WUI Structure / Parcel / Community 

Fire Hazard Mitigation Methodology (HMM)

April 5, 2022

NIST

Alexander Maranghides 

Eric D. Link

Shonali Nazare

CAL FIRE

Steven Hawks 

Jim McDougald

IBHS

Stephen L. Quarles

Daniel J. Gorham
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

• Primary Drivers influencing fire losses

• Traditional WUI definitions

• Adding Structure Separation Distance (SSD) in redefined WUI categories

• Relationship between exposure and hardening

• Fire Spread pathways

• Effects of parcel centric approach of fire losses

• Effects of fuels agglomeration on fire losses

• Fuel Loading in actual WUI settings

• Technical foundation for the HMM

• HMM ember hardening 

• HMM fire hardening including the principles of Fuels Reduction, Relocation, Removal and Structure Hardening options. 

• Fuel Spacing, Parcel layouts and Fuels Placement

• HMM Tables A, B and C

• Factors Influencing structure survivability

• Defensive actions

• Partial structure hardening

• Partial community hardening

• Housing density (i.e., SSD)

• Effects of Housing Density on Resident Participation and Complete Structure and Parcel Hardening 

• HMM comparison to WUI Codes 

Presentation Outline

2

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

• SSD refines the WUI Definitions in Context of Hazard and Hazard 

Mitigation

• Fire and Embers are different hazards with impacts over different 

spatial dimensions

• Ember hardening is “required”, fire hardening is spatially dependent –

HMM provides an effective and cost-efficient pathway to address both 

hardening needs for existing communities

• Protection provided by partial hardening of the structure and parcel is 

not linearly related to hazard mitigation

• Community implications of partial structure and parcel hardening are 

related to community housing density

Presentation Themes

3

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

WUI Fire Hazard Mitigation Components

4

HARDEN 

STRUCTURE

REDUCE 

EXPOSURE

WUI fire hazard mitigation is a balance between two input dials —

reducing exposure and increasing structure hardening

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

1. Fuel ignition potential

2. Density of vegetative and structural fuels 

3. Wind and terrain

4. Extent/size of fire front reaching the communities

Primary Drivers of WUI Fire Losses

5

Communities and residents can only control the 

density of vegetative and structural fuels 

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
WUI Definitions

6

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Traditional WUI Definition

NIST TN 2205 (HMM), Table 1. 7

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

WUI by SSD and Parcel Size

NIST TN 2205 (HMM), Table 2. 8

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

WUI Type 2 

9

High Density Interface – Interior

Coffey Park in Santa Rosa, CA

NIST TN 2205 (HMM), Figure 1.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

WUI Type 2

10

• An example of structure density 

of the Mountain Shadows 

neighborhood in Colorado 

Springs, CO 

• Insets show the highest density 

area (yellow) and associated lot 

size distribution, most <0.2 ac.

NIST TN 2205 (HMM), Figure 2.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

WUI Type 2 – SSD

11

• Varying structure separation 

distances in the Mountain 

Shadows neighborhood of 

Colorado Springs, CO 

NIST TN 2205 (HMM), Figure 3.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

WUI Type 6

12

Low Density Intermix Rancho Santa Fe, CA.

NIST TN 2205 (HMM), Figure 4.

Rancho Santa Fe, CA 

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

WUI Type 6/5

13

Some structure-to-structure distances are representative of Moderate Density 

Intermix (lower part of image). Also note limited high density non irrigated 

vegetative loading.

The Trails, Rancho Bernardo, CA 

Irrigated lawns, long paved driveways, pools

NIST TN 2205 (HMM), Figure 5.

Type 5

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

WUI Type 7

14

Concow, CA. Note the overall large SSD and 

structures located within the wildland vegetation. 

Note the imagery is taken post-fire.

NIST TN 2205 (HMM), Figure 6.

