NIST Technical Note 2205

WUI Structure/Parcel/Community
Fire Hazard Mitigation Methodology
Alexander Maranghides
Eric D. Link
Steven Hawks
Jim McDougald
Stephen L. Quarles
Daniel J. Gorham
Shonali Nazare

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NIST Technical Note 2205

WUI Structure/Parcel/Community
Fire Hazard Mitigation Methodology
Alexander Maranghides
Eric D. Link
Shonali Nazare
Engineering Laboratory, NIST
Steven Hawks
Jim McDougald
California Department of Forestry and Fire Protection (CAL FIRE)
Stephen L. Quarles
Daniel J. Gorham
Insurance Institute for Business & Home Safety (IBHS)

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March 2022

U.S. Department of Commerce
Gina M. Raimondo, Secretary
National Institute of Standards and Technology
James K. Olthoff, Performing the Non-Exclusive Functions and Duties of the Under Secretary of Commerce
for Standards and Technology & Director, National Institute of Standards and Technology

Certain commercial entities, equipment, or materials may be identified in thisdocument 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 theentities, materials, or equipment are necessarily the best available for the purpose.
NIST policy is to use the International System of Units (SI) in all publications.
However, in this document, units are presented in the system prevalent in the relevant
discipline, although in some cases more than one system of units may be presented.

National Institute of Standards and Technology Technical Note 2205
Natl. Inst. Stand. Technol. Tech. Note 2205, 77 pages (March 2022)
CODEN: NTNOEF
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Abstract

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In the last twenty years, wildland-urban interface (WUI) fires have grown in severity andsize. The structures destroyed by WUI fires have devastated entire communities and havecost billions of dollars while significantly impacting the social fabric and economic wellbeing of entire regions. Structure losses are attributed to exposures from both embers(firebrands) and fire (radiation and/or convection). As structure losses continue to increase,
there is a growing need for a comprehensive hazard assessment and mitigation methodology
to harden appropriate structures and parcels effectively and efficiently against ember and fire
exposures. To address this need, the National Institute of Standards and Technology (NIST),
the California Department of Forestry and Fire Protection (CAL FIRE), and the InsuranceInstitute for Business & Home Safety (IBHS) embarked on a sixteen-month collaborative
effort, culminating in this Hazard Mitigation Methodology (HMM).
The HMM provides an implementable path forward by considering the spatial relationships
between fuels, exposures, and hardening at the structure and parcel levels. The HMM
demonstrates how complex structure hardening is, and how and why hazards associated withboth fire and ember exposures need to be mitigated. By describing the relationships between
exposure and hardening within the methodology, HMM highlights situations where structure
hardening does not provide sufficient protection in the absence of parcel hardening. The
HMM also addresses housing density, structure separation distance, and parcel layouts. The
methodology was explicitly designed to address the current building stock, i.e., to solve
retrofit challenges, and efforts were made to limit retrofit expenses. While the methodology
was developed primarily for retrofits, the presented strategy can also be applied to new
construction.
This science-based methodology uses the knowledge collected from post-fire fieldobservations spanning a dozen years and tens of thousands of hours of field data integration
and analysis. Additionally, the HMM utilizes the latest technical knowledge gained fromlaboratory and large-scale research in fire propagation and hazard mitigation in the WUI.
This report documents the methodology and addresses the critical issues of mitigation
effectiveness at the parcel and community levels. The impacts of partial mitigation at theparcel and community level were addressed for different types of WUI communities.
Key words
community hazard reduction; disaster resilience; ember; firebrand; hazard mitigation; large
outdoor fires; parcel hardening; retrofit; structure separation distance; wildfire; wildlandurban interface; WUIiii

Table of Contents
Abstract ................................................................................................................................... iii
Table of Contents ................................................................................................................... iv
List of Figures ......................................................................................................................... vi
Glossary ................................................................................................................................ viii
1. Introduction ......................................................................................................................... 1
2. WUI Definition .................................................................................................................... 3
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3. Defining WUI by Structure Separation Distance ............................................................ 7
4. WUI Hazard Mitigation Methodology ............................................................................ 16
4.1. Introduction .................................................................................................................. 16
4.2. Quantifying Ember and Fire Exposures ....................................................................... 17
4.2.1. Embers ................................................................................................................... 17
4.2.2. Fire ......................................................................................................................... 18
4.3. Structure Ignition Pathways ......................................................................................... 19
4.3.1. Impact of Parcel Boundaries on Code Application ............................................... 20
4.3.2. Impact of Linear Features on Fire Spread ............................................................. 21
4.3.3. Impact of Fuel Agglomeration on Fire Spread ...................................................... 23
5. Hazard Mitigation at the WUI......................................................................................... 27
5.1. Using the HMM ........................................................................................................... 30
5.2. Fuels Spacing ............................................................................................................... 30
5.3. Fuel Density in Different WUI Types .......................................................................... 34
5.4. Factors Influencing Structure Survivability ................................................................. 40
5.4.1. Effects of Defensive Actions on Structure Survivability ...................................... 40
5.4.2. Impacts of Structure Mitigation Compliance on Structure Resilience .................. 41
5.4.3. Impacts of Community Mitigation Compliance on Community Resilience ......... 42
5.4.4. Effects of Housing Density on Mitigation Strategies ............................................ 43
6. Frequently Asked Questions ............................................................................................ 45
7. Comparison of this Hazard Mitigation Methodology with National and International
WUI Codes ............................................................................................................................. 47
8. Summary............................................................................................................................ 48
Acknowledgments ................................................................................................................. 52
References .............................................................................................................................. 53
Appendix A.

Home Hardening Requirements ................................................................ 57

Appendix B.

Comparison of Existing Codes and Standards ......................................... 65iv

List of Tables
Table 1. WUI definitions.......................................................................................................... 6
Table 2. WUI Types classified by structure separation distance (SSD) and typical parcel size.
................................................................................................................................................... 8
Table 3. Structure and parcel hardening effectiveness. .......................................................... 29
Table 4. Parcel layout and fuel placement in different WUI Types visualized in Figures 17
through 20. .............................................................................................................................. 34
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Table 5. Defensive actions identified in NIST post-fire case studies. ................................... 40
Table 6. HMM items completely included in selected existing WUI building codes. .......... 47
Table A. Structure and attached combustible hardening against ignition from embers. ....... 58
Table B. Surrounding parcel hazard mitigation – hardening structure and attachedcombustibles against ignition from fire (radiation, convection). ............................................ 62
Table C. Primary parcel hazard mitigation – hardening structure and attached combustibles
against ignition from fire (radiation, convection). .................................................................. 63
Table D. Structure hazard mitigation – hardening structure and attached combustibles against
ignition from fire (radiation, convection). .............................................................................. 64
Table E. Comparison of selected existing WUI building codes. ........................................... 66v

List of Figures
Figure 1. The neighborhood of Coffey Park in Santa Rosa, CA is an example of WUI Type 2(High Density Interface – Interior). For SI: 1 ft = 0.305 m, 1 ac = 0.4 ha. ............................... 9
Figure 2. An example of structure density of the Mountain Shadows neighborhood inColorado Springs, CO is shown in Map Figure 13 from NIST TN 1910 [4] (units modified).
Insets show the highest density area (yellow) and associated lot size distribution. ............... 10

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Figure 3. Varying structure separation distances in the Mountain Shadows neighborhood ofColorado Springs, CO are shown in Map Figure 14 from NIST TN 1910 [4] (units modified).
The inset highlights an area of very low SSD. For SI: 1 ft = 0.305 m, 1 ac = 0.4 ha. ............ 11
Figure 4. WUI Type 6: Low Density Intermix Rancho Santa Fe, CA. .................................. 12
Figure 5. WUI Type 6: Low Density Intermix, The Trails, Rancho Bernardo, CA. Noteirrigated lawns, long paved driveways, pools and tennis courts. Some structure-to-structure
distances are representative of Moderate Density Intermix (lower part of image). Also note
limited high density non-irrigated vegetative loading. ........................................................... 13
Figure 6. WUI Type 6: Low Density Intermix: Concow, CA. Note the overall large SSD andstructures located within the wildland vegetation. Note the imagery is taken after the 2018
Camp Fire................................................................................................................................ 14
Figure 7. WUI fire hazard mitigation is a balance between two input dials—reducing
exposure and increasing structure hardening. ......................................................................... 17
Figure 8. Parcel-level combustible features that can contribute to various structure ignition
pathways. Highlighted in white are hazards that NIST and IBHS have performed extensive
study of.................................................................................................................................... 20
Figure 9. Existing codes are parcel centric. Residential structures are similarly located in
Scenario A and B; however, the parcel division in Scenario A allows placement of an
auxiliary structure too close to the neighboring primary structure when the parcels are
considered independently........................................................................................................ 21
Figure 10. Illustration of numerous fire spread pathways among neighboring parcels via
linear features and other combustibles. Inset photographs are from field observations andexperiments. ............................................................................................................................ 22
Figure 11. Parallel western red cedar fences burning with an applied wind of 13 mi/h (6 m/s)
(left two images) compared to a single burning fence (right). ................................................ 23
Figure 12. Spatial relationships between fuels on two adjacent parcels. Sheds are spaced
appropriately from the residences and other fuels, except fences, on their respective lots.
However, Shed A-1 is too close to Residence B, and both sheds act as an agglomerated fuelpackage along the property border. The sheds and fences will substantially increase theexposures from the red highlighted area, igniting the structures. ........................................... 24
Figure 13. Fuel on Lot B is adequately spaced from Residence B; however, fuel isagglomerated at the fence, which opens the pathway to Residence A. Ignition on Lot B may
still result in pathway shown, gazebo→fence→RV→Residence A, despite noncombustible
fence attachments on both residences. .................................................................................... 25

vi

Figure 14. Illustration, based on an actual WUI community, showing real world fuelagglomeration and nominal 8 ft SSD on high-density parcels. Illustrated conditions extend
across the street and throughout the community (not pictured). Fuel continuity is present
across the community of hundreds of homes. ......................................................................... 26
Figure 15. Three scenarios with a range of SSDs between the primary residence andauxiliary structure >120 ft2: 1) greater than 50 ft, 2) between 25 ft and 50 ft, and 3) under
25 ft. Structure hardening to increase ignition resistance is illustrated in blue (Scenario 2).
Note that hardening is directional and is not necessarily required around the entire structure.
Hardening for embers is required in all scenarios. ................................................................. 31
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Figure 16. Three scenarios with a range of SSDs: 1) greater than 50 ft, 2) between 25 ft and
50 ft, and 3) under 25 ft. Structure hardening to increase ignition resistance illustrated in blue
(Scenario 2). Note that hardening is directional and is not required completely around both
structures. Hardening for embers is required in all scenarios. ................................................ 33
Figure 17. Scenario A, WUI Type 2. Small parcel with small SSD provides minimal space
for auxiliary fuels. The shed is located closer to the neighboring residence than the owner’s
residence. ................................................................................................................................ 36
Figure 18. Scenario B, WUI Type 4. Moderate density interface. In some areas there islimited space for auxiliary fuels, shown by the agglomeration on the property boundaries (redhighlights). The area highlighted in blue presents reduced impact on residential structures.
The lower figures show before and after fire imagery with the actual fire spread pathway
highlighted. ............................................................................................................................. 37
Figure 19. Scenario C, WUI Type 4 or Type 6. While the structures are placed on large
parcels with extensive backyards, the primary structures are very large with considerably
smaller SSD. Selective hardening may be required in this scenario despite the large parcels.
................................................................................................................................................. 38
Figure 20. Scenario D, WUI Type 6. This community is characterized by large parcels withmore extensive space available for auxiliary fuels. Avoidance of agglomeration of fuels
within the outlined areas would influence the total number of auxiliary fuels allowed in thesespaces. ..................................................................................................................................... 39vii

Glossary
Abbreviations
AHJ ......................... authority having jurisdiction
CA Chapter 7A ....... a chapter in the California Building Code establishing minimum
standards for construction in state-designated areas, titled Materials andConstruction for Exterior Wildfire Exposure
CAL FIRE ............... California Department of Forestry and Fire Protection
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FRAP....................... Fire and Resource Assessment Program (CAL FIRE)
FSD ......................... fuel separation distance – the spatial distance between a fuel (source)
and a target
FSR ......................... fuel separation range – the spatial distance between a fuel and target inwhich hardening of the target will significantly reduce target ignition
potential
ICC .......................... International Code Council
IBHS ....................... Insurance Institute for Business & Home Safety
NFPA ...................... National Fire Protection Association
NIST........................ National Institute of Standards and Technology
SSD ......................... structure separation distance – the distance between two structures(primary or auxiliary)Terms
cladding ................... the exterior materials or assembly of materials of a structureember....................... a hot or burning (flaming or smoldering) particle generated/broken off
from the source (due to impact, wind, or the reduced structural integrityof the solid fuel due to burning)
exposure .................. the heat energy from flames, radiation, and/or embers that are generated
from a burning object, feature, or structure
firebrand .................. see ember; particularly an airborne ember
parcel ....................... a specific plot of land with delineated boundaries, typically containing ahouse or building
source ...................... an object, feature, or structure that, when burning, generates an exposureof heat, flames, and embers
target ....................... the recipient of an exposure – may be a structure (hardened or nonhardened) or other parcel features
wildlands ................. any undeveloped land with vegetative fuels that can generate fire andember exposures (may also be located on partially developed parcels)viii

1. Introduction

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Fires in the wildland-urban interface (WUI) pose a growing threat to communities across thenation and around the world. Fires have grown in intensity and size in the last 20 years [1-3].
Additionally, a new threat has emerged from fire storms, events where tens of fires can occur
simultaneously, creating challenging conditions for first responders that result in reduced
response coverages. Recent events include the October 2017 North Bay Fires and the August2020 lightning fires in California, as well as the September 2020 fires in Oregon. As losses
mount, a paradigm shift of the hazard mitigation approaches that have been widely used todate is needed.
Fire and embers vary over small geospatial scales in WUI fire events. Post-WUI fire casestudies have identified that these exposures can vary on a sub-parcel scale, on the order of
50 ft to 100 ft, highly dependent on localized fuel, wind, and topographic conditions [4-6].
Embers can travel several miles, and initial low to moderate ember exposures can impact
entire communities [7, 8]. These initial wildland exposures can significantly contribute to the
spread of fire into the community directly, or through the development of significant fire
exposures from the ignition of fuels within the community. Subsequent ignitions withincommunities can generate localized very high ember exposures. Fire (radiation andconvection) exposures can also exhibit significant variations occurring over several feet. This
is not only a function of the spatial distribution of fuels but also is associated with the rapid
decay in radiation intensity with increasing distance and the limited extent of flaming
combustion (i.e., flame length) compared to the widespread extent of ember exposures.
National Institute of Standards and Technology (NIST) WUI case studies have repeatedly
identified the need to couple exposures to structure hardening [4-6]. A Hazard Mitigation
Methodology (HMM) was developed to address this specific need and was a sixteen-monthcollaborative effort by NIST, the Insurance Institute for Business & Home Safety (IBHS),
and the California Department of Forestry and Fire Protection (CAL FIRE).
All WUI communities are not the same. This HMM goes beyond the traditional WUI
definitions and introduces the concept of structure separation distance (SSD) for thedevelopment and implementation of hazard reduction. The proposed HMM accounts for both
ember and flaming exposures in the WUI and addresses the spatial relationships between
fuels that drive the ignition of structures and fire propagation. This work represents thetechnical evolution of the NIST WUI Hazard Scale [9] published in 2013. Scientificdevelopments and field observations have resulted in the increase in spatial resolution of the
proposed methodology as compared to the WUI Hazard Scale.
The goal of the HMM is to reduce the overall implementation burden by assessing local
exposures and applying the latest laboratory research findings and knowledge from post-firefield observations to effectively harden structures in a cost-effective way. The HMM is
primarily aimed at retrofitting the existing building stock. The proposed methodology also
highlights the needs and requirements for implementation at the community level.

