding

The WUI-scale can be used to guide reconstruction in communities that have been 

affected by a WUI fire. The reconstruction at the community scale will begin with a 

community evaluation. Post WUI fire data will be used to determine the likely evolution 

of the fire scenario and to quantify fire and ember exposure across the entire community. 

E/F-zones will be identified and reconstruction beyond E1 and F1 zones will be guided 

by the proposed improved multi-tier building construction codes system.

Future post fire reconstruction plans may be developed using guidelines similar to those 

used for rebuilding in flood prone areas, earthquake zones or other disaster prone areas.

18

The WUI-scale will enable more sustainable rebuilding of WUI communities by 

integrating fire resistant structures and vegetation with community planning and design.

7.0 Future Work

The above framework outlines the fundamentals of the WUI-hazard scale methodology 

and how it can be used to improved building codes and standards. Table 7 summarizes 

the WUI-scale framework, including the E and F scale construction considerations and 

threats to homeowners and first responders. The four construction classes, WUI 1 

through WUI 4 will enable targeted and cost effective hazard mitigation implementation. 

While the full characteristics and resulting ember fluxes for the different ember 

exposures are not currently known, initial values will be obtained by a combination of 

field and laboratory experiments, NIST and the US Forest Service (USFS) are working 

on conducting both types of experiments to provide initial ember and heat flux exposure 

values across a range of wildland fuels and weather conditions. Computer modeling will 

be used to augment the field and laboratory experiments for conditions that are not be 

evaluated experimentally. Figure 9 illustrates the proposed technical framework for 

developing the WUI-scale. 

The work necessary to implement the proposed WUI-scale falls into three categories:

1. Develop building construction and landscaping guidance for the building and fire

codes and standards. This work should be conducted in partnership with national and 

international standards and codes organizations. Emphasis will be given to design 

guidance and or requirements for both new construction and retrofit solutions of 

existing structures. The technical coupling of the exposure to construction 

requirements set forth in building codes and standards is one of the cornerstones of 

the proposed approach. 

2. Create a guide for field application of the WUI-scale methodology in new and 

existing communities including application to retrofit and post fire reconstruction 

needs. This is important for two reasons. A field application guide will enable 

communities to assess their vulnerabilities and take proactive steps to reduce their 
20

WUI risks. The guide will aid for post fire reconstruction for communities affected 

by WUI fires. 

3. Refine the guidance and expand the applications through research to better define fire 

and ember exposure levels based on fuel, topography and local weather. This is the 

technical foundation of the E/F-scale system. Work will be done with different 

wildland fuel types and fuel loadings. This work will be done with technical input 

from laboratory and field experiments as well as through the use of two and threedimensional fire models. The technical approach to the development of the E/F scale

has four components:

• Post-fire data collections, such as conducted by NIST,

vi,xvi Texas Fire Service 

(TFS) and USFS. The post fire data collections is needed to identify construction

and landscaping vulnerabilities as well as validate the performance of implemented 

hazard reduction technologies. 

• Field data collections from prescribed burns are needed to quantify the fire and 

ember exposures for different fuel topography and local weather conditions.

• Laboratory work is needed to characterize the ignition response of structure 

materials and assemblies to exposures representative of real conditions. 

Guidance on representative exposure conditions and candidate structure 

vulnerabilities will be obtained from field data. Representative building 

constructions will be evaluated for all four building classes identified in Table 7. 

NIST has developed an ember generator that can be used to generate specific 

ember size and quantify distributions. The capabilities of this experimental 

apparatus have been demonstrated on initial experiments with roof assemblies, 

vents and decks. xvii[What about collaborating work with IBHS?]