Concow, CA

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Zoom Area
WUI Fire Exposures

embers + fire

15

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

WUI Fire Hazard Mitigation Components

16

HARDEN 

STRUCTURE

REDUCE 

EXPOSURE

WUI fire hazard mitigation is a balance between two input dials —

reducing exposure and increasing structure hardening

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Fire and Ember Exposures

17

* Ignition is driven by the combination of duration and flux intensity

up to ~10,000+ ft

Ember Exposures 

Intra or Inter Community

n=1 to ~1,000,000

diameter: 0.1 to ~5+ inches

material: variable

other: accumulation, energy content

ignition conditions: variable

up to ~100 ft

Fire Exposures 

Intra or Inter Parcel

Flux: up to ~100 kW/m2

Duration: seconds to tens of minutes*

Ignition heat flux: variable, exposed combustibles: ~15 kW/m2

Source

WIND

Two Different Problems with Different Spatial Scales 

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Fire and Ember – Different Spatial 

Exposure Hazards

18

Wildlands

WUI Community

Individual

Parcels

Potential Widespread Ember Exposure

Arrival of Fire Front

Spot Ignition of Structure

Locally High Fire Exposure

New Ember Generation

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Structure Ignition Pathways and Parcel 

Level Combustible Attributes

19

Highlighted in white 

are hazards that 

NIST has performed 

extensive study.

Local conditions will drive ember exposures to specific structure 

elements. Extreme variability influence actual exposures.

Source placement (and local wind) will drive fire exposures to specific 

structure elements.

All burning hazards (sources) generate embers and fire exposures

NIST TN 2205 (HMM), Figure 8.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Structure Ignition Pathways and Parcel 

Level Combustible Attributes

20

Local conditions will drive ember exposures to specific structure 

elements. Extreme variability influence actual exposures.

Source placement (and local wind) will drive fire exposures to specific 

structure elements.

All burning hazards (sources) generate embers and fire exposures

NIST TN 2205 (HMM), Figure 8.

Photo courtesy of CAL FIRE, used with permission

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Fire Spread Pathways

structures and parcels

21

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Fire Spread within and Across Parcels

22

1.Parcel boundaries typically limit the continuity of 

protection between parcels

2.Linear features can carry fire very efficiently within 

and in between parcels

3.Fuel agglomeration has significant impact on energy 

release and fire spread

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Parcel boundaries typically limit the 

continuity of protection between parcels

23

Residential structures are similarly located in Scenario A and B, 

however the parcel division in Scenario B allows placement of 

an auxiliary structure too close to the neighboring primary 

structure when the parcels are considered independently.

NIST TN 2205 (HMM), Figure 9.

Existing codes are parcel centric and can 

offer limited protection between parcels 

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Impact of Linear Features on Fire 

Spread

24

A single ignition B on a fence line can carry fire 

to 6 lots and destroy (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 direct connection to Residence 3, the 

result is still 2 destroyed residences within the 

figure: 

Ignition B → shed → Structure 3 → Structure 4.

NIST TN 2205 (HMM), Figure 10.

Inset photographs are from field observations and experiments. 

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles 25

Introduction Impact of Linear Features on Fire Spread

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles 26

Impact of Linear Features on Fire Spread

Destroyed

Destroyed

Destroyed

B7

B1

B5 B2

B

D

structure-to-structure

ignition

NIST Structure Separation Experiment.

See NIST Tech. Note 1600.

NIST Structure Separation 

Experiment (2021).

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Impact of Fuel Agglomeration on Fire 

Spread 

27

Parallel western red cedar fences vs. Single burning fence

NIST TN 2205 (HMM), Figure 11.

Fences burning with an applied wind of 13 mi/h (6 m/s)

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Deck Deck

Combustible Fence

Lot A Lot B

Residence A

Residence B

Gazebo

Shed

A-2

Shed

A-1

Shed

B

Spatial Relationships Between Fuels on 

Two Adjacent Parcels

28

Shed A is too close to Residence B, and both sheds 

are agglomerated along the property border. 

Sheds are spaced 

appropriately from 

the residences and 

other fuels, except 

fences, on their 

respective lots. 

NIST TN 2205 (HMM), Figure 12.

The sheds and fences will substantially increase the exposures from the red 

highlighted area, igniting the structures.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

✓ Low/reduced hazard

Hazardous condition

Likely ignition condition

!

!

✓

✓

✓

✓

✓

!