1HMM was designed to be applied in communities located in hazardous fire prone WUI areas.
The benefits of community-wide mitigation can be viewed as proportional to the overall
community fire hazard, including fire exposures from surrounding and occluded wildlands.
While the implementation of HMM in a high-hazard community will likely substantiallyreduce losses, significant benefits can also be achieved in lower hazard scenarios. Recent
WUI fires have demonstrated that significant losses can occur in low-hazard WUI or urban
settings. One example is the impact of the 2017 Tubbs Fire on the Coffey Park community in
Santa Rosa, CA.
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NIST and IBHS are non-regulatory entities, while CAL FIRE has regulatory authority in thestate of California. This HMM was developed to provide authorities having jurisdiction(AHJs) and homeowners with the latest comprehensive understanding of fire behavior andstructure response in the WUI. When terms such as “required” and “should” are used, they
are only in the context of highlighting the necessary components identified for the HMM towork. AHJs will ultimately choose where to implement any or all of the components
presented in the HMM.

2

2. WUI Definition

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The common conceptual definition of wildland-urban interface (WUI) is the geographicalarea where human development, including structures and other infrastructure, meets orintermixes with undeveloped wildlands [10, 11]. Communities in such areas may be grouped
into one of three categories—interface, intermix, or occluded 1—depending on the density ofdevelopment, coverage of wildland fuels, and population density [12]. This definition has
been adopted by several governing federal- and state-level authorities and codes/standards
organizations, including the US Fire Administration, the International Code Council (ICC)
International Wildland-Urban Interface Code (IWUIC), and the National Fire Protection
Association (NFPA).
Beyond the conceptual definition, establishing a uniform operational definition has provendifficult [13-15]. WUI areas are often delineated using three primary components:
1. human presence, often quantified by housing density or population density
2. wildland vegetation, often quantified by percentage of land coverage
3. proximity to wildlands, often quantified as a buffer distance between wildlands anddeveloped land or structures.
The introduction of an official Federal WUI definition is presented in the Federal Register in2001 [11] in support of Federal wildland fuel management efforts in response to the fireseason of 2000. This definition quantified human presence through either housing density or
population density. However, these definitions only qualitatively describe the quantity ofwildland vegetation and proximity to wildlands.
The Healthy Forests Restoration Act (HFRA) of 2003 [16] slightly expanded the WUIdefinition to include “at-risk communities”—groups of homes or structures with basic
infrastructure and services where there is a significant threat, to human life or property, fromlarge-scale wildland fires. 2 The WUI extends at least 0.5 mi (0.8 km) from the boundary ofsuch areas and includes any additional area within 1.5 mi (2.4 km) consisting of topography
or fuel accumulation conditions with increased potential for elevated fire behavior, or land
suitable for effective firebreaks. Furthermore, the HFRA also deemed areas adjacent toevacuation routes from such communities as WUI.
Several studies have implemented the federal definition into practice to identify, quantify,
and map WUI areas [14, 17-21]. The generally accepted defining criteria are presented inTable 1. However, even while adhering to the spirit of the Federal Register definition,slightly differing assumptions, interpretations of the thresholds, differing data sources, andintermediate data processing steps yield different outcomes [20-23]. Detailing specifics of the
Occluded community refers to a situation where developed areas surround an island of wildland fuels generally < 1000 ac (400 ha) in size[11]. These communities are typically treated and defined similar to interface communities.
2
Note that both the Federal Register [11] and the HFRA [16] specify communities within or adjacent to federal lands. However, it isimportant to note that this reflects jurisdictional limitations and should not be understood to limit the broader sense ofqualification/definition as WUI.
1

3

various implementations are beyond the scope of this document. However, the focus of the
mapping studies, and the federal definition in general, is to provide a high-level overview anda sense of scale to help progress to the next step—directing the additional focus needed tohone in on and support direct mitigation efforts in areas where it is needed.
It is important to note that not all communities in the WUI have the same fire risk level, andrisk or threat is not quantified or defined in the Federal Register or the HFRA. However, theFederal Register does suggest the consideration of the following three broad categories whenevaluating risk factors:
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1. Fire behavior potential (e.g., fuel types and distribution, slopes, prevailing winds)
2. Values at risk (e.g., structures, watershed, cultural/historical values), and3. Infrastructure (e.g., road access/egress, water supply, firefighting capacity).
While the above three risk factors provide useful information for assessing risk, the specificimplementation criteria impact the final perimeters of WUI areas determined by local
jurisdictions. Historically, WUI fire losses have occurred within these defined boundaries; it
has been shown that a significant fraction of buildings destroyed by wildfires are located in
the WUI [24]. However, it is also essential to keep in mind that hazardous conditions may
extend beyond the “defined” WUI. For example, the Coffey Park neighborhood of Santa
Rosa, CA, which was destroyed in the 2017 Tubbs Fire, sits outside the edge of the interface
definition implemented by Radeloff et al. [17]. Once into the high-density structures, the fireburned 0.9 mi beyond the interface limit into developed lands. Coffey Park losses illustrate
the potential of a WUI fire to morph into an urban conflagration. Similar losses fromstructure-to-structure fire spread have been observed during many other large-loss WUI fire
events whether the locations are designated WUI or not.
HMM was designed to be applied in communities located in hazardous fire prone WUI areas.
State and local fire hazard severity mapping systems can be used to identify communities thatcan benefit from the application of this HMM. Some jurisdictions, such as the State ofCalifornia, characterize hazard in their definition of WUI. The CAL FIRE Fire and Resource
Assessment Program (FRAP) classifies land into Fire Hazard Severity Zones (FHSZ) [25,26]. There are 3 levels of FHSZ—Moderate, High, and Very High—which are determined byanalyzing several factors, including vegetation, fuel loading, topography, weather(temperature, humidity, and wind), fire history, and ember potential. The state uses thedefined FHSZs to require building code provisions (i.e., California Building Code (CBC)
Chapter 7A [27], California Fire Code Chapter 49 [28]) and hazard mitigation actions [29].
Efforts to define and map WUI hazard areas are necessary to assist local agencies inunderstanding their jurisdiction’s wildfire hazards and to prepare their communities byfocusing broad mitigation strategies and land use planning. However, fire behaves at many
scales, and fire hazard at the structure or property level is often difficult to quantify usingcoarse scale data such as LANDFIRE [30].

4Structure survivability is the result of the relationship between the structure construction andlocal intensity and duration of fire and ember exposures. Communities should assess bothcomponents in the process of structure and community hardening. As this HMM details,
mitigation and hazard assessment distill down to a structure-level issue, and thus, considering
structure separation distance as a defining component of WUI fire hazard is a logicalprogression for determining necessary hazard mitigation.

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Table 1. WUI definitions.

Interface

Definition Component
Conceptual

6

Housing density

Federal Register
There is a clear line of demarcation between
residential, business, and public structures andwildland fuels; wildland fuels do not generally
continue into the developed area
≥ 3 structure/ac (741 structure/km2)

Population density
Vegetation cover
Buffer from wildland≥ 250 people/mi2 (96 people/km2)
Structures directly abut wildland fuels
Up to 1.5 mi (2.4 km) from community border

Infrastructure

Fire protection of the structures from both aninterior fire and an advancing wildland fireprovided by the local fire department.

Conceptual

There is no clear line of demarcation; wildland
fuels are continuous outside of and within thedeveloped area
≥ 1 structure/40 ac (6.18 structure/km2)

Intermix

Housing density

Population density
Vegetation cover(28 to 250) people/mi2 [(11 to 96) people/km2]
Structures are scattered throughout a wildlandarea

Buffer from wildland
Infrastructure
Note: HU = housing units

Fire protection districts provide life andproperty protection and may also have wildland
fire protection responsibilities

Common Implementation
Developed land not dominated(i.e., < 50 %) by vegetation

CAL FIRE
High-density development adjacent toundeveloped wildland vegetation≥ 1 HU/40 ac (6.18 HU/km2)>1 HU/20 ac (12.4 HU/km2) inModerate, High, or Very High FHSZ

< 50 %< 1.5 mi (2.4 km) from landwith > 75 % vegetative coverNot dominated by wildland vegetation
Wildfire susceptible vegetation up to
1.5 mi (2.4 km) from interface

Developed land dominated (i.e.,> 50 %) by vegetation

Lower-density housing mingled with
undeveloped wildland vegetation≥ 1 HU/40 ac (6.18 HU/km2)

1 HU/20 ac to 1 HU/5 ac(12.4 HU/km2 to 50 HU/km2) OR>1 HU/5 ac (50 HU/km2) whendominated by wildland vegetation,
in Moderate, High, or Very High FHSZ

> 50 %

Dominated by wildland vegetation
Wildfire susceptible vegetation up to
1.5 mi from intermix

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The WUI environment is complex and multifaceted. Several definitions of the WUI havebeen presented and used to meet specific documentation goals or to broadly identify hazard
areas. For more focused and effective mitigation efforts, namely the enumeration of required
structure hardening, many of these definitions are not sufficient on their own as they do not
focus on the parcel-level details that characterize parcel-to-parcel and structure-to-structure
fire spread. In this document, the definition of the WUI is subdivided into seven distinct
types based on structure separation distance (SSD) and “traditional” WUI categories (i.e.,interface or intermix). The Types are listed in Table 2. These definitions provide perspective
and can help characterize overall community housing density and lot sizes. Table 2 can beapplied at the community or the parcel level; however, Table 2 is not sufficient to implement
the HMM.
The range of structure densities defined as WUI are extremely broad. A typical high-densitycommunity with 8 HU/ac (housing units per acre) is 320 times denser than the lowest density
typically considered WUI, 0.02 HU/ac (1 HU on 40 acres). The agglomeration of structures 3
seen in high-density communities (frequently resulting in small SSD) has significant impact
on fire behavior [4, 31]. The use of SSD as a key metric in characterizing WUI areas
specifically with respect to fire is twofold. First, structures represent a significant density offuels that impact fire spread and, in many cases, directly contribute to the ignition of
additional structures, propagating fire throughout the community. Secondly, existing
structures pose a unique challenge in hazard management—they are immobile. While they
can be hardened (see Section 5 for mitigation methodology), they cannot be readily removed
or displaced like many other WUI fuels.
The traditional WUI categories (interface/intermix) describe the general community
classification and hazard profile. However, effective structure-level mitigation must be
driven by parcel-level assessments and localized evaluation of potential fire exposures. Thiscan be aided through identification of a parcel-level WUI Type. The Types listed in Table 2
are primarily defined by SSD, with secondary consideration of parcel size. Even if structures
are located on a large lot but are clustered together on the edge of the property with small
SSD, the parcel WUI Type would correspond to a smaller SSD.

All structures, not just housing units, including primary (residential and commercial) and auxiliary (e.g., auxiliary dwelling units (ADUs),
garages, sheds) structures impact fire spread.

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Table 2. WUI Types classified by structure separation distance (SSD) and typical parcel size.
TypicalTypicalHousing
Type
Parcel Size
Density#WUI Type Name
SSD (ft)(ac)(struct/ac)
High Density Interface –1
6a to 30
< 0.5
2 to 8 +Perimeter
High Density Interface –2
6a to 30
< 0.5
2 to 8 +Interiorb
Medium Density Interface –3
30 to 100
0.5 to 1+<2
Perimeter
Medium Density Interface –430 to 100
0.5 to 1+<2
Interiorb
5

Medium Density Intermix

30 to 100

0.5 to 1+<2

6

Low Density Interface

100+

1+

<1

7

Low Density Intermix

100+

1+

<1

For SI: 1 ft = 0.305 m, 1 ac = 0.4 haarepresentative of parcels with a 3 ft setback (common for new construction of sprinklered residences)binterior of community defined as > 0.25 mi (400 m) from wildlands

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WUI Type 1 and Type 2

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WUI Type 1 and Type 2 represent high-density interface communities. These communities
have a high fraction of their fuel load in the form of residential structures. SSDs in thesecommunities are often as little as 6 ft (minimum 3 ft building setback from property line) andup to 30 ft. These types of WUI can be found around the country including in some very high
WUI fire hazard areas. These WUI communities typically have parcel sizes significantly lessthan 0.5 ac (0.2 ha). Communities like Coffey Park in Santa Rosa, CA (shown in Figure 1)
or the Mountain Shadows area of Colorado Springs, CO are examples of these high-densitycommunities. Typical lot sizes in these two communities are less than 0.15 ac, and typical
SSD is 6 ft (see maps and data in Figure 2 and Figure 3). The Mountain Shadowsneighborhood can be characterized as Type 1 High Density Interface – Perimeter while
Coffey Park can be characterized as a Type 2, High Density Interface – Interior community.

Figure 1. The neighborhood of Coffey Park in Santa Rosa, CA is an example of WUI Type 2(High Density Interface – Interior). For SI: 1 ft = 0.305 m, 1 ac = 0.4 ha.
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Figure 2. An example of structure density of the Mountain Shadows neighborhood inColorado Springs, CO is shown in Map Figure 13 from NIST TN 1910 [4] (units modified).
Insets show the highest density area (yellow) and associated lot size distribution.
For SI: 1 ac = 0.4 ha; 1 structure/ac = 2.47 structure/ha.
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Figure 3. Varying structure separation distances in the Mountain Shadows neighborhood ofColorado Springs, CO are shown in Map Figure 14 from NIST TN 1910 [4] (units modified).
The inset highlights an area of very low SSD. For SI: 1 ft = 0.305 m, 1 ac = 0.4 ha.

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WUI Type 3, Type 4, and Type 5
WUI Types 3, 4, and 5 are all characterized by medium-density construction with SSD
ranging from 30 ft to 100 ft. Parcels are typically larger in the medium (or moderate) density
WUI, with many being over 1 ac. These WUI Types are further subdivided; interface (Types
3 and 4) and intermix (Type 5). Like WUI Type 1 and Type 2, Medium Density Interfaces
can be located on the perimeter abutting wildlands (Type 3) or in the interior of WUIinterface communities (Type 4). WUI Type 5 represents medium density intermix
communities with similar SSD found in Types 3 and 4 of 30 ft to 100 ft.
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WUI Type 6 and Type 7
WUI Types 6 and 7 are characterized by low structure density at the interface and intermix,
respectively. These are low structure density locations where residential structures are
typically over 100 ft apart. 4 These communities are comprised of larger lots, typically over 2
ac. Note that even within a WUI Type, structure and vegetation characteristics may be quite
different. Figure 4, Figure 5, and Figure 6 show different examples of low-density intermix.

Figure 4. WUI Type 6: Low Density Intermix Rancho Santa Fe, CA.

Auxiliary structures may be present and can have significant impact on fire spread. These local SSD (between all structures and otherfuels) are explicitly addressed through the HMM.

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Figure 5. WUI Type 6: Low Density Intermix, The Trails, Rancho Bernardo, CA. Noteirrigated lawns, long paved driveways, pools and tennis courts. Some structure-to-structure
distances are representative of Moderate Density Intermix (lower part of image). Also note
limited high density non-irrigated vegetative loading.

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Figure 6. WUI Type 6: Low Density Intermix: Concow, CA. Note the overall large SSD andstructures located within the wildland vegetation. Note the imagery is taken after the 2018
Camp Fire.

14

The seven WUI types described above can be used to help characterize WUI communities.
These definitions are intended to provide general structure spacing information for acommunity or part of a community. The hazard mitigation and retrofit guidance provided inthis document is parcel-centric and specifically accounts for local fuel spacing conditions.
SSD and vegetation type, quantity, and location also relate to potential fire exposure.
Exposures in the WUI are a combination of two components—fire and embers. These two
exposure types are further discussed in Section 4.2. The impacts of fuel densities on
exposures are discussed in Section 4.3, and the mitigation methodology to address thevarying fire and ember exposures is described in Section 5.
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4. WUI Hazard Mitigation Methodology
4.1. Introduction
The proposed methodology was developed to meet two primary objectives:
1. Reduce structural losses in WUI fires by hardening structures and parcels, and
2. Prioritize mitigation efforts to reduce overall mitigation costs.