• Computer modeling using tools such as the NIST and USFS developed wildlandurban interface fire dynamics simulator (WFDS)

xviii will be initially used to create 

the zones. WFDS will be used to create simpler tools as technical expertise and 

model validation mature. Models will also be used to identify ignition threshold 

zones for the different types of construction and therefore yield the E3 though E1 

and F3 though F1 zone boundaries. Representative buildings will be created in the 

model based on the laboratory experiments. An example of a WFDS calculation 

for an existing community is shown in Figure 10.
21

Table 6: WUI-scale Framework Summary

Escale/zoneEmber

Exposure

Ember Flux

(grams/m

2

-sec)

Building 

Construction 

Class

Building Construction 

Attributes for Protection 

against Embers

Threat to

Homeowners

Threat to Fire

Fighters

E-1 None None WUI 1 Normal Construction Requirements

- Maintained Landscaping

- Local AHJ-Approved Access for firefighting 

equipment

No No

E-2 Minor ** WUI 2 Low Hardening Construction Requirements:

- Treated combustibles allowed on structure

- Attached treated combustibles allowed

- Treated combustibles allowed around structure

- Low flammability plants

- Irrigated and well maintained Landscaping

- Local AHJ-Approved Access for firefighting 

equipment

Moderate No

E-3 Significant ** WUI 3 Intermediate Hardening Construction Requirements:

- No exposed combustibles on structure

- Combustibles placed well away from Structure

- Low flammability plants

- Irrigated and well maintained Landscaping

- Local AHJ-Approved Access for firefighting 

equipment

Severe No

E-4 Extreme ** WUI 4 High Hardening Construction Requirements:

- No exposed combustibles

- All vents, opening must be closed

- Windows and doors must be covered with 

insulated non-combustible coverings.

- Irrigated and well maintained low 

flammability landscaping - not

- Local AHJ-Approved Access for firefighting 

equipment - not

Severe No

** further work needed 
22

WUI-scale Framework Summary (Continued)

F-scale/zone Fire

Exposure

Heat Flux

(W/cm

2

)

Building 

Construction 

Class

Building Construction 

Attributes for Protection 

against Fire

Threat to

Homeowners

Threat to Fire

Fighters

F-1 None 0 WUI 1 Normal Construction Requirements

- Maintained Landscaping

- Local AHJ-Approved Access for firefighting 

equipment

No No

F-2 Low heat 

flux**

<0.8 WUI 2 Low Hardening Construction Requirements:

- Treated combustibles allowed on structure

- Attached treated combustibles allowed

- Treated combustibles allowed around structure

- Low flammability plants

- Irrigated and well maintained Landscaping

- Local AHJ-Approved Access for firefighting 

equipment

Severe No

F-3 Moderate

heat flux**

0.8-2 WUI 3 Intermediate Hardening Construction Requirements:

- No exposed combustibles on structure

- Combustibles placed well away from Structure

- Low flammability plants

- Irrigated and well maintained Landscaping

- Local AHJ-Approved Access for firefighting 

equipment

Severe Moderate

F-4 Severe heat

flux** 

(prolonged 

direct flame 

contact)

>2 WUI 4 High Hardening Construction Requirements:

- No exposed combustibles

- All vents, opening must be closed

- Windows and doors must be covered with 

insulated non-combustible coverings.

- Irrigated and well maintained low 

flammability landscaping

- Local AHJ-Approved Access for firefighting 

equipment

Severe Severe

** Experiments needed to quantify the flux levels. 
23

Figure 9: Technical framework for developing the WUI Hazard Scale11

8.0 Conclusions

The WUI-scale and zone concept offers a framework for evaluating the fire and ember 

exposure of proposed and existing WUI communities. The coupling of the WUI-hazard 

scale concept with a multi-tiered codes and standards framework will enable an efficient 

reduction in WUI structural losses for both new and existing communities. Additionally,

the framework links fire and ember exposure and resident and fire fighter safety. While 

there is significant technical work that is necessary to fully implement the proposed 

framework, this coupled exposure-vulnerability technical approach could offer a datadriven cost effective way to reduce losses from future WUI fires. 