!

!

!
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Deck

Gazebo

Deck

Combustible Fence

RV

Lot A Lot B

Residence A

Residence B

Non-combustible 

Fence

Fuel Agglomeration Combined with 

Linear Features

29

Fuel on Lot B is adequately spaced from Residence B, however fuel is 

agglomerated 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.

NIST TN 2205 (HMM), Figure 13.

Non-combustible 

fence attachments.

Good spacing for 

Residence B.

Non-combustible

fencing

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

✓

✓

✓

✓

!

!

!

!

✓ Low/reduced hazard

Hazardous condition

Likely ignition condition

!

!
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Fuel Loading and Spacing of Actual 

WUI Community

30

Real world fuel agglomeration 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. 

NIST TN 2205 (HMM), Figure 14.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Structure Hardening

31

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Hardening for Embers 

32

• Ember exposures are uncontrollable and 

unpredictable.

• A total of 40 separate vulnerabilities have been 

identified along with associated mitigation actions. 

All structures must be hardened for high 

ember exposures

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Hardening for Fire

33

a. Reduce the fuels

This approach reduces the number of combustibles and therefore the expected 

exposures. 

b. 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.

c. Remove the fuel

This approach removes the exposure all together and is essential when there is limited 

space for fuel displacement.

d. Harden structures for fire exposure

If the above three approaches cannot be implemented, hardening the structure for fire 

may be necessary. The hardening option increases structure survivability but may not be 

as effective as hazard removal, and therefore, is the least desirable option. Additionally, 

hardening is typically the most expensive option. 

NIST TN 2205 (HMM), Section 5.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Effectiveness of Hardening for Fire 

directly related to Structure Density

NIST TN 2205 (HMM), Table 3. 34

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

HMM Implementation

35

A. Harden for embers

B. Surrounding parcel hazard mitigation

C. Primary parcel hazard mitigation

D. Harden for identified fire exposures

Always

f (fuels, 

spacing)

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Table A. Hardening Against Ignition from Embers

NIST TN 2205 (HMM), Appendix A, Table A. 36

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Table A. Hardening Against Ignition from Embers

NIST TN 2205 (HMM), Appendix A, Table A. 37

• 40 identified vulnerabilities

• 57 options to implement

• 8 major categories

• Roof

• Cladding

• Vents

• Windows

• Doors

• Attachments

• Mobile home 

skirting/crawl spaces

• Optional 

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Table B. Surrounding Parcel – Hazard Mitigation

NIST TN 2205 (HMM), Appendix A, Table B. 38

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Table C. Primary Parcel – Hazard Mitigation

NIST TN 2205 (HMM), Appendix A, Table C. 39

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Table D. Structure – Hazard Mitigation

NIST TN 2205 (HMM), Appendix A, Table D. 40

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Scenario 2

SSD between 

25 ft and 50 ft

Residence

Aux. 

Str.

SSD

Scenario 3

SSD less 

than 25 ft

Residence

Aux. 

Str.

SSD

Scenario 1

SSD greater 

than 50 ft

Residence

Aux. 

Str.

SSD

Using HMM – Fuel Spacing (relocation) 

41

Three scenarios with a range of SSDs between the primary residence and 

auxiliary 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). 

Hardening for fire is directional and is not necessarily required around the 

entire structure. 

Hardening for embers is required in all scenarios.

NIST TN 2205 (HMM), Figure 15.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

!

! ✓

✓

✓ Low/reduced hazard

Hazardous condition

Likely ignition condition

!

!
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Neighboring

Residence

Using HMM – Fuel Spacing (relocation) 

NIST TN 2205 (HMM), Figure 15. 42

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Fuel relocation 

must consider 

neighboring parcels

Scenario 1

Residence

Aux. 

Str.

SSD

✓

SSD

Recall previous parcel-centric discussion... !

✓ Low/reduced hazard

Hazardous condition

Likely ignition condition

!

!
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Using HMM – Fuel Spacing (fixed spacing) 

43

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). 

Hardening for fire is directional and is not necessarily required around the 

entire structure. 

Hardening for embers is required in all scenarios.