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WUI fire hazard mitigation is often thought of as either an “every little bit helps” or a “onesize-fits-all” voluntary endeavor. The first approach is a good start and can provide realized
benefits in low-exposure conditions where defensive actions 5 are also very effective [4, 32].
However, such an approach often leaves significant gaps and a false sense of security (seeSections 5.4.2 and 5.4.3 on partial compliance). This approach has significant shortcomings
in conditions where defensive actions are not possible due to high exposures or limited
resources. There is a fundamental difference in communication for public awareness andhardening for standalone protection. Public education is a critical component of a hazard
communication strategy but does not go far enough to solve the larger-scale problem. The
second approach, “one-size-fits-all” mitigation, is very expensive if all vulnerabilities are to
be addressed. The goal of the methodology developed in this document is to bridge the gap
through selective hardening actions in response to potential exposures at the parcel level,
while considering exposures to/from neighboring parcels. This quasi-performance-based
design approach can reduce the overall burden of mitigation in both effort and expense.
The mitigation strategy outlined in this document was primarily developed to address theneeds for retrofit hardening of existing communities; however, the information can also be
used for the design and construction of new communities. The proposed methodology isdesigned to augment defensible space [33]. The strategy relies on defensible space principles
specifically related to the removal and maintenance of vegetative fuel and other combustible
materials. The strategy was developed over a period of 16 months through a collaborative
effort between NIST, CAL FIRE, and IBHS and leverages the most current science and data
together with more than ten thousand hours of pre- and post-WUI fire field data collection
conducted by the collaborating agencies. Discussions with the building industry werenecessary to clarify and improve different implementations for retrofit solutions. The HMM
will continue to be adapted and revised as additional science and technical findings become
available.
Structures in the WUI can experience two different types of hazardous exposures—embers
and fire (i.e., flames via radiation and/or convection) [34]. To survive without contributionsof defensive actions (see Section 5.4.1 on the impact of defensive actions in the WUI)
structures must be able to withstand both ember and fire exposures. This adaptive
methodology is designed to harden the structure and the parcel against both hazards.

5

Defensive actions include fuel displacement, fire suppression, fire containment, and exposure protection by first responders.

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There is a direct relationship between exposure and the structure hardening required forsurvival, and effective mitigation is a tuned balance of the two components. This relationshipis illustrated in Figure 7, where the two knobs or dials represent exposure level and structure
hardening. There are as many relationships that will result in structure ignition as there are
relationships or settings that will result in no ignition. At the extreme, if the exposures (bothfire and embers) are eliminated then no hardening is required. Conversely, if a structure ishardened to a windowless concrete bunker, it could survive an extreme exposure. It is
acknowledged that the bunker approach is not realistically implementable or desirable,highlighting the need for a balanced strategy between exposure reduction and structure
hardening.

Figure 7. WUI fire hazard mitigation is a balance between two input dials—reducing
exposure and increasing structure hardening.

4.2. Quantifying Ember and Fire Exposures
4.2.1. Embers
In a WUI fire, structures will likely experience ember exposures. Embers can cause direct
ignitions of structures and parcels. Ignitions of parcel-level features may result in fire
exposures to the structure and generation of additional embers. Structure ignition will dependon the vulnerabilities present on the structure and the intensity/duration 6 of local ember
exposures.
Ember exposures can vary over several orders of magnitude in number flux (count/m2/s) andin energy content, as embers can range from sub-millimeter scale to many centimeters in size
[4, 35, 36]. In WUI fire events the initial embers arriving to a structure often originate frombeyond the parcel that is being protected/hardened. Depending on the origin of the embers
and the ownership of the ember generating fuels, options may be available to limit ember
generation. While ember exposures can be reduced locally by defensive actions, the presenceof first responders cannot be relied upon particularly during large incidents. Therefore,
structures must be hardened to withstand various ember exposures ranging from very low tovery high. Because the exposure intensity and duration cannot be controlled, or in manycases even predicted, ember hardening must address significant ember exposures andIntensity of ember exposures will be a function of several factors including size distribution, number density, and energy content, which allvary in both space and time.

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associated local ember accumulations. While scientific work is in progress to measure andquantify ember fluxes (e.g., [37-39]), visual observations and ignition occurrences are thebest documentation at this time. The technical goal is to prevent ignition, not merely delay it.
This is accomplished through the use of noncombustible materials in vulnerable areas of thestructure. A detailed list of ember hardening requirements is discussed in Appendix A.

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On the parcel, embers can cause the ignition of different parcel-level features. The hardeningof these features can fall outside the current regulatory practices of fire departments and local
AHJs. One example is recreational vehicles (RVs). The ignition resistance of RVs to embers
will impact the potential contributions of RVs to fire spread within and across parcels in the
WUI. RVs and other parcel-level features will generate both ember and fire exposures.
4.2.2. Fire
Fire exposures can originate from within the respective parcel or from adjacent parcels or
wildlands. Like ember exposures, fire exposures can vary in intensity and duration. However,unlike embers that can travel several miles, fire exposures impact potential fuel in the rangeof tens of feet, as radiation and convection decay rapidly with distance.
There is a significant connection to be made between fire exposure and hardening. A
structure with limited hardening can tolerate a low fire exposure relatively easily. Examples
of low fire exposures are a mulch fire or a shrub fire. While these exposures can generateheat fluxes of 15 kW/m2 locally, these exposures are very limited in space and have shortfinite durations. These types of low exposures are not a challenge to hardened structure
cladding (e.g., stucco, cementitious board) and do not cause cladding failure as current test
methods (e.g., ASTM E2957) are designed to accommodate them. While non-threatening tohardened structure cladding, these exposures can still pose a significant ignition hazard toother non-hardened exposed combustibles, which may further increase the total exposure to
the structure. Depending on materials and assembly construction, a low fire exposure canresult in local ignition and subsequent structure loss.
Hardening for these types of exposures requires localized changes in building cladding andreinforcement of the assembly. As an example, raising the combustible cladding from theground 7 uncouples potential exposures from a mulch bed or an accumulation of wind-blown
flammable materials (e.g., pine needles and leaf litter). This strategy is easier and more costeffective to implement than replacing the entire siding for that very targeted low intensity fire
exposure. It should be noted that while low intensity fire exposures can be treated with local
hardening solutions of exposure uncoupling (e.g., fuel displacement) or structure hardening,they must be addressed if structure survivability is to be achieved.
Fire exposures are a continuum in both intensity and duration. The above example illustrates
the low end of that continuum. An example at the other end of the fire exposure spectrum
Table A, item 10, in Appendix A requires a noncombustible cladding for the first 2 ft from ground level. Field observations frequently
identify wind-blown debris piles exceeding 1 ft in height.

7

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may be encountered in a high-density construction environment where structures are
separated by 6 ft (i.e., 3 ft setback from property line). A fully involved residential structure
will expose the neighboring residence to very high heat fluxes for a prolonged duration.
High-exposure fires can also be generated by auxiliary structures. Such high heat fluxes will
challenge almost all typical residential construction. A structure that can withstand thesetypes of exposures can be thought of as built like an inside-out furnace. While this istechnically achievable with current construction materials, this level of hardening is very
difficult to retrofit existing construction.
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It should be noted that wildland vegetation at the edge of an interface community can also
generate very high heat fluxes with very long flame lengths (much longer than what is
generated from a residential structure). These wildland fire heat fluxes can be very large andeasily result in structure ignition even if they are not as long lived as fluxes from a fully
involved residence. These impacts can be further accentuated in the presence of complex
terrain (i.e., slope, aspect, canyons).
In between these two fire exposure extremes are scenarios where hardening the cladding of
the primary residence will provide increased resilience and significantly reduce or eliminate
the structure’s ignition potential. This is further described in Section 5.2.
The primary technical approach to dealing with the fire exposure problem is to uncouple theexposure by implementing fuel displacement (e.g., moving firewood away from othercombustibles) or fuel removal (e.g., remove a shed from a property) if space is not available
for fuel displacement.
4.3. Structure Ignition Pathways
There are multiple structure ignition pathways from both ember and fire exposures in the
WUI [4, 5, 7, 34, 40]. Figure 8 illustrates the parcel-level combustible features that cancontribute to fire spread within a parcel as well as across parcels. The list is not all-inclusive
but is used here to illustrate the potentially large number of combustibles. Fire can moveacross one or multiple combustible features (becoming new sources of fire and embers) andacross multiple parcels. The NIST Camp Fire Case Study [7] showed 16 different pathways
that resulted in structure ignitions during that event.
The ignitability and energy release (in terms of both embers and fire) of these combustiblefeatures is a function of materials, assembly and design particulars, and size. Of the featuresin Figure 8, only firewood (wood piles), vegetation, and mulch have inherent combustible
characteristics. To some extent, all of the other features can either be made of, or utilize
cladding of, noncombustible materials. This is readily achievable for simply assembled
features like fences but is significantly harder or impossible to achieve for complex fuels like
RVs and other vehicles (e.g., cars and boats).

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Figure 8. Parcel-level combustible features that can contribute to various structure ignition
pathways. Highlighted in white are hazards that NIST and IBHS have performed extensive
study of.
There are three technically important considerations when assessing fire spread within andacross parcels. These are that:
1. parcel boundaries typically limit the continuity of protection between parcels,
2. linear features 8 can carry fire very efficiently within and between parcels, and3. fuel agglomeration has significant impact on energy release and fire spread.
4.3.1. Impact of Parcel Boundaries on Code Application
Historically, codes have been parcel-centric and address exposures in the context ofprotecting a residence from the exposures on the same parcel. An example of how code
compliance is typically interpreted/applied is illustrated in Figure 9, which shows a diagramof the placement of an auxiliary structure (e.g., a shed). Codes provide a minimum separation
distance requirement for auxiliary structures. In some cases, the code requirement places theauxiliary structure at a minimum distance from the residential structure (30 ft), however, it
provides very little for the protection of structures in adjacent parcels as the auxiliary
structure setback from the property line does not reflect the potential hazard to adjacentparcels. A 5 ft setback is often allowed if the auxiliary structure is not sprinklered, placing
the auxiliary structure as close as 10 ft from a non-sprinklered residence on the other side of
the property line, seen in Figure 9a. Both structures are protected by the necessary SSD
when the two residences are located on the same lot, as in Scenario B. When property lines
are considered as a limit of code application, the hazard imposed from the auxiliary structure
to the neighboring residence in Scenario A is unmitigated.
8

combustible features such as fences, retaining walls, and landscaping railroad ties

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Figure 9. Existing codes are parcel centric. Residential structures are similarly located in
Scenario A and B; however, the parcel division in Scenario A allows placement of an
auxiliary structure too close to the neighboring primary structure when the parcels are
considered independently.
4.3.2. Impact of Linear Features on Fire Spread
The impact of linear features was first documented in the Witch/Guejito Case Study [8, 32]
and quantified during the Amarillo Case Study [5, 6]. This was further observed during the
Waldo Fire Case Study [4]. Based on these field observations, NIST has conducted over 100
experiments to assess fire spread among fences and the impact of fences on fire propagation[41].
Figure 10 illustrates how fire can burn along linear features to spread from a single ignition
point to multiple lots and impact multiple residences. For example, following ignition andfire spread pathway B shows that a single ignition on a fence line can carry fire to 6 lots anddestroy (directly or indirectly) 3 residences depicted within the extent of the figure. It is also
important to note that even with a partial improvement, such as removing the fence’s directconnection to Residence 3, the result is still 2 destroyed residences within the figure:
Ignition B → shed → Structure 3 → Structure 4.

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Figure 10. Illustration of numerous fire spread pathways among neighboring parcels via linear features and other combustibles. Inset
photographs are from field observations and experiments.

4.3.3. Impact of Fuel Agglomeration on Fire Spread
NIST experiments [41] have confirmed field fire observations about the role of fuelagglomeration on fire and ember exposures generated from combustibles in close proximity
to one another. Figure 11 illustrates the intense burning and significant energy release fromtwo fences placed 8 in (20 cm) apart. The observed increase in intensity compared to theburning of a single fence remained even when fences were spaced by 36 in (92 cm).

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Figure 11. Parallel western red cedar fences burning with an applied wind of 13 mi/h (6 m/s)
(left two images) compared to a single burning fence (right).
The impact of this increase in fire intensity on structure ignitions in the WUI is explicitly
addressed in the methodology outlined here by requiring spacing between combustibles. This
strategy decreases exposures to the asset to be protected (i.e., residence or other WUIfeature), increases life safety of the public during evacuation, and creates a more defensible
space for first responders to conduct rescue and firefighting operations.
Fuel agglomeration can occur unintentionally, as in the case of two neighbors putting up
fences, significantly increasing the fire and exposure potential (see Ignition C in Figure 10).
However, another path to fuel agglomeration has been observed to be an indirect result of
mitigation guidance to WUI residents—when fuel relocation is encouraged rather than fuelremoval. This becomes a significant fire hazard in moderate- and high-density construction
on medium to small parcels ranging from 1 ac down to 0.2 ac or less. One observed example
is firewood piles moved as far away from the residence as possible to the edge of these small
properties, typically near combustible fences, creating high energy pathways for fire to movethrough these high-density communities. Fuel relocation is a valid hazard reduction
approach; however, it should not be used as the default approach on small parcels. Thisapproach is better suited to larger lots that can adequately separate all combustibles.

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Figure 12. Spatial relationships between fuels on two adjacent parcels. Sheds are spaced
appropriately from the residences and other fuels, except fences, on their respective lots.
However, Shed A-1 is too close to Residence B, and both sheds act as an agglomerated fuelpackage along the property border. The sheds and fences will substantially increase theexposures from the red highlighted area, igniting the structures.
Figure 12 illustrates how the issue of fuel agglomeration and increased exposures isparticularly important in high- and moderate-density construction. The two red dashedoutlines represent areas of high fire exposure potential from the sheds and illustrate thechallenge from the fire exposures of high fuel loading on small parcels. The area represented
by the overlapping dashed outlines represents an area of potential high energy release, andthe red/white arrow illustrates the direct impact from this high energy release to the primaryresidence. Backyards like those illustrated in Figure 12 represent a significant defensible
space challenge to first responders. The highlighted example of the interactions between thetwo sheds 9 and the combustible fences is one of several possible similar scenarios in Lots A
and B in Figure 12.
Fuel agglomeration can present fire spread challenges even if the fuel spacing on individual
parcels is adequate to prevent fire spread within the parcel. Figure 13 illustrates a scenario
where the gazebo on Lot B (right) is located sufficiently far from the primary residential
structure. However, agglomeration on the edge of the lot increases the fire spread hazard.
Combined with the linear fence feature, a fire spread pathway to the neighboring parcel
exists, even in this case where both structures have noncombustible fence attachments to theNote: smaller sheds (e.g., under 120 ft2 in CA) are typically not as spatially regulated in terms of their placement as larger sheds. Current
codes often allow these smaller sheds to be as close as 5 ft from a residence and/or property line.

924

primary structure. Additional fuel agglomeration on Lot A presents a hazard to the primaryresidence, indicated by the red and white arrows. The inclusion of an RV is to illustrate how
combustible components that do not require a building permit can impact fire spread. RVowners may consider spatial separation distances similar to those for auxiliary structures tomitigate exposures to other combustibles.

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Figure 13. Fuel on Lot B is adequately spaced from Residence B; however, fuel isagglomerated at the fence, which opens the pathway to Residence A. Ignition on Lot B may
still result in pathway shown, gazebo→fence→RV→Residence A, despite noncombustible
fence attachments on both residences.
Figure 14 depicts the agglomeration of residential structures and other fuels in a high-densitycommunity. This figure illustrates an actual WUI community and is drawn to scale. No fuelbreaks are present through a community of hundreds of homes (extending beyond theillustrated parcels). Residential structures, with nominal SSD of 8 ft, and the parcel-level
fuels including combustible fences, decks, sheds, and vehicles, can allow a single ignition todestroy significant portions of the community.
Spacing requirements to prevent direct high fire exposure to residences, together with theneed to reduce overall high fire exposure conditions to enhance life safety, require a detailed
spatial parcel-level fuel loading assessment. The methodology described in this reportprovides the tools to address this issue. The fuel/spatial relationships mentioned here aredetailed in Table B and Table C in Appendix A. The impacts of non-compliance withspacing requirements on structure ignitions and community resilience will be addressed in
Section 5.4.3.

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Figure 14. Illustration, based on an actual WUI community, showing real world fuelagglomeration and nominal 8 ft SSD on high-density parcels. Illustrated conditions extend
across the street and throughout the community (not pictured). Fuel continuity is present
across the community of hundreds of homes.