A quantified and easily implemented exposure scale is needed. Without it, efforts to 

reduce the structural losses from WUI fire incidents will likely have only limited 

success. This paper has outlined the framework for the initial development of the 

WUI-hazard scale concept. Future research will guide further development of the 

concept for the effective implementation of the WUI-hazard scale/zone concept and 

WUI building construction class system.

 11 Prescribed burns will be conducted in different fuel, topographical and weather conditions.

Structure 

ignition 

vulnerabilities 

Fire and 

ember 

exposures

Prescribed 

burns(with 

and without 

fuel 

treatments)

Post fire data 

collections

Representative 

structures for WUI 1, 

WUI 2, WUI 3 and 

WUI 4 constructions 

Laboratory 

experiments and 

modeling

Building Codes 

and Standards

Community Design 

Codes and Standards
24

Figure 10: Snapshot from a computer simulation, using the Wildland-urban 

interface Fire Dynamics Simulator (WFDS),18 of a WUI fire in Worley, Idaho. Tree 

locations and heights were obtained from LiDAR data.

9.0 Acknowledgments

This work was made possible though 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. 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”.
25

10.0 References

 i

International Code Council WUI Fire Statistics, June, 15, 2011 draft.

ii Technical discussion with Texas Forest Service, October 2011.

iii Paragraph Citation, 66 Federal Register 753, January 4, 2011 

iv Fourmile Canyon Fire, Preliminary Findings, U.S. Department of Agriculture Forest 

Service Rocky Mountain Research Station, Revised October 12, 2011,

http://www.fs.fed.us/rmrs/docs/fourmile-canyon-fire/preliminary-findings.pdf

v Mega Fires: The Case for Mitigation, Institute of Business and Home Safety, July 2008, 

http://www.disastersafety.org/content/data/file/megafires_full.pdf

vi A Case Study of a Community Affected by the Witch and Guejito Fires, Maranghides

and Mell, Fire Technology, 47(2), April 2011, pp. 379-420.

vii California Wildland-Urban Interface Code Information, 

http://www.fire.ca.gov/fire_prevention/fire_prevention_wildland_codes.php

viii

http://www.fire.ca.gov/fire_prevention/downloads/ICC_2009_Ch7A_2007_rev_1Jan09 

Supplement.pdf

ix Jack Cohen, http://www.firewise.org/Information/Research-and-Guidance/WUI-HomeIgnition- Research/The-Jack-Cohen-Files.aspx

x International Code Council, International Wildland-Urban Interface Code, 2012.

xi The wildland-urban interface fire problem-current approaches and research needs, Mell, 

Manzello, Maranghides, Butry and Rehm,, International Journal of Wildland Fire 2010, 

19, 238-251

xii Based on threshold window breakage values identified in NIST-GCR-98-751, Window

Breakage Induced by Exterior Flames, Frederick W. Mower, June 1998.

xiii Buschman, A.J., ‘‘Ignition of Some Woods Exposed to Low Level Thermal 

Radiation,’’ NBS Report 7306, Natl. Bur. Stand., Washington, DC, 1961.

xiv JFSP FY2011, Request for Applications, Area Of Interest 9: Fuel Treatments and the

Wildland Urban Interface, http://www.firescience.gov/JFSP_AFPs.cfm

xv Fighting Fire with Fire: estimating the efficacy of wildfire mitigation programs using 

propensity scores, Butry, Environmental and Ecological Statistics 16:291-319 (2009)

xvi Initial Reconnaissance of the 2011 Wildland-Urban Interfaces Fires in Amarillo, 

Texas, Maranghides, Mell, Ridenour, McNamara, NIST Technical Note 1708, June 2011.
26

 

xvii Manzello, S., Suzuki, H., “Summary of Full Scale Experiments to Determine 

Vulnerabilities of Building Components to Ignition by Firebrand Showers”, NIST SP 

1126, January, 2012.

xviii http://www.fs.fed.us/pnw/fera/research/wfds/