NIST TN 2205 (HMM), Figure 16.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

✓

✓

✓

!

!

!

✓ Low/reduced hazard

Hazardous condition

Likely ignition condition

!

!
Multi-Parcel Layout Examples

44

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Parcel layout and fuel placement in 

different WUI Types 

NIST TN 2205 (HMM), Table 4. 45

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Parcel Layout and Fuel Placement 

in Different WUI Types –Scenario A 

46

WUI Type 2. Small parcel with small SSD provides very 

limited space for auxiliary fuels. The shed is located closer 

to the neighboring residence than the owner’s residence.

NIST TN 2205 (HMM), Figure 17.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Parcel Layout and Fuel Placement 

in Different WUI Types – Scenario B 

47

• WUI Type 4. Moderate density 

interface. In some areas there 

is limited space for auxiliary 

fuels, shown by the 

agglomeration on the property 

boundaries (red highlights). 

• The area highlighted in yellow

presents reduced impact on 

residential structures. 

• The lower figures show before 

and after fire imagery with the 

actual fire spread pathway 

highlighted (Camp Fire, NIST 

TN 2135).

NIST TN 2205 (HMM), Figure 18.

Imagery: Butte County GIS. Overlays by NIST.

Available space for 

auxiliary fuels with 

limited impact on 

residential structures 

Imagery: Google Landsat/Copernicus, Overlays NIST

Limited space for 

auxiliary fuels

Imagery: Google, Landsat/Copernicus. Overlays by NIST.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles 48

• 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.

NIST TN 2205 (HMM), Figure 19.

SSD: 31 ft

SSD: 34 ft

Available space for 

auxiliary fuels with 

limited impact on 

residential structures

Imagery: Google, Landsat/Copernicus. Overlays by NIST.

Parcel Layout and Fuel Placement 

in Different WUI Types – Scenario C 

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Parcel Layout and Fuel Placement 

in Different WUI Types – Scenario D

49

• WUI Type 6. This 

community is 

characterized by large 

parcels with more 

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 these spaces.

NIST TN 2205 (HMM), Figure 20.

Available space for auxiliary 

fuels with limited impact on 

residential structures 

Imagery: Google, Landsat/Copernicus. Overlays by NIST.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Structure Survivability

50

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Factors Influencing Structure 

Survivability

51

• Defensive actions

• Partial structure hardening

• Partial community hardening

• Housing density (i.e., SSD)

NIST TN 2205 (HMM), Section 5.4.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Defensive Actions 

52

Data from the NIST Waldo Canyon Case Study shows 39% of 

undamaged structures within the fire perimeter were defended

NIST TN 2205 (HMM), Table 5.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Defensive Actions, Defensible Space and 

Structure Response Design Threshold 

53

• Occurrence of large WUI fires, like the Tubbs and 

Camp Fires

• Numerous fire storms that have occurred in 

California since 2000

• Number of exposed structures can quickly 

outnumber statewide resources 

NIST TN 2205 (HMM), Section 5.4.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Defensive Actions, Defensible Space and 

Structure Response Design Threshold 

54

• Defensible space contributes to making structures 

defensible by first responders 

• Defensible space is not a substitute for homes 

standing completely alone throughout a large WUI 

fire event or a firestorm 

NIST TN 2205 (HMM), Section 5.4.

Structures must be designed to stand alone

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Partial Structure Hardening

55

• Partial hardening of structures has a reduced impact on structure 

survivability that is not directly proportional to the fraction of hardening 

actions implemented.

• The effectiveness of partial hardening represents how much benefit is 

provided by fractional structure hardening. 

• Implementing half of the ember hardening specified in Table A does not 

automatically translate to a 50 % increase in ignition resistance. 

Impacts of Structure Mitigation Compliance on Structure Resilience

NIST TN 2205 (HMM), Section 5.4.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Effectiveness of partial hardening is inversely proportional to local fire and ember exposures, 

incident size, and number of simultaneous incidents.