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5. Hazard Mitigation at the WUIThe primary goal of the proposed structure and parcel hardening methodology is to minimize
the chance of structure ignitions. The technical information presented here can be used for
new construction; however, the methodology is focused on the retrofit of existing structures.
In order to reduce overall retrofit costs, removal or displacement of fuels should be addressed
first to reduce or eliminate the need for any structure hardening for fire. Factors influencing
the effectiveness of the mitigation strategy are discussed in the following subsections.
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A significant increase in structure ignition prevention is achieved by hardening structures
against the complete range of expected exposures. This necessitates that all structures are
hardened for high ember exposures. Ember showers are uncontrollable and often
unpredictable because of airflow circulation patterns, unknown variability of primary winddirection, and ember characteristics. Hardening to prevent structure ignition from embers isachieved by addressing the multitude of known structure-level vulnerabilities. A total of 40
separate vulnerabilities have been identified along with associated mitigation actions. Thesevulnerabilities include the roof, gutters, windows, doors, vents, and attached combustibles.
The full list of structure hardening requirements to prevent ignition from embers can befound in Table A in Appendix A.
A spatial analysis is necessary to identify potential exposures to the structure of interest. Thisconsists of characterizing the extent of the potential exposure of the source and assessing its
location with respect to the residential structure and other combustibles. If the placement has
the potential to generate hazardous exposures, action must be taken to mitigate the exposure.
This is accomplished by the approaches listed below:
a. Remove the fuel—this approach removes the exposure all together and is essential
when there is limited space for fuel relocation/displacement.
b. Reduce the fuels—this approach reduces the number of combustibles and therefore theexpected exposures.
c. Relocate the source—this approach allows the fuel to remain on the parcel but moves
it away from the primary residence to reduce fire exposures.
d. Harden structures for fire exposure—if the above three approaches cannot be
implemented, hardening the structure for fire will likely be necessary. The hardening
option increases the likelihood of structure survivability but may not be as effective as
hazard removal, and therefore, is the least desirable option. Additionally, hardening istypically the most expensive option.
The first approach of fuels removal (item a. above) is the most effective at hazard reduction
and is the only implementable fuel reduction option on very small lots. Fuel removal works
particularly well by replacing hazardous features with noncombustible alternatives,27

particularly when the combustibility of the feature does not add any functional component to
its use. 10 This approach is the most effective and can be used even on larger lots.
Reducing the quantity of fuels will directly reduce the exposures generated. The extent of
exposure reduction will depend on the fuel type, fuel geometry, and local conditions. An
example of fuel reduction would be keeping less firewood on the property.

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Fuel relocation has been a key anchor of many hazard mitigation strategies. As discussed in
Section 5.4.4, this is a valid approach for medium and large lots (medium and low housing
density WUI). However, this approach can cause unanticipated problems when neighboringparcels are not considered in the hazard risk assessment, particularly on small lots.
Hardening for fire exposures may be necessary in certain spatial situations where fuelremoval or displacement is not possible (e.g., primary residential structure spacing 25 ft to
50 ft, see Table B Item #1). The various components of structure hardening are presented inAppendix A (Table B and Table C) and must be evaluated for each structure. The potential
effectiveness of structure hardening is dependent on the potential fire exposure fromneighboring structures, wildlands, and other sources. Table 3 shows the effectiveness of
hardening for fire in the various WUI Types.
The table illustrates that the effectiveness of hardening for fire is directly related to structure
density. The coloring used in Table 3 is summarized for the following three structure density
scenarios:a. Low Density (SSD on the order of >50 ft) – lower potential losses from individual
structure ignitions, low potential for urban conflagration,b. Moderate Density (SSD on the order of 25 ft to 50 ft) – greatest impact on structure
ignition resistance by hardening for fire exposures from neighboring structures, andc. High Density (SSD on the order of < 25 ft) – must not have any structures ignite as
risk of entire community loss is very high due to structure-to-structure fire spread.
The above strategy is based on field fire behavior observations as well as controlledlaboratory and field fire experiments [4, 31]. Fire behavior at the parcel level is very dynamic
and, in a sense, very opportunistic, leveraging ignition vulnerabilities of different fuels andexposures between fuels. The above exposure/distance hardening strategy offers a pathforward that creates discontinuities for fire exposures and selectively hardens structures
where clear benefits can be achieved in ignition resistance.
The best available knowledge was utilized to determine the spacing distances to make themethodology implementable. As is the case with other guidance documents, ongoing
research will continue refining the spacing guidance included here.

10

Note: noncombustible features may not always provide identical aesthetics

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Table 3. Structure and parcel hardening effectiveness.

# WUI Type

Probability ofStructure
Survivability if
Neighboring
Structure Ignites

Potential Firea
Exposure from
Burning
Neighboring
Structure

Exposure from
Other Parcel
Fuels

Exposureb
from
Wildlands

Impact ofLikelyCommunity/
Structure Ignition Effectiveness ofon Fire spread in Partial Structure/ Neighborhood
Community
Parcel Hardening Participation

HD Interface –Perimeter

Low

High

f (fuels, dist.)c

Variable

High

Low

Necessary

2

HD Interface –Interior

Low

High

f (fuels, dist.)c

Low

High

Low

Necessary3

MD Interface –Perimeterf (hardening)

Moderatef (fuels, dist.)e

Variable

Moderatef (wildland fuels,
parcel fuels)

Desired

4

MD Interface –Interiorf (hardening)

Moderatef (fuels, dist.)e

Low

Moderatef (parcel fuels)dDesired5 MD Intermixf (hardening)

Moderatef (fuels, dist.)e

Variable

Moderatef (wildland fuels,
parcel fuels)

Desired

6 LD Interface

f (hardening)Low

f (fuels, dist.)e

VariableLowf

f (parcel fuels)

Desired

7 LD Intermixf (hardening)Low

f (fuels, dist.)e

VariableLowf

f (parcel fuels)

Desired29

1

HD = high density, MD = medium density, LD = low densityf (X) indicates “a function of X” (e.g., the level of exposure from other parcel fuels is a function of the fuels and distance from the target structure)aflames and radiationb
based on fire history, fuel loading, wind, and topography/aspect; wildland fuel treatments may not be at the control of the communitycparcel-level mitigation will have limited impact if nearby upwind structures catch on firedwould be a function of wildland fuel treatment AND hardening of most/all perimeter structures and parcels
eparcel-level mitigation, including wildland fuel treatment, together with home hardening, will enhance structure ignition resistancefignitions due to embers from burning residential structures have been observed as far as 200 ft to 300 ft downwind

5.1. Using the HMM
The Hazard Mitigation Methodology (HMM) described in this report is summarized in four
tables that can be found in Appendix A. The tables include all identified vulnerabilities;
however, not all vulnerabilities may be present on every structure and/or parcel.
Communities and homeowners need to address all vulnerabilities present on each individual
structure to enhance ignition resistance significantly.

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Table A, in Appendix A, describes all 40 structure hardening actions that have been
identified to date and that are needed to significantly reduce the ignition of structures fromvarying ember exposures. Table A contains information on the structure component,
assembly, or attached combustible, the hardening action, the goal, and the applicableconditions (when that hardening action is needed). The table also contains notes and a
relative cost column. Structural features with large surface area (e.g., other than Class A roof
coverings) or interior corners can contribute disproportionately to ember accumulation andignition, especially in low-ember exposure situations. Because of the variability in actual
experienced ember exposures, all 40 actions must be addressed if a significant reduction in
structure ignition probability is to be achieved. More details on partial use of the HMM areprovided in Sections 5.4.2 and 5.4.3.
Fire exposures and reduction in structure ignitions from radiation and convection are
addressed by three interconnected tables in Appendix A. Table B and Table C evaluate thespatial relationships of fuels on surrounding parcels and on the parcel where the structure ofinterest is being protected. Here the concept of Fuel Separation Range is used. This notion is
described next in Section 5.2 and Figure 15 and Figure 16. Table B and Table C containcolumns describing the fuel (hazard), minimum required fuel separation distance, the fuelseparation range, the hardening action, notes, and whether the structure will need to behardened against flames. Additional consideration may be needed when complex topography
can result in higher exposures. The intent is to displace or remove the fuels to reduce or
eliminate any structure hardening for fire in order to reduce overall retrofit costs. It should be
noted that structure hardening is directional based on the expected exposures, and an allaround (full 360-degree) hardening is not required by default.
If any hardening is required by Table B or Table C, the user is directed to Table D where
structure hardening is explicitly addressed. Table D contains a list of 10 specific vulnerable
structural components that need to be addressed to significantly reduce the ignition potentialof the structure from radiative and/or convective exposures.
5.2. Fuels Spacing
Specific hardening needs for structure protection against fire are addressed through therelationship of the exposure intensity and distance between the exposure source and theresidence. The rapid decrease in radiation and convection with distance is leveraged here to
optimize the required level of structure hardening. To illustrate different structure hardening
needs, three scenarios of a residential structure with different structure separation distance30

(SSD) to an auxiliary structure >120 ft2, and therefore different fire exposure levels, areshown in Figure 15 and described below. The term SSD is used in Figure 15 because boththe source and the target are structures. The same concept applies for non-structure fuels, andin those cases the term fuel separation range (FSR) is used (Table B). Note that in all three
scenarios the residence must also be fully hardened against ember exposures, regardless of
the required fire hardening.

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Figure 15. Three scenarios with a range of SSDs between the primary residence andauxiliary structure >120 ft2: 1) greater than 50 ft, 2) between 25 ft and 50 ft, and 3) under
25 ft. Structure hardening to increase ignition resistance is illustrated in blue (Scenario 2).
Note that hardening is directional and is not necessarily required around the entire structure.
Hardening for embers is required in all scenarios.
Scenario 1
This scenario represents exposure/distance combinations where the fire exposures do notreach the structure to be protected. A distance of 50 ft or greater is used in this example ofauxiliary structures with floor area greater than 120 ft2 (Table C Item #3). Based on fieldobservations, the residence will not experience significant fire exposures at this distance(radiation and convection) and no hardening for fire is required.
Scenario 2This scenario represents exposure/distance combinations where the fire exposures can ignite
a structure. Hardening the residence can significantly mitigate the likelihood of structure
ignition. Another way to say this is that there is a “sweet spot” in the spacing between thesource and the target (residential structure or other asset to be protected) where hardening
will add value. To follow the example of the auxiliary structure in Scenario 1, this range inspacing is when the auxiliary structure is between 25 ft and 50 ft from the target. Thisdistance range is a function of the source’s energy content and spatial configuration. In thiscase, hardening the auxiliary structure (the source) would also reduce its ignition potential,
and consequently, exposure to the primary structure.

31

Scenario 3
In many ways this is the most challenging scenario. This spacing range represents
exposure/distance combinations where the very high fire exposures (e.g., direct flame contactfrom a fully involved residence) will ignite a structure and mitigation in the form of
hardening will have limited or no effect in reducing the ignition potential of the structure.
This represents scenarios with very high fuel densities. In the case of the auxiliary structure
greater than 120 ft2, this occurs when the auxiliary structure to target (residence or other asset
to be protected) separation is less than 25 ft.
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In this scenario, the path forward to increase the survivability (i.e., ignition resistance) of thetarget is to prevent the ignition of the source. For auxiliary structures this can be achieved byalso applying the hazard mitigation methodology to the auxiliary structure. The best practiceis to displace or remove the shed. Relying on ignition prevention of auxiliary structuresenhances primary structure survivability under certain conditions, however, does not provide
the same protection provided by displacement or removal.
The same approach applies to fire spread between residential structures and is illustrated in
Figure 16. Unlike small auxiliary structures that can be removed or relocated in many cases,
the spacing between existing residential structures is fixed and the applicable scenario will
depend on the SSD.

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Figure 16. Three scenarios with a range of SSDs: 1) greater than 50 ft, 2) between 25 ft and
50 ft, and 3) under 25 ft. Structure hardening to increase ignition resistance illustrated in blue
(Scenario 2). Note that hardening is directional and is not required completely around both
structures. Hardening for embers is required in all scenarios.

33

5.3. Fuel Density in Different WUI Types
In Section 5.2 the fuel spacing concept was introduced. This section explores theimplications of fuel spacing/density across multiple parcels and different WUI Types. WUITypes can be viewed macroscopically; however, assessments need to be conducted at theparcel level. There may be considerable variability between two parcels with the same WUIType. Lot size alone is not sufficient to determine availability of space for combustiblefeatures. Residence placement, lot geometry, and structures on surrounding parcels will
impact space availability for combustible features. This is illustrated in the following
example scenarios listed in Table 4.
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Table 4. Parcel layout and fuel placement in different WUI Types visualized in Figures 17
through 20.
Lot
Backyard
WUI
Lot Size
Dimensions
Backyard
Dimensions
ScenarioType
SSD (ft)(ac)(ft × ft)
Size (ac)(ft × ft)
A

2

10

0.14

110 × 55

0.06

55 × 25B

441 to 55

0.45

150 × 150

0.14

150 × 35C

6/4

31 to 341

330 × 130

0.6

200 × 130

D

6

> 50

1 to 1.2

variablevariablevariable

For SI: 1 ft = 0.305 m, 1 ac = 0.4 haCompare SSDs for Scenario B (WUI Type 4) and Scenario C (Type 4 or Type 6). While
SSDs for Scenario B are larger, the lots are smaller which is the exact opposite of Scenario
C. Figure 17 through Figure 20 illustrate how much space may be available on individual
parcels or between parcels from select locations in the WUI. The blue shapes in these figures
represent general locations where high hazard combustibles such as sheds, RVs, gazebos, andother parcel-level auxiliary fuels may be placed without directly compromising thesurrounding structures from radiative and convective fire exposures. The shapes areapproximate and used for illustration purposes only; exact dimensions would be a function of
the combustibles present. The actual placement of the combustibles to prevent structure
ignition from fire can be determined using the tables in Appendix A.
Scenario A, in Figure 17, shows a high-density interface community with SSD of 10 ft
between residences. The area highlighted in red depicts the limited space available for
auxiliary fuels in the backyard. In this scenario, the shed located in the highlighted area iscloser to the neighboring residence (20 ft) than the owner’s residence (27 ft). Additional
hazards include the tight spacing of fences between residences, the shed placed against thefence, and interconnected fences joining multiple parcels. Conditions like this negatively
impact defensible space, safe operation of first responders, and structure survivability.

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An example from a medium-density interface community (Type 4) is illustrated in Scenario
B, Figure 18. However, within this community, locations may be classified as Type 5 due toincreased wildland vegetation. This scenario illustrates the effect of structure placement (onresidential lots) on the available space for auxiliary fuels. The red highlighted area between
the two residences contains fuels that can result in high exposures to both parcels. The area
contained by the blue outline is selected here to illustrate portions of the parcel wherecombustibles might be safely placed away from both residences. The connectivity of the twoareas outlined in red increase the number of available fire spread pathways to both primaryresidences. The lower two figures depict the actual fire progression pathway(shed→fence→shed→house) between parcels which resulted in the destruction of thestructure on the left even while defensive actions were taking place.
Larger lots, in Scenario C for example, are often found in lower density communities (Type6) and typically have space to accommodate additional combustible fuels. However, structure
positioning on the parcels can result in locally high structure densities with reduced SSD,
reclassifying to WUI Type 4 or 5 (medium-density interface or intermix). An example isshown in Figure 19, where parcels have large backyards (0.6 ac) and the structures areplaced well away from the wildland boundary. However, SSD is a key driver to fire
propagation between residences and will require targeted hardening in the illustrated case.
While the SSD in this scenario is greater than in high density communities, fuel placement
between structures can still impact fire spread and defensive actions (fuel agglomeration
effectively reducing SSD).
Scenario D (Figure 20) shows another case with large lots similar in size to Scenario C.
However, structure placement towards the center of the lot increases the average SSD andcreates auxiliary fuel zones, or areas within the community that are well away from all
surrounding residential structures, highlighted by the blue outlines. Avoiding agglomerationof fuels within the outlined areas would influence the total number of auxiliary fuels allowedin these spaces.

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Figure 17. Scenario A, WUI Type 2. Small parcel with small SSD provides minimal space
for auxiliary fuels. The shed is located closer to the neighboring residence than the owner’s
residence.

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Figure 18. Scenario B, WUI Type 4. Moderate density interface. In some areas there islimited space for auxiliary fuels, shown by the agglomeration on the property boundaries (redhighlights). The area highlighted in blue presents reduced impact on residential structures.
The lower figures show before and after fire imagery with the actual fire spread pathway
highlighted.

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Figure 19. Scenario C, WUI Type 4 or Type 6. While the structures are placed on large
parcels with extensive backyards, the primary structures are very large with considerably
smaller SSD. Selective hardening may be required in this scenario despite the large parcels.

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Figure 20. Scenario D, WUI Type 6. This community is characterized by large parcels withmore extensive space available for auxiliary fuels. Avoidance of agglomeration of fuels
within the outlined areas would influence the total number of auxiliary fuels allowed in thesespaces.

39

5.4. Factors Influencing Structure Survivability
Four primary factors influence the impact of structure hardening/mitigation on structure
survivability in WUI fires:

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defensive actions,•partial structure hardening,•partial community hardening (i.e., fraction of structures/parcels within the communitythat are fully hardened), and•housing density (i.e., SSD).

5.4.1. Effects of Defensive Actions on Structure Survivability
NIST post-fire WUI case studies, conducted together with state and local stakeholders, haveidentified that defensive actions by first responders have a significant impact on structure
survivability. NIST WUI fire reconstructions have determined that most (> 90 %) damaged
structures were defended and saved by explicit first responder actions. Table 5 summarizes
the defensive action/damaged structure relationship for the Witch/Guejito, Tanglewood
Complex, and Waldo Canyon Fire case studies. Remote sensing has also identified theextensive contributions of defensive actions and their impact on fire containment [42, 43].
Table 5. Defensive actions identified in NIST post-fire case studies.

% ofDamaged
Structures
Identified as
Defended
94 %Structures inCase Study
245

Damaged
Structures
16

Damaged
Structures
Identified as
Defended
15

Tanglewood Complex

179

13

11

85 %

Waldo Canyon

1455

101

94

93 %

Case Study
Witch/Guejito

The second important finding between defensive actions and structure survivability is thatmany structures are often defended that are not damaged or destroyed. This was observed
during both the Witch/Guejito and the Waldo Canyon Fire case studies. During the Waldo
Canyon case study 154 of the 397 undamaged structures within the fire perimeter wereidentified as being defended (39 %). This number does not include additional actions thatwere taken at the parcel level which might have indirectly saved structures by reducing
exposures and stopping fire spread further from the structure.
These two findings have significant implications on assessing the WUI built environment.
The percentages of identified first responders that extinguished and saved the ignited40

structures suggests that if a home in a WUI event is damaged it was more than likely
defended and that an ignited structure will not survive unless actively defended. Both the
Witch/Guejito and the Waldo Canyon Fire case studies confirmed that not only were a large
fraction of the undamaged homes defended but also that, in many cases, these defensive
actions were not always readily identifiable in the field (during or after WUI fires). The NISTWUI data collection methodology has enabled the identification of such actions throughdetailed technical discussions with large fractions of the first responders involved in each
incident.
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Exposures have been demonstrated to vary at a sub-parcel level in all NIST case studies [4-8,32]. Additionally, information from field data collection does not indicate that the varying
exposures and defensive actions “average out” throughout an incident. All these findings
suggest that post-fire analysis of damage and survivability should not be conducted in theabsence of defensive action data. Any assessments not accounting for defensive actions will
likely be incomplete and possibly inaccurate.
The occurrence of large WUI fires, like the Tubbs and Camp Fires, together with thenumerous fire storms that have occurred in California since 2000, suggest that structures
must be designed to stand alone as the number of exposed structures can quickly outnumber
statewide resources. Defensible space contributes to making structures defensible by first
responders; however, this is not a substitute for homes standing completely alone throughout
a WUI fire event.
5.4.2. Impacts of Structure Mitigation Compliance on Structure Resilience
The impact of partial hardening on structure survivability is not directly proportional to thefraction of hardening actions implemented. As an example, doing half of the ember
hardening specified in Table A (see Appendix A) does not automatically translate to a 50 %increase in ignition resistance. The relationship between effectiveness of partial hardening
and local exposures can be represented by the following statement:
Effectiveness ofpartial hardening∝

1
local fire andember exposuresincident size

number ofsimultaneous
incidents

If the structure of interest receives very limited exposures, then the effectiveness of partial
hardening is increased. Considering the 50 % ember hardening scenario discussed
previously—if a structure is in a location where only very few embers land on it, there is abetter chance of survival. However, the high exposure scenario is quite different. If very highember exposures impact the structure, it is improbable that the structure will survive unless
all the ember hardening identified in Table A had been implemented. That is because in highember exposures, where thousands and thousands of embers hit the structure, it is likely that41

embers will “find” one or more of these vulnerabilities and cause the structure to ignite. The
same concept applies to hardening structures for fire exposures. If a structure is only partiallyhardened and the weak spot ignites, the otherwise hardened components will have limited tono effect on structure survivability once the structure has ignited. In this situation only timely
defensive actions will result in a damaged, rather than destroyed, structure.

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The increase in incident size has a negative effect on partial structure hardening. As theincident gets larger (in terms of area and number of structures ignited), the number ofavailable first responders per residence or parcel decreases. This also holds true in the eventof a fire storm where the finite number of first responders will have to respond to multiple
WUI fires, further reducing the number of responders per residence or parcel.
It should be noted that among local fire and ember exposures, incident size, and number ofsimultaneous incidents, the only potential control that can be exercised is the reduction of fire
exposures at the parcel level. Fuel reduction, displacement, and removal will all impact local
exposures. However, the ember exposures will only be partially impacted as embers cantravel from far beyond residential parcel boundaries; therefore, hardening is required.
The above can be summarized by saying partial ember hardening only adds some value inlow exposure conditions, and almost no value in high exposure conditions. The localized
variability of fire behavior results in unpredictable ember exposures. Full ember hardening isnecessary to enable structures to stand alone during high ember exposures. The same holds
true for fire exposures. Suppose a combustible, and readily ignitable, auxiliary structure or
RV is located too close to a home, and there is significant fire or ember activity on the parcel.
In that case, there is a likelihood that the auxiliary structure or RV will ignite and proceed toignite the structure. Therefore, to stand alone, the structure must be evaluated and fully
hardened for embers, and the structure and parcel hardened for fire where necessary.
5.4.3. Impacts of Community Mitigation Compliance on Community Resilience
The impact of a partially hardened structure on the community is inversely related to SSD;
partially hardened structures need to be further apart.
In low density communities, a structure ignition will increase local ember exposures;
observations have shown these ember exposures to be significant in the first 300 ft downwind[4]. 11 Locally there will also be increased exposures from fire (radiation and/or convection).
These exposures may impact parcel-level combustible features; however, there should not be
any direct fire exposures to adjacent properties due to larger SSD. The impact to the adjacent
properties and community will be indirect in terms of fire, and possibly low for ember
exposures (because of the large SSDs). Therefore, in low density communities with large
SSDs the impact of structure ignition on the surrounding structures is low.

11
Data collected from the Waldo Canyon Fire case study showed embers from burning structures resulted in structure ignitions 200 ft to
300 ft downwind.

42

In moderate density communities, a structure ignition can ignite an adjacent property under a“favorable” wind direction and other local conditions. The embers generated from a burning
structure will generate significantly higher exposures to the downwind structures relative tothe low-density case due to separation distance. Compared to low density, the relative impactof a structure ignition to the surrounding structures can be seen as medium.

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However, high density communities are in a very different exposure category. Here theignition of a structure will almost invariably result in the ignition of one or more adjacent
properties and will likely result in the loss of a significant fraction of the community, as
evidenced by several large loss WUI fires in the U.S. This is not only because fire spreadoccurs very easily between tightly spaced structures, but also because it is very difficult tocontain a fully involved structure fire, even when only moderate winds of 10 mi/h to 15 mi/hare present. Fire spread is difficult to stop as it is very challenging to remove/block the heat
between residential buildings when they are constructed 6 ft to 10 ft apart. 12 In high-density
construction, a single ignition can have a disproportionate impact on the overall community
losses.
5.4.4. Effects of Housing Density on Mitigation Strategies
The fire spread information provided in this report can be used to develop an overall
community WUI mitigation strategy. The community response specifically in terms ofcommunity participation is summarized in Table 3. The primary technical messages for thedifferent structure density and WUI types are listed below.
1. High Density Communities
Community participation is necessary in high-density communities. This is due to thedisproportionate impact of a single structure ignition on the community. There aremany ways to accomplish this via voluntary or mandatory community hazard
mitigation programs. Different programs can be implemented using HOA, county, or
state rules and regulations. The design and implementation of such programs arebeyond the scope of this report. What is important in these communities is to harden
the structures and parcels for ember exposures, as these can originate from outside thecommunity, and also ensure that there are no fire spread pathways that can generate
fire exposures to residences. In certain scenarios, additional protection may beprovided by structure cladding (siding and roofing) hardening requirements (such as
included in CA Chapter 7A). While these requirements add significant value forprotecting against low and moderate exposures, they will frequently not be able tocope with the full exposure assault of a residence burning 6 ft to 10 ft away becauseof limitations of one of the components in the wall or roof assemblies. Parcel-level
hardening is a critical requirement in these high-density communities, and auxiliary
fuel removal (instead of displacement) will likely be necessary on very small lots. A
key issue that will need to be addressed is the presence of vehicles in the community,
as they can generate locally high fire exposures resulting in structure ignitions.
12

Common building setback requirements are often 3 ft to 5 ft from the property line.

43

Ongoing research by NIST, CAL FIRE, and IBHS aims to provide guidance on
minimum spacing between vehicles and residences in the future.

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2. Moderate Density Communities
Moderate density communities offer more options to residents for community
participation in hazard mitigation. In moderate density construction, the ignition of asingle residence may directly impact one or more adjacent residences. However,direct fire (via radiation and convection) progression across the community will
depend on local construction, spacing, and fuels more than in high density. Here there
is an opportunity to slow down that fire spread and reduce the structure ignition
pathways by selectively hardening structure sides that may see increased exposures.
This selective structure hardening reduces overall community hardening program
costs while increasing community resilience. Hardening of all structures for embers isstill necessary in these communities. Fuel displacement, reduction, and removal may
also need to be implemented to reduce fire exposures.
3. Low Density Communities
These community types, by the nature of their structure density, do not present a
direct structure to structure fire spread pathway as can be expected in high density
and certain moderate density configurations. These larger parcels also offer asignificantly large number of fuel displacement options to residents, as auxiliary
parcel level fuels can be placed where they will not impact other fuels, the primaryresidence, or neighboring structures. The notion of sacrificial auxiliary
structures/features can be introduced in this setting; a resident with a large lot may bewilling to accept the loss of an auxiliary structure that is located far from theirresidence or other features to be protected. In a low-density community this may be areadily acceptable hazard if the residence is hardened for ember exposures and allother potential fire propagation pathways near the residence have been disrupted(using the provided methodology).

44

6. Frequently Asked Questions
A few common scenarios are presented in a question-and-answer format to provide examplesof the thought process behind applying the HMM to each case. Questions are posed as if they
are from homeowners with an interest in personal and community mitigation.
Q: I live in a high-density community. What can I do to prevent my structure fromburning during a WUI fire?
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A: There are three steps that you can take to increase the likelihood of your home surviving a
WUI fire:
1. Apply the methodology in this report to your home and parcel. You will need toharden your home against embers and evaluate and address any potential fire
exposures within your parcel. This may require you to reduce, relocate, replace, andpossibly remove some auxiliary parcel-level fuels.
2. Work with your neighbors and get them to engage and follow the same practices.
3. In your high-density community you will want to get as many residents as possible tofollow the hazard mitigation methodology. To accomplish this, you could engage yourcommunity (via a homeowner’s association (HOA), if present) as well as your local
and county representatives (e.g., FireSafe Councils, Firewise USA, and local
town/city/county jurisdictions) about implementing community-wide tools for hazard
reduction. Whether the tools are mandatory or voluntary, complete community
participation is essential.
Q: I have a shed on my 2-acre lot that is far from any other fuels on my property. Can I
keep that shed where it is?
A: There are two parts to this answer:
1. Consider clearing the zone within 5 ft of your shed to minimize the chance of a surface
fire or embers directly igniting the shed. You can always work on hardening theexterior building materials of the shed to make it more ignition resistant; however,these actions may not be necessary if the shed itself is an acceptable loss.
2. You should also consider how your shed may impact neighboring properties. What
you want to avoid is your shed posing a significant exposure to your neighbor’s
residence. If your shed is close to a neighboring residence, consider moving your shedelsewhere on your property. If that is not possible, consider hardening the exteriorbuilding materials of the shed to make it more ignition resistant. You can apply themethodology to harden your shed against embers and increase the likelihood that yourshed will not ignite. By preventing the ignition of your shed, you are contributing topreventing the ignition of your neighbor’s residence. Fire does not care about propertylines, and WUI fire mitigation is a community-level effort. You want to work withyour neighbors to protect their home, as you may need their help to protect your own
home.
45

Q: I have a half-acre lot and my neighbors are 60 feet away. My home is older and does
not have a hardened wall cladding (siding). Do I need to replace my wall cladding(siding)?
A: There are three parts to this answer:
1. First make sure that your residence is hardened for ember exposures. You should
address all 40 vulnerabilities identified in this methodology.
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2. Look at your parcel and make sure you do not have any parcel-level pathways thatcan bring fire to your residence. Address any potential fire spread pathways (e.g.,fence to house, fence to vehicle to house) by reducing, relocating, replacing, andpossibly removing, auxiliary parcel-level fuels.
3. The exposure from your neighboring structure is beyond the special limit outlined in
the Hazard Mitigation Methodology. However, if your cladding is old and you areconcerned that local conditions may cause significant fire exposures, you canconsider replacing the cladding only in the direction (side) that is facing yourneighbor’s residence. This will reduce the overall mitigation expense.

46

7. Comparison of this Hazard Mitigation Methodology with National and International
WUI Codes
None of the current codes/best practices implement the breadth of mitigation components
considered in the HMM, and none of them harden as extensively as the HMM requires.