Partial Structure Hardening

56

Impacts of Structure Mitigation Compliance on Structure Resilience

• Homeowner has limited control on 

ember exposures

• Must harden to protect against embers

Availability of Defensive Actions

• Resident has limited control on fire exposures from outside their parcel

• Resident has significant control on exposures within their parcel: 

Fuels Reduction, Relocation, Removal and Structure Hardening options 

NIST TN 2205 (HMM), Section 5.4.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Partial Community Hardening

57

Impacts of Community Mitigation Compliance on Community Resilience

The impact of a partially hardened structure on the community is 

proportional to the inverse of SSD

High Structure Density (Low SSD)

One Structure Ignition Large Losses

Moderate Structure Density (Moderate SSD)

One Structure Ignition Variable Losses

Low Structure Density (High SSD)

One Structure Ignition Limited Additional Losses

NIST TN 2205 (HMM), Section 5.4.

Partially hardened structures need to be further apart 

to prevent cascading losses 

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Effect of Housing Density on 

Mitigation Approach

58

High Structure Density (Low SSD)

One Structure Ignition Large Losses

Low Structure Density (High SSD)

One Structure Ignition Limited Additional Losses

Moderate Structure Density (Moderate SSD)

One Structure Ignition Variable Losses

Resident Participation and Complete 

Structure and Parcel Hardening 

“Desired”

“Desired”

“Required”

NIST TN 2205 (HMM), Section 5.4.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Comparison of HMM to WUI Codes 

NIST TN 2205 (HMM), Table 6. 59

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Summary

60

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Summary (1 of 3)

61

• Looked at traditional WUI definitions; added Structure Separation Distance in redefined WUI categories

• Outlined the relationship between exposure and hardening

• Fire Spread pathways

• Effects of parcel centric approach 

• Effects of fuels agglomeration

• Fuel Loading in actual WUI settings

• Outline the technical foundation for the HMM

• HMM ember hardening (Table A)

• HMM fire hardening including the principles of Fuels Reduction, Relocation, Removal and Structure 

Hardening options. (Table B, C, D)

• Fuel Spacing, Parcel layouts and Fuels Placement

• Factors Influencing Structure Survivability

• Defensive actions 

• Partial structure hardening

• Partial community hardening

• Housing density (i.e., SSD)

• Effects of Housing Density on Resident Participation and Complete Structure and Parcel Hardening 

• HMM comparison to WUI Codes 

NIST TN 2205 (HMM), Section 8.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Summary (2 of 3)

62

• Field Observations, Case Studies and Laboratory and Field Research 

were used to develop the HMM.

• HMM provides a path forward to reduce structural losses from WUI 

fires. 

• HMM is designed to leverage the spatial component of exposures to 

effectively and cost efficiently mitigate hazards at the structure and 

parcel level.

• While high density communities pose a significant hazard mitigation 

challenge, HMM demonstrates why it is essential to get full resident 

buy-in and complete structure and parcel hardening to prevent 

ignitions that can cascade throughout the community with 

catastrophic results. 

NIST TN 2205 (HMM), Section 8.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Summary (3 of 3)

63

• HMM also demonstrates why the strict compliance requirements 

are not necessary and can be laxed as housing density decreases. 

• The comparison to existing codes should be treated with caution as 

the codes contain many of the components of HMM. 

• Where implemented HMM will provide a significant improvement in 

structure and community resiliency to WUI fires. The effectiveness 

of partial implementation will be a function of local exposures 

(driven by SSD and parcel fuels) and defensive actions. 

NIST TN 2205 (HMM), Section 8.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
64

Contact Information:

Alexander Maranghides

alexm@nist.gov

202-567-1634

Thank You

NIST

Alexander Maranghides 

Eric D. Link

Shonali Nazare

CAL FIRE

Steven Hawks 

Jim McDougald

IBHS

Stephen L. Quarles

Daniel J. Gorham
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Summary Of Technical Principles

65

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 attributes and specific combustion considerations can 

decrease ember generation.

3. A property owner cannot stop or realistically control ember exposures.

4. Large ember exposures can be generated and observed throughout WUI fire incidents.

5. The potential for high exposures drives the need to completely harden structures against 

ember exposures.

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.

NIST TN 2205 (HMM), Section 8.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Summary Of Technical