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Table A includes 40 items with a total of 57 options to specifically address structure
hardening for ember exposures. The list encompasses all currently identified vulnerabilities;
however, some structures may not have all of the vulnerabilities identified, andvulnerabilities to embers not currently identified in the HMM may also exist. All of theember ignition vulnerabilities present on each individual structure need to be addressed tosignificantly enhance ignition resistance. An additional 10 structure ignition vulnerabilities
are identified in Table D to harden for fire exposures identified in Table B and Table C.
Table 6 below summarizes the number of options completely included in each of the selected
WUI building codes for both fire and ember vulnerabilities. While a lot of ember and fire
hardening information is included in the compared codes, the exact requirements outlined in
HMM are only identified in the existing codes in limited numbers. Fire progression andstructure hardening at the component and assembly level requires very detailed and exact
guidance. The detailed information presented in this document is based on the latest scienceand field observation data. Appendix B contains a tabulated list of detailed comparisons ofprimary solutions listed in the analyzed building codes, in Table E.
The differences between the compared codes and best practices and HMM may be due todifferent goals and objectives. Some codes may emphasize defensible space and explicitly
rely on first responders to save structures, while other best practices may focus on engaging
residents, providing education, and/or starting community participation. The purpose of the
HMM is to present implementable guidance to significantly increase the likelihood of
structure survival without external intervention in the form of defensive actions. This was set
as the structure protection criteria because the predominance of large WUI events and firestorms in the last twenty years point to that need.
Table 6. HMM items completely included in selected existing WUI building codes.
Number of
Number ofember items
fire items fromfrom Table A% of ember
Table D% of fireWUI Code(out of 57)
items(out of 10)
items
7A/1140/IWUIC

5 to 139 to 23

0 to 50 to 50

All 3 codes

3

5

0

0

None

42

74

5

50

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8. Summary

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WUI fires have grown in intensity, frequency, and devastation in the past twenty years.
NIST, CAL FIRE, and IBHS have developed the Hazard Mitigation Methodology detailed inthis report. The goal of HMM is to reduce the vulnerability of structures and parcels in a
cost-efficient and implementable way. The HMM was conceived to allow structures in the
WUI to “stand alone” and survive fire and ember exposures without contributions from first
responders. This was deemed necessary as field data has demonstrated that wildfire
progression can quickly outpace the efforts of first responders during large and/or concurrent
WUI fires. NIST WUI reconstruction data has also shown the efficient and effective structure
protection actions by first responders; very little or no meaningful improvement can beachieved in this area. The emphasis must be shifted to making structures stand alone.
This HMM has outlined a detailed structure hardening strategy to resist ignitions from ember
exposures. The 40 identified structure ignition vulnerabilities illustrate how detailed structure
hardening must be for a structure to stand alone. Structures in the WUI need to be protectedagainst ember exposures independent of WUI Type and housing density. This list goessignificantly beyond what is outlined in many best practice documents currently available to
WUI residents. The spatial analysis used in the HMM was developed specifically to address
fire (radiation and convection) exposures within and across parcels. The latest science andexpert knowledge combining decades of field observations were used to develop the spatial
relationships outlined in the HMM. Discussions with the building industry were necessary to
clarify and improve different implementation solutions.
Full community participation will be critical to hardening existing high-density communities.
This document has outlined a technical path forward to accomplish this. The methodology
was also developed specifically to reduce the overall financial burden of mitigation. The
preferential/partial use of this methodology is undesirable and provides only very limited
actual structure ignition resistance.
HMM was developed based on how fire spreads between and across parcels, as fire behavior
does not respect parcel boundaries. HMM was developed utilizing the principles of fuelreduction, relocation, and removal. By implementing the HMM, residents and communities
can significantly reduce structural losses from future WUI fires.
NIST and IBHS are non-regulatory entities, while CAL FIRE has regulatory authority in thestate of California. This HMM was developed to provide AHJs and homeowners with thelatest comprehensive understanding of fire behavior and structure response in the WUI. This
report has demonstrated the benefits of using HMM as it is designed and presented here, as
well as the limitations of partial implementation. Regulatory agencies and homeowners will
choose where to implement any or all of the components presented in the HMM. Future
hazard mitigation research (including laboratory and field observations) will be incorporated
into the methodology to further enhance its effectiveness.

48

Summary of Key Technical Principles
This section contains two types of technical principles. The first is a list of specific principles
followed by two lists of generalized relationships between exposures, structure hardening,and community survivability.
In the WUI, structures can experience two distinct fire exposure problems/hazards—embers
and fire. Following are three sets of technical principles specific to fire, embers, andparcel/community hazard mitigation implementation.
This publication is available free of charge from: https://doi.org/10.6028/NIST.TN.2205

Embers
1. Ember exposures to the residence/parcel to be protected can originate from adjacent
and/or far field parcels and are beyond the control of the owner of the parcel being
protected.
2. Increased ignition resistance of features and specific combustion considerations candecrease ember generation.
3. Large surface area combustibles (e.g., combustible roof) contribute significantly tothe ember ignition hazard.
4. Large ember exposures can be generated and observed in WUI fire incidents. Ember
exposures can vary significantly in space and time.
5. The potential for high ember exposures drives the need to harden structures against
ember exposures completely.
6. Partial structure hardening for embers does not relate linearly to hazard reduction.
90 % ember hardening does not translate to 90 % risk reduction.
7. In high ember exposures, complete ember hardening (100 % compliance) is necessary
to significantly reduce the structure ignition potential from embers.
Fire
8. Fire represents a direct and indirect exposure hazard to residences, commercial
structures, and infrastructure in the WUI. Direct exposures occur when a source (itemburning) directly impacts a target, in this case a residence or commercial structure.
Indirect exposures occur when a source ignites a secondary fuel, impacting the target.
9. Fire exposure increases via fuels agglomeration. Increased fire exposures can thenpotentially impact the residence/commercial structure. Increased exposure also
negatively impacts defensible space, making it more hazardous for first responders to
conduct firefighting and rescue operations.
10. A property owner can control and reduce fire exposures to their residence/commercial
structure by managing their parcel-level combustibles.
11. A property owner may not be able to control fire exposures from adjacent parcels.

49

Parcel/Community
12. The parcel size and placement of structure(s) will determine structure-to-structure
exposures within the parcel and to adjacent parcels.
13. Structure separation distances have significant impact on fire propagation in the WUI.
14. Fuel relocation, reduction, and/or removal should be considered when addressing
parcel hardening independent of WUI housing density.

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15. Even if certain parcel-level combustible features are seen as expendable/disposable inthe context of parcel hardening, the impact of these features must be considered in thecontext of fuels agglomeration and exposures to structures and other nearby
combustibles.
16. In high-density WUI communities, fuel removal may be necessary to comply with the
HMM and reduce structural losses.
The following lists contain information intended to capture the relationships between
exposures, parcel and structure hardening, and community structural losses. The relationships
illustrated here are intended to provide relative performances and highlight trends and criticalthresholds. Actual conditions, including construction, parcel sizes and fuel loading, structure
separation distances, parcel and structure hardening, local weather, ignition sequencing, anddefensive actions, will impact actual fire spread and community resilience.
General Relationships between Exposure and Hardening
1. Low fire exposures are relatively easy to address by hardening the structure.
2. High fire exposures (direct flame impingement from large sources such as a burning
residence) are very difficult to address by hardening the structure. This is because
both the cladding (siding and roofing) material and the assembly need to be hardened
not only to withstand the exposure but also to withstand it for the entire exposure
duration, therefore potentially propagating energy into the assembly past the external
cladding component.
3. Fire exposures from a fully involved single family residence will result in fire
propagation that will be very difficult to stop in the presence of wind in high densitycommunities.
4. High fire exposures can readily cause direct ignition of exposed combustibles.
5. The ember hardening and structure survivability relationship is not linear. Hardening
80 % of the vulnerabilities will not necessarily result in 80 % decrease in structure
ignition potential. While the actual values will vary based on local conditions andspecifics of hardening, the relationship will apply to almost all scenarios.
6. In situations where a potential fuel source is located near a residential/commercial
structure and when fuel reduction, relocation, or removal cannot alleviate severe fire
exposures to that structure, hardening the structure for fire may frequently add limited
value. In these situations, ignition prevention of the fuel source will be the critical
path to reducing the ignition of the residential/commercial structure.
50General Relationships between Structure/Parcel Hardening Compliance and Community
Survivability
1. Partial community compliance, in the form of incomplete structure and parcel
hardening, has limited impact beyond the partially hardened properties in a lowdensity community.
2. Partial community compliance, in the form of incomplete structure and parcel
hardening, has moderate impacts beyond the partially hardened properties in amoderate-density community.
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3. Partial community compliance, in the form of incomplete structure and parcel
hardening, has very significant impacts across the entire community in high- densitycommunities. Just a few partially hardened properties can jeopardize an entire highdensity community.

51

Acknowledgments

This publication is available free of charge from: https://doi.org/10.6028/NIST.TN.2205

The authors would like to acknowledge the productive collaboration among CAL FIRE,
IBHS, and NIST. Additionally, the authors would like to thank the following individuals forproviding input to the report and participating in the review process; Derek McNamara(Geospatial Measurement Solutions, LLC), Kuma Sumathipala (American Wood Council),Karl Fippinger (International Code Council), Karen Jackson (Texas Forest Service, Retired),Jonathan Griffin (NIST Public Affairs Office), Glenn Forney (NIST Fire Research Division),
Tom Cleary (NIST Fire Research Division), Jiann Yang (NIST Fire Research Division), andthe NIST Editorial Review Board.

52

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56

Appendix A. Home Hardening Requirements
The following tables identify the various potential structure and parcel ignition
vulnerabilities and their associated mitigation options.
Table A specifically addresses hardening against ember exposures. Table B and Table C list
hazards from adjacent parcels and hazards from within the parcel containing the residence
being protected. These tables are linked spatially to Table D, which contains structure
hardening requirements for exposures from fire (flames and radiation).
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Table A is divided into 8 major categories (e.g., roof, walls, vents, etc.) listing specificstructural components or assemblies. The table contains columns listing the hardening action,
the performance goal of the action, and the applicable conditions associated with eachcomponent. Also included are any brief notes, and a column estimating relative cost of eachaction. The final column indicates which of the selected existing WUI building codes (CABuilding Code Chapter 7A, NFPA 1140, and ICC IWUIC) address the specific hazard to thenecessary detail.
Table B addresses parcel fire exposures to the primary residence from far-field fuels
included in adjacent parcels. For each fire-emanating source (e.g., neighboring structure,
wildland fuels) a minimum required fuel separation distance (MFSD) or fuel separation range(FSR) is provided. If the hazard is located closer than the listed MFSD, or within the listed
FSR, structure hardening must be addressed via Table D. If the fuel is further than the listed
distances, no hardening action is required for fire exposures. If the fuel is closer to thestructure being protected than the listed FSR, hardening for fire will have limited impact due
to the expected severity of the exposure, and is therefore not required in the HMM for costsavings. AHJs or homeowners may choose to add an additional level of protection byhardening in these situations.
Table C addresses parcel hazard mitigation and lists 9 separate types of vulnerabilities. The
table contains columns on hardening actions, goals, and the applicable conditions. Similar to
Table A, notes and expected relative cost are included. As in Table B, MFSDs are provided,and links to Table D when spatial conditions require mitigating action.
Table D lists 10 separate structure components requiring hardening for fire exposures. The
use of Table D is driven by the spatial requirements identified in Table B and Table C.
Similar to the other tables, columns are included for hardening action, the performance goal,
and the applicable conditions of the component/assembly. Columns for notes and expected
relative cost are also provided. As in Table A, comparison to selected existing codes is also
included.

57

Table A. Structure and attached combustible hardening against ignition from embers.
Item
#
Roof

Structure Component, Assembly, orAttached Combustible

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1

Skylights

2

Roof to skylight flashing

3

Roof assembly details (i.e., dormer
and other roof-to-wall intersections)

Hardening Action
Replace plastic skylight withmultipaned glass with tempered glass
outer pane. If skylight opens, install
metal screen on the inside. If screen is
non-metal replace with metal.
Check for standard metal flashing andthat no exposed wood is present, repair
as necessary.
Metal flashing at roof-to-wall
Replace with noncombustible siding inthat area only (e.g., dormer or splitlevel residence)

Expected
Cost Range($, $$, $$$)

Matched in
Existing
Code

Performance Goal

Applicable Condition(s)Notes

Minimize embers with enough energy tocause ignitions

Plastic skylight pane, nonmetal screen,no screen

Screen is needed only on openable
skylights$

Chapter 7A
NFPA 1140
ICC IWUIC

Prevent ignition of combustible skylightframing

Lack of metal flashing around skylight

n/a

$

None

Prevent ignition of roof from burning
debris accumulation

Combustible siding at roof-to-wallintersection

Alternative option: Add the 6 inch "tall"
flashing over the existing siding or
remove the siding and put flashing on.
Preferred option: remove combustible
siding and replace with noncombustible
siding.
No additional baffles or screening should
be installed as they may impact PV
cooling

$

None

$$

None

$

None

4

Solar panels

Minimize debris accumulation underand next to solar panels

Prevent ignition of debris, solar panels, androof

Solar panels on roof

5

Roof covering – old wood shake

Replace with Class A (withnoncombustible birdstopping, if
needed)Prevent ignition of roof material

Non-fire retardant treated wood shake
roof

n/a

$$$

Replace with Class A when needed

Prevent ignition of roof material

Degraded or end of life Class B roof
and needs replacement
Degraded or end of life Class C roof
and needs replacement

Chapter 7A
NFPA 1140
ICC IWUIC

n/a

$$$

Chapter 7A

Close/plug with noncombustible
material

Prevent accumulation of debris between
roof covering and roof deck

Style of roof (tile or metal) thatcreates openings

n/a

$-$$

Chapter 7A
NFPA 1140
ICC IWUIC

Noncombustible gutter cover a

Limit accumulation of ignitable debris ingutters$-$$

ICC IWUICNFPA 1140

Metal drip edge

Prevent ignition from embers, protect facia
and sheathing from flames

$

None

No-gutter

Prevent ignition from embers

$$$

None

Add metal flashing if fascia does notcover roof sheathing

Prevent accumulation of embers at fasciasheathing intersection

Exposed sheathing (i.e., not coveredby fascia)

$

None

Replace exterior wall covering with
noncombustible material for the bottom
2 ft (from ground); add metal flashing
to protect bottom edge of sheathing

Prevent windblown debris and local fuelsfrom igniting the wall

All siding within 2 ft of ground

Metal flashing is required for all
claddings, including noncombustible

$$

None

Minimize embers with enough energy tocause ignitions

Plastic ridge vent AND installable
metal baffle
Plastic ridge vent AND non-installable
metal baffle

Metal flashing

$

None

n/a

$$

None

6

7

8

9

Roof covering – Class B
Roof covering – Class C
Roof covering with openings between
roof covering and edge or ridge

Gutters

No gutter

Cladding (Siding)
10

Height of wall assembly from thegroundVentsb11

Ridge vent

Add metal baffle
Replace ridge vent w/ metal ridge vent58

Combustible gutter cover or no gutter
cover

If metal cover cannot be installed onplastic gutter, then replace gutter withmetal gutter and noncombustible coverCertain gutters already have drip edge aspart of the gutter
Very expensive solution, less expensive
options are available
May require subsurface-surface drainage
such as ICC 11.01.6

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Item#

Structure Component, Assembly, orAttached Combustible

12

Off -ridge vent

13

Gable vent14

Under eave and any vents in theunder-eave area on the rake (gable)
end of the building

15

Crawl space vents

Hardening Action
Replace with ember and flame-resistantvent
Remove gable ventReplace with ember and flame-resistantvent
Replace with ember and flame-resistantvent, add fire caulking around all
blocking
Create a soffited eave (horizontal) or
enclose eave (angled) using
noncombustible material
Replace with ember and flame-resistantvent

Performance Goal
Minimize embers with enough energy tocause ignitions

Applicable Condition(s)Notes

Expected
Cost Range($, $$, $$$)

Non-ember/flame resistant vent

n/a

$$

Chapter 7A

Minimize embers with enough energy tocause ignitions

Combustible screen and/or meshgreater than 1/8 inch

n/a

$$

None

$

Chapter 7ANon-ember/flame resistant vent and/or
uncaulked eaves blocking

n/a

$-$$

Chapter 7ANon-ember/flame resistant vent and/or
open eave

1. All sides of the residence
2. Angled implies following the roof line

$-$$

None

n/a

$-$$

Chapter 7AReplace with ember and flame-resistantvent and add moisture barrier

Minimize embers with enough energy tocause ignitions

$-$$

CA
Building
Code

$$$

None

$

None

$

None

Minimize embers with enough energy tocause ignitions

Non-ember/flame resistant vent

Convert to unvented crawl space

Can use plastic ground cover to reduce
venting requirement; addition of moisturebarrier reduces required vent area by 10x
Energy benefit, very expensive solution,less expensive options are available
n/a
Needed in rooms within occupied portionof residence where gas appliances arelocatedMatched in
Existing
Code

16

Dryer vents

Metal flapper (closed unless in use)

Limit ignition of lint and ducting

No metal flapper in place

17

Makeup air intake

Replace with ember and flame-resistantvent

Minimize embers with enough energy tocause ignitions

Combustible screen and/or meshgreater than 1/8 inch

Minimize embers with enough energy tocause ignitions

Gaps around siding penetrations
greater than 1/8 inch

n/a

$

None

Limit ignitions under structure

Open space under residence (e.g., pierand post foundation)

n/a

$$

None

Limit mobile home ignition

Mobile home with attachment(s)

n/a

$$

Chapter 7A
NFPA 1140

Limit ember intrusion and accumulation

Penetrations

n/a

$

None

Minimize embers with enough energy tocause ignitions

Opening window

n/a

$

None

Prevent ignition of combustible frame

Exposed wooden frame single pane

Moisture removal benefits, energy benefit- seek cost sharing

$$

None

Prevent ignition of combustible frame

Exposed wooden frame double pane

n/a

$$

None

Limit ember ignition of door and frame

Wood door frame and/or wooden door
and/or wooden threshold

n/a

$-$$

None

$$

None

18

Other penetrations (electrical, water)
to roof and walls
Close up open space under residence

19

Other attachments to mobile homes(portico/car port)

20

Other penetrations

Windows
21

Window screens

22

Exposed wooden frame single pane

23

Exposed wooden frame double pane

Doors
24

Exterior door (non-sliding)

Close with fire caulking and inspectduring routine maintenance and close
gaps as needed
Install noncombustible skirting on all
sides. Add vents as required. Vents
must be ember and flame resistant.
6 inches of metal flashing at the wallintersection
Seal around penetrations with firecaulking
Add metal screen
Replace with noncombustible frameand double pane tempered glass
window
Replace with noncombustible frameand double pane tempered glass
window
Install metal door jamb kit and metal
threshold; add metal kick plate, andmetal door bottom
Replace with metal door with metalframe and threshold

59

Item
#

Structure Component, Assembly, orAttached Combustible
Exterior Door (sliding)25

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26

27

Wood-frame screen door

Plastic screen in screen door

Garage door

Hardening Action
Replace with non-wood containing
slider with dual pane tempered glass
and with a metal screen
Replace wooden screen door with metal
door and frame

Performance Goal

Applicable Condition(s)Notes

Expected
Cost Range($, $$, $$$)

Limit ember ignition of door and frame

Wood sliding door

n/a

$$

None

Limit ember ignition of screen door andframe

Wooden screen door

$$

None

Replace with metal screen (1/16 inch)

Prevent ignition from embers

If screen door and frame are metal,
replace screen or entire system(whichever is more cost effective). If
screen door or frame is wood, replaceentire screen door (see line above)Primary door hardening option that does
not require primary door replacement
n/a

$-$$

None

Prevent ignition from embers

Wooden garage door

n/a

$-$$

None

Prevent ignition from embers

Wood garage door frame

n/a

$-$$

None

Prevent ember intrusion and ignition fromembersNo gasketing

n/a$Chapter 7A

1. Area under the deck footprint must be
maintained with no combustibles,
addressed through defensible spacerequirement
2. Replacing boards that are not parallelto the residence may require
additional deck framing

$-$$

None

n/a

$$$

NFPA 1140
ICC IWUIC

If garage door is not metal, go overwith metal flashing around the bottomof the door (both inside and outside forfirst 6 inches (using a metal "C"channel)
Add metal flashing at base of framing
for first 6 inches [go as close to theground as possible (< 1/4 inch desired),raise wood and extend flushing tolower than bottom of wood]
Add gasketing

Attachments to Residence

28

Decks, stairs, and landings attached toresidence29

Deck-to-wall intersection

30

Combustible decks with combustibles
present in 0 ft to 5 ft zone around deck31

Fence to residence

32

Steps connected to residence

Replace walking surface deck boards
with noncombustible deck board forfirst 1 ft away from residence
Replace entire deck with
noncombustible option (metal or otheroption)Replace bottom 2 ft of combustible
siding with noncombustible (e.g., fiber
cement) and add metal flashing toprotect exposed sheathing
Remove combustibles in 0 ft to 5 ftzone around deck c
Replace with noncombustible option(metal or other option), minimum 8 ft
Introduce noncombustible
barrier/section between steps andhouse, minimum 1 ft
Introduce noncombustible
barrier/section between handrails andresidence or replace with
noncombustible handrail, minimum 1 ft

60

Prevent ignition of deck and limit spread offire to residence

Combustible deck, stairs and landings
attached to residence

Matched in
Existing
Code

Limit ignition of wall on residence

Combustible siding

n/a

$$

None

Prevent flames from adjacent fuels from
igniting the deck

Combustibles in 0 ft to 5 ft zone
around deck

n/a

$

None

Prevent ignition of combustible fence

Combustible fenceThis applies only to single fences; see
Table B Item #10 for double fences

$-$$

None

Combustible steps

This addressed top of deck ember ignited
scenario (not fire under steps/deck)

$

None

Combustible handrails

n/a

$-$$

None

Limit spread of fire to houseThis publication is available free of charge from: https://doi.org/10.6028/NIST.TN.2205

Item#

Structure Component, Assembly, orAttached Combustible

33

Other attachments

34

Attached retaining walls

35

Combustible furniture

36

Pergola/trellis

Mobile Home Skirting/Crawl Spaces

Hardening Action
Introduce noncombustible
barrier/section between combustible
attachment and residence or replacewith noncombustible, minimum 1 ft
Replace retaining wall length equal totwo times retaining wall height with
noncombustible components
Replace furniture with noncombustible
framing materials or move away 5 ft
(place cushions inside residence before
egress)Remove vegetation, detached fromresidence, and have a 2 ft open
space/separation from residence

37

Skirting

Install mobile home noncombustible
skirting on all sides

38

Crawl space access hinged door withclasp (door construction consistent
with cladding)Install crawl space access ember
intrusion protection

39

Crawl space vents

Install ember and flame-resistant vents
if venting is required

Optional Work

Performance Goal

Applicable Condition(s)Notes

Expected
Cost Range($, $$, $$$)

Limit spread of fire to house

Combustible attachment

n/a

$-$$

None

Prevent flames from retaining wall from
igniting residence

Combustible retaining wall within twotimes the retaining wall height fromthe residence

Structural requirements of the retaining
wall must be maintained (must meet code)

$$

None

Prevent flames from combustible furniturefrom igniting residence

Combustible furniture on deck

Homeowner action item, required action

$

None

Prevent ignition of pergola/trellis from
igniting residence

Attached combustible pergola/trellis

Structural integrity of pergola/trellis must
be maintained

$$

None

Mobile home without skirting or
without noncombustible skirting

n/a

$$

None

Access to crawl space

n/a

$-$$

None

New skirting with vents if needed
Non-ember- and flame-resistant vents
installed

n/a

$-$$

Chapter 7A

$-$$

None

Prevent embers from igniting underside ofmobile home and prevent accumulation of
flammable (windblown) debris
Prevent embers from igniting materials incrawl space and prevent accumulation of
flammable (windblown) debris
Minimize embers with enough energy tocause ignitions

n/aInsert metal flashing between deck
Limit ignition of deck
Combustible deck and joists
n/aboards at joistsa
If noncombustible gutter cover cannot be installed on combustible gutter, gutter must be replaced with a noncombustible material and fitted with noncombustible cover. (Item # 8)b
Vents need to account for reduction in air flow. If a vent is not fire and ember resistant it should be replaced with one that is.
c
Well-irrigated lawn may be allowed in the 0 ft to 5 ft around deck. (Item # 30)
40

Between deck boards

61

Matched in
Existing
CodeTable B. Surrounding parcel hazard mitigation – hardening structure and attached combustibles against ignition from fire (radiation, convection).
Item#

Structure Separation Distance (SSD) or
Neighboring Parcel Exposure Distance(NPED)a

Slope and location of
structure on the terrain(low, mid, high slope)

Hardening Action(on neighboring parcel)bThis publication is available free of charge from: https://doi.org/10.6028/NIST.TN.2205

Notes
Hardening Structure is required
only if neighboring structure
falls within Fuel Separation
Range
https://www.fire.ca.gov/progra
ms/communications/defensiblespace-prc-4291/
https://www.fire.ca.gov/media/
umkhhdbs/fuels-reductionguide-final-2021-print.pdf

Minimum Required
Fuel Separation
Distance (MFSD) (ft)

Fuel Separation
Range (FSR) (ft)Hardening Structure and Attached
Combustibles Against Ignition from
Flames (radiation, convection)

25 to 50Required (in Table D) if 25<SSD<50

1

Proximity of closest neighboring primaryresidence(s) – SSD

2

Proximity to untreated wildland fuels –NPED

Slope of concern is between
wildland fuel and structure (notabsolute slope of wildland fuel)3

Proximity to treated wildland fuelsc –NPEDAssumes that slope has beenfactored in the fuel treatment

4

Proximity to auxiliary buildings > 120 ft2
in size (primary or neighboring parcel) –SSD

5Proximity to auxiliary buildings 64 ft2 to
120 ft2 (primary or neighboring parcel) –SSD

6Proximity to auxiliary buildings <64 ft2 insize (primary or neighboring parcel) –SSD

7

Proximity to vegetative fuels notcompliant with defensive spaced – NPED

100

Required (in Table D) if
NPED<MFSD

8

Proximity of large auxiliary structuresand fuels (e.g., gazebo, RVs, boats) –
NPED

50

Required (in Table D) if
NPED<MFSD

9

Proximity of small combustible auxiliary
structures (e.g., single fence) – NPED

10

Required (in Table D) if
NPED<MFSD

10

Proximity of small combustible auxiliary
structures (e.g., double combustible
fences) – NPED

20

Required (in Table D) if
NPED<MFSD

11

Proximity of detached retaining wall –
NPED

10

Required (in Table D) if
NPED<MFSD

Aux. building hardening will need to
be treated as primary structure toprevent ignition from fire (radiation)and embers
Aux. building hardening will need to
be treated as primary structure toprevent ignition from fire (radiation)and embers
Aux. building hardening will need to
be treated as primary structure toprevent ignition from fire (radiation)and embers

Retaining wall hardening will need
to be treated as primary structure toprevent ignition from fire (radiation)and embersaFrom the edge of the house closest to the exposure.
If neighboring parcel is not part of the program, certain hardening actions may not be implementable and structure hardening may be necessary.
cWildland fuel treatment must have occurred within 3 years; must meet local, state, or federal guidance/standard.
dRefers to vegetative fuels on adjacent properties.
b

62100, 150, 200

Required (in Table D) if
NPED<MFSD

100

Required (in Table D) if
NPED<MFSD
25 to 50Required (in Table D) if 25<SSD<50

20 to 40

Required (in Table D) if 20<SSD<40

15 to 30

Required (in Table D) if 15<SSD<30Table C. Primary parcel hazard mitigation – hardening structure and attached combustibles against ignition from fire (radiation, convection).

Item
#
1
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2

3

Parcel Feature –Exposure Distance (ED) or
Structure Separation Distance(SSD)
Firewood – ED

Vegetative fuels not compliant
with defensible space – ED

Auxiliary buildings (> 120 ft2) –SSD

4

Small (64 ft2 to 120 ft2) auxiliarybuildings – SSD

5Very small (< 64 ft2) auxiliarybuildings – SSD

Hardening Action
Replace firewood with other heating
source, displace firewood 30 ft away frommain residence and other Table C features,or store in a noncombustible enclosure 15 ft
from Table C features
Treat vegetation
Remove or separate 50 ft from main
residence and other Table C features or
harden the auxiliary building constructionin Table C and incorporate a 0 ft to 5 ft
ember-resistant zone.
Remove, displace 40 ft from main
residence, or harden the auxiliary building
construction (between 5 ft to 40 ft) fromresidence and other items in Table C and 0
ft to 5 ft ember-resistant zone
Remove, displace 30 ft from main
residence, or harden the auxiliary building
construction (between 5 ft and 30 ft) fromresidence and other items in Table C and 0
ft to 5 ft ember-resistant zone
Remove, displace 50 ft from main
residence and other Table C features, or
harden the auxiliary building construction
to noncombustible (If < 50 ft) from otheritems in Table C and incorporate 0 ft to 5 ft
ember-resistant zone

Hardening Structure
and Attached
Combustibles
Against Ignition from
Flames (radiation,
convection)

Performance Goal

Applicable Condition(s)NotesPrevent firewood fromdirectly (flames) igniting
residence or other Table C
items

If closer than recommended
separation distance

Defensible space expanded toaccount for all other Table C items$

30

Required (in Table D)
if ED<MFSD

Defensible spacecompliance

Not in compliance with defensible
space

If vegetative fuels reduction is notpossible, and ED is less than
distance specified in Table B for
untreated wildland fuels, residence
hardening will be required

$-$$

100
(See Table B)

Required (in Table D)
if ED<MFSD

Prevent auxiliary building
from directly (flames)
igniting residence or other
Table C items

If closer than recommended
separation distanceIf building removal, displacement,or hardening of auxiliary structure
is not possible, residence
hardening will be required$$-$$$

50

Required (in Table D)
if SSD<MFSD

If closer than recommended
separation distanceIf building removal, displacement,or hardening of auxiliary structure
is not possible, structure hardening
will be required

$-$$$

40

Required (in Table D)
if ED<MFSD

If closer than recommended
separation distanceIf building removal, displacement,or hardening of auxiliary structure
is not possible, structure hardening
will be required

$-$$$

30

Required (in Table D)
if ED<MFSD$$-$$$

50

Required (in Table D)
if SSD<MFSD$

50

Required (in Table D)
if ED<MFSD$

10(See Table B)

Required (in Table D)
if ED<MFSD$

5

Required (in Table D)
if ED<MFSD

Prevent auxiliary
structures from directly(flames) igniting main
residence and other Table
C items
Prevent auxiliary
structures from directly(flames) igniting main
residence and other Table
C items

If building removal, displacement,or hardening of auxiliary structure
Prevent fuels fromIf closer than recommended
Other combustible structures
is not possible; remove or displace6
directly (flames) igniting
separation distance and non(> 120 ft2) (e.g., gazebo) – SSD
the other Table C items or replace
other (combustible) items hardenedwith a new structure made ofnoncombustible material.
Prevent vehicles fromIf vehicle removal or displacement
Remove, displace 50 ft from main structure
directly (flames) igniting
If closer than recommended(beyond 50 ft) is not possible,
Large vehicles (e.g., RVs, boats)7
and other Table C features, and create a 0 ft
main residence and otherseparation distancestructure hardening will be– ED
to 5 ft ember-resistant zone
Table C items
requiredPrevent fuels fromIf replacement with
Replace with noncombustible/ignitionFences on property (see Table B
directly (flames) igniting
If combustible and closer thannoncombustible/ignition-resistant8
resistant materials (See Table B forfor distances) – ED
main residence and otherrecommended separation distancematerials is not possible, structure
distances)
Table C items
hardening will be required
Prevent fuels fromIf replacement with
Small combustibles within 5 ft of
directly (flames) igniting
If combustible and closer thannoncombustible/ignition-resistant
Replace with noncombustible/ignition9
residence (e.g., door mat, planter,main residence and otherrecommended separation distancematerials is not possible, structure
resistant materials or move away 5 ft
garden hose) – ED
Table C items
hardening will be requireda
MFSD must account for all exposed structure combustible appendages, such as decks. For example, MFSD is measured from the edge of the deck, not the wall of the structure it is attached to.

63

Expected
Cost Range($, $$, $$$)

Minimum
Required Fuel
Separation
Distancea(MFSD) (ft)

Table D. Structure hazard mitigation – hardening structure and attached combustibles against ignition from fire (radiation, convection).
Item
#
1

Structure Component –Exposure Distance (ED) or Structure
Separation Distance (SSD)
Roof covering and roof design (assembly)
including dormer and bump out roofs

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2

Dormer side

3

Dormer under eave

4

Dormer window

Hardening Action
Replace non-Class A roofs by assemblyor by covering alone
Replace all combustible siding with
noncombustible options
Replace all under eave construction with
noncombustible options or cover with
noncombustible material
Replace with dual pane window withboth being tempered

Performance Goal

Applicable Condition(s)Notes

Expected
Cost Range($, $$, $$$)

Matched in
Existing
Code

Prevent ignition of roof from flames

Non-Class A roof

n/a

$$$

None

Prevent ignition of dormer fromflamesCombustible dormer siding

n/a

$$

None

Prevent ignition of dormer fromflamesCombustible dormer eave

n/a

$$

NFPA 1140

Prevent complete window failurea

Single pane or dual pane non-tempered

n/a

$$

NFPA 1140
ICC IWUIC

$$$

None

Exterior wall(s) including bump out(s)Add on top (of existing cladding) or
replace with noncombustible cladding.
Trim must be noncombustible

Prevent siding ignition

Combustible cladding facing exposuresin Table B and/or Table C

Bump out bottom (underside)

If exposed framing, enclose with
noncombustible material. If enclosedwith combustible material, replace or
cover with noncombustible material.
Trim must be noncombustible and extend(vertically) to account for added material

1. Add on top option is available only
for residences with cladding thathave a flat profile.
2. Add moisture barrier under new
cladding.

prevent ignition under bump out(s)Combustible bump out(s) undersidematerial or exposed framing

n/a

$$

None

7

Glazing in doors and glass sliding doors

Double pane windows (both panes
tempered)

Vinyl frames must have reinforcement
to prevent panes from being dislodged
because of frame deformation (due toheating)$$$

NFPA 1140
ICC IWUIC

8

Under eave(s) - overhanging eave andoverhanging rake (gable end)

n/a

$$

NFPA 1140
ICC IWUIC

9

Screens

n/a

$$

None

$$$

NFPA 1140
ICC IWUIC

5

6

Create a soffited eave (horizontal) or
enclose eave (angled) using
noncombustible material
Screen over entire window(s) (even if
window does not open) and other glazed
surfaces. Framing for screens must be ofnoncombustible material

Prevent complete window failureaPrevent ignition in under eave areaand entry of fire in residence
Reduce radiative exposures to glass
and possibly to parts of frameSingle pane non-tempered, non-tempered
double pane, or double pane with one
tempered facing exposures in Table B
and/or Table C
Under eave(s) combustible construction
facing exposures in Table B and/or TableCAll glazed surfaces facing exposures inTable B and/or Table C

Single pane non-tempered, non-tempered Vinyl frames must have reinforcement
double pane, or double pane with one
to prevent panes from being dislodged
10
Windows
Prevent complete window failurea
tempered facing exposures in Table B
because of frame deformation (due to
and/or Table C
heating)
Note: All sides of a structure must be hardened for fire and radiation unless a field inspection identifies more localized exposures that demonstrate directional hazard (with no fire exposure to other side(s)).
aComplete glazing failure will open up residence and allow embers and fire to enter.
Replace with dual pane window withboth being tempered

64

Appendix B. Comparison of Existing Codes and Standards
This appendix contains a tabulated list (Table E) of detailed comparisons of primarysolutions listed in three selected WUI building codes commonly used in the United States.
The comparison is made with the most recent (2018 or 2019) editions of the International
Wildland-Urban Interface Code, NFPA 1140/NFPA 1144, 13 and the California Building
Code Chapter 7A. Note that each of these listed codes are regularly revised and improved
varying temporal cycles.
This publication is available free of charge from: https://doi.org/10.6028/NIST.TN.2205

There are multiple programs available to the public that can be used to reduce fuels andharden structures in the WUI [40]. Examples include NFPA’s Firewise 14 and CAL FIRE’s
Ready, Set, Go! 15 programs which encourage defensible space and structure hardening.
Depending on jurisdiction, these types of programs may be voluntary or mandatory andtypically complement local, state, and national codes.

NFPA codes are in transition to consolidate related codes. NFPA 1144 is a component of the new consolidated WUI code, NFPA 1140.
https://www.nfpa.org/Public-Education/Fire-causes-and-risks/Wildfire/Firewise-USA
15
https://www.readyforwildfire.org/prepare-for-wildfire/ready-set-go/
13
14

65

Table E. Comparison of selected existing WUI building codes.
Building Component

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2018 IWUIC (Ignition Resistant Construction Class 1)

2018 NFPA 1144

2019 California Building Code Chapter 7A

RoofSpecifies Class A fire rated covering (IR 2 specifies Class B, IR 3
specifies Class C). Requirements for "bird-stopping" / "fire stopping",
including 72-pound cap sheet alternate method specified in Chapter 7A,
also applies here. Requirements for thickness of metal when used in avalley. Valley using metal flashing also requires use of 72-pound cap
sheet underlayment.

Roof covering assemblies tested and rated as Class A in accordance withASTM E108 (or UL 790). Roof covering tested with all assemblycomponents representing as-built condition (e.g., between panel joints
must be present, and joints shall be in alignment with burning brandduring test). When a roof profile provides for a gap between the roof
covering and a combustible roof deck assembly shall include a 72-poundcap sheet that complies with ASTM D3909 (Asphalt Roll Roofing(Glass Felt) Surfaced with Mineral Granules). The cap sheet shall berolled out over the entire roof deck. In lieu of the use of such a rollroofing product, fire-retardant treated plywood can be used. Such roof
coverings shall also be blocked at eaves, ridges, and hips with anoncombustible material. A metal drip edge shall be installed at all rake
and eave edges.

Refers to Chapter 15 of the CBC. Chapter 15 specifies Class A coveringin VHFHSZ, Class B in HFHSZ and Class C in MFHSZ. Specifies thatany gaps between roof covering and roof deck be fire-stopped (e.g., birdstopped, fire-stopped). An alternate way to comply would be to install aminimum 72-pound cap sheet material over the decking. Specifies that(metal) valley flashing will be installed over a minimum 36-inch wide
cap sheet that runs the full length of the valley.

Under-Eave

Eaves and soffits protected on exposed underside by one of four options,
including 1) use of an Ignition Resistant Material or materials approved
for a minimum one-hour fire-resistant-rated construction, 2) nominal 2inch lumber, or 1-inch nominal fire-retardant-treated lumber, or 3/4-inch
nominal fire-retardant-treated plywood [fire-retardant materials rated forexterior use]. Open-eave or boxed in "ok", but both must comply with
"underside" requirements. Fascia protected on backside by ignition
resistant material or materials approved for one-hour fire-resistancerated construction or nominal 2-inch lumber

Specifies that eaves be enclosed with either 1) exterior FRT wood orignition resistant material, or 2) a noncombustible material, or 3) amaterial that, when tested to ASTM E2957 (under eave flame exposuretest) complies with provisions provided in Section 5.3.4.1 (absence offlame penetration, absence of structural failure, absence of sustained
combustion (flame or smoldering) after 40 minutes).

Provisions are consistent with the provisions of the exterior wall
requirements. Soffited eaves must be enclosed with either 1)
noncombustible material, 2) ignition-resistant material, 3) use of 5/8inch Type X gypsum sheathing in the assembly, 4) the exterior portionof a one-hour fire resistive wall assembly, or 5) complies with SFM
Standard 12-7A-3. States that traditional open-eave framing using
nominal 2x dimension lumber is allowed (prescriptively).

Underfloor Enclosure

Where attached structure projects over a descending slope greater than
10%, the area below the structure shall be enclosed to within 6-inches of
the ground with materials that comply with exterior wall requirements.

Gutters/Downspouts

Specifies gutter made of noncombustible materials (vinyl gutters notallowed). Gutters shall be provided with an approved means to prevent
accumulation of debris (i.e., use of gutter cover device is required).

Noncombustible material. Gutters covered by approved noncombustiblemeans to minimize accumulation of debris.

Specifies that gutters be provided with the means to "prevent" theaccumulation of leaves and debris. This section has generally beeninterpreted to mean "install a gutter cover device."

Vents

Attic vent: Cannot be located in soffit, eave overhangs, between rafters,
or in other overhang areas. Gable end and dormer located minimum 10feet from property lines. Foundation vents: located as close to grade aspractical. Individual vent openings not to exceed 144 in2 and covered
with corrosion resistant mesh not to exceed 1/4-inch or designed andapproved to prevent flame and ember penetration.

Vents screened with corrosion-resistant noncombustible wire mesh,mesh opening not to exceed 1/8-inch, or a vent that, when tested inaccordance with ASTM E2886 comply with provisions found in Section
5.3.3 (2). These are the same provisions provided in Chapter 7A. This
information applies to all openings / penetrations, with the exception ofdryer vents.

General requirement for vents to "resist building ignition from theintrusion of burning embers and flame through ventilation openings."
Specific requirements: 1) Tested in accordance with ASTM E2886 andwith specified items (no flaming ignition of cotton [ember intrusion], noflaming ignition [flame intrusion], and maximum temperature on
unexposed side of vent < 350 °C [662 °F]), or mesh openings between
1/16-inch and 1/8-inch, noncombustible and corrosion resistant material.
Vents not allowed on underside of eave unless 1) listed tested to ASTM
E2886 and complying with stated provisions (above), or 2) AHJ permits
vent as being resistant to flames and embers, or 3) attic is fully
sprinklered or exterior cladding and underside of eave is noncombustible
or approved IRM, and vent is more than 12 feet from ground or walking
surface.

66Building Component

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2018 IWUIC (Ignition Resistant Construction Class 1)

2018 NFPA 1144

2019 California Building Code Chapter 7AExterior Walls

Exterior walls with one five methods, including 1) One-hour fireresistant-rated construction, 2) Approved noncombustible materials, 3)
Heavy timber or log wall construction, 4) Fire-retardant-treated wood on
exterior side (rated for exterior use), or 5) Ignition-resistant materials on
exterior side. Materials must extend from top of foundation to undersideof roof sheathing.

An approved ignition resistant material, or exterior fire-retardant- treatedwood, or noncombustible material, or be a wall assembly exhibiting aminimum 1-hour fire resistance rating (with tested to ASTM E119) andexhibiting a minimum Class B flame spread index (ASTM E84).
Provisions allow for AHJ to require enhanced protection. --- 6-inch
vertical noncombustible zone (i.e., foundation)

Objective is to resist building ignition and/or safeguard against intrusionof flames resulting from embers or short-term direct flame contact
exposure. Included in "exterior wall" is under-eave and other horizontal
projections (with exception of attached deck). Exceptions include trim
and fascia. Open eave construction also excepted as long as blocking andrafters have minimum dimension of nominal 2-inches [need to reconcile
this statement with Sections 707A.4, 707A.5, 707A.6, 707A.7, 707A.8,
707A.9]. Options for complying include: 1) noncombustible material, or2) ignition resistant material, or 3) "heavy timber" defined as smallest
minimum nominal dimension of 4-inches (glu-lam ok), or 4) log wallconstruction assembly, or 5) test to ASTM E2707 or SFM Standard 127A-1 and complying with specified provisions. Two deemed-to-comply
prescriptive options are also available, including 1) incorporation of onelayer 5/8-inch Type X gypsum board behind exterior cladding on
exterior side of framing, or 2) exterior portion of 1-hour fire resistive
exterior wall assembly. Assemblies listed in Gypsum Association Fire
Resistance Design Manual are okay.

Windows

Exterior glazing (glass in windows) shall be, 1) tempered glass,multilayered glazed panels or 2) glass block, or 3) have a 20-minute fireresistance rating. These requirements also apply to skylights.

Applies to exterior windows, windows within exterior doors andskylights. All shall be tempered glass, multilayered glazed panels, glass
block or have at a minimum a 20-minute fire-resistance rating. Windows
shall be screened using noncombustible material.

Exterior windows and exterior glazed door assemblies and skylights
shall comply with one of the following: 1) multipane glazing with aminimum of one tempered pane meeting requirements of Section 2406
Safety Glazing, 2) glass block units, 3) fire resistance rating of not lessthan 20 minutes (NFPA 257 - using vertical furnace and timetemperature curve also described in ASTM E 119) or 4) meeting
performance requirements of SFM 12-7A-2. Skylights also neednoncombustible screening with minimum 1/8-inch mesh.

Doors

Doors shall be 1) Noncombustible, 2) solid core wood (thickness of solidcore wood not less than 1 3/4-inch), or 3) have at a minimum a 20minute fire rating. Vehicle access (garage) doors are excluded.

Solid-core wood, > 1 3/4-inch thick, or be construction with
noncombustible materials, or a "fire protection" rating of no less than 20
minutes.

Exterior doors shall comply with one of the following: 1 and 2) exterior
surface shall be noncombustible or ignition-resistant material, or 3)
construction of solid core wood, stiles and rails not less than 1 3/8-inch
thick; raised panels not less than 1 ¼-inch thick except exterior perimeterof panel may taper to tongue not less than 3/8-inch thick, or 4) fire
resistance rating on 20 minutes when tested in accordance to NFPA 252,or 4 and 5) comply with performance requirements of SFM 12-7A-1 /ASTM E2707. Provision in Section 708 A.4 for minimizing intrusion ofembers via inclusion of weather stripping (complying flammability
rating V-2 or better via UL 94, Standard for Tests for Flammability of
Plastic Materials for Parts in Devices and Appliances.

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This publication is available free of charge from: https://doi.org/10.6028/NIST.TN.2205

Building Component

2018 IWUIC (Ignition Resistant Construction Class 1)

Appendages (Decks)

Unenclosed accessory structures attached to buildings with habitable
spaces and projections shall have 1) One-hour fire resistant-rated
construction, or 2) heavy timber, or either a) noncombustible material, b)
exterior rated fire-retardant treated wood, c) ignition resistant building
materials. When an attached structure (such as a deck) extends over adescending slope greater than 10%, the area below the structure must be
enclosed to within 6-inches of the ground. Enclosing materials
complying with "exterior walls" requirements.

Including all projections (exterior balconies, carports, decks, patio
covers, unenclosed roofs and floors) shall be constructed of heavy
timber, or noncombustible materials, or fire retardant treated wood,
ignition resistant materials or be an assembly with a 1-hour fire
resistance rating (ASTM E119).

This section only applies to the walking surface of the deck (porch,balcony, stairs, landings, included). Specifies that decking material
comply with one of the following: 1) Materials that are tested inaccordance with both ASTM E2632 (underdeck flame) and ASTM
E2726 (top of deck brand) and comply with provisions found in Section
709.4, or 2) qualifies as Ignition Resistant Material (extended ASTM
E84), or 3) complies with provisions of both SFM Standard 12-7A4 and
12-7A-5 (similar to 1 above), or 4) exterior fire retardant treated wood,or 5) noncombustible material, or 6 and 7) complies with provisions in
12-7A-4A (underdeck flame, minimum PHRR only), or tested to ASTM
E2632 with acceptance criteria provided in Section 709A.5), withexception that Class B flame spread allows siding that complies with 7A
to be used. Allowable PHRR is 25 kW/ft2 (~276 kW/m2).

Projections

1) Enclosed to ground ["exterior wall" requirements], or 2) "one-hour
construction, or heavy timber, or exterior rated FRT wood.

When projections are attached to an exterior wall, the construction of the
projection will be built to maintain the "fire-resistive" integrity of thevertical wall.

Provisions given for general case in Section 707A.2. Specific provisions
are included for exterior porch ceilings, floor projections, underfloor
projections, and the underside of appendages. Requirement options forthese components include 1) noncombustible material, 2) ignitionresistant material, 3) one layer of Type X gypsum sheathing appliedbehind an exterior covering, 4) the exterior portion of an approved 1hour fire resistive exterior wall assembly or 5) complying withperformance criteria of SFM 12-7A-3. When used in underfloor
projections and appendages, heavy timber columns and beams do not
require additional protection. When used in porch ceilings and floor
projections, architectural trim boards do not need to comply. The
minimum size to be considered heavy timber depends on the component.

Detached Accessory
Structures

If located less than 50 ft from primary building, exterior wall of building
to conform to "exterior walls" section. "Topography" language applies
here (i.e., if portion of structure projects over descending slope).
Enclosure not required if underside of exposed floors and exposed
structural members protected by one-hour fire rating, heavy timber, or
exterior rated FRT lumber.

Constructed according to provisions outlined in this standard, or
separated from primary building by at least 30 ft.

No requirements for buildings more than 50 ft from applicable building.
Specifies that attached accessory structures shall comply with thissection (states that structures shall be constructed of noncombustible orignition-resistant materials); detached accessory structures within 50 ftof the "applicable" building shall comply when required by the AHJ.

Unaddressed Issues that
Need Resolution

1) Fence / fencing; 2) Clarification between "projections" "detached accessory structures" and "appendages"; 3) where to include small area out-buildings, such as a tool shed, that is located close to the primary building; 4)
make sure the feature of being "attached" is addressed (structural or non-structural not so much an issue - proximity is the issue)"; 5) how much into "defensible space" issues do codes reach (e.g., providing requirements fornoncombustible retaining walls in down-slope areas) and other features that can enhance protection of the primary building of interest.

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2018 NFPA 1144

2019 California Building Code Chapter 7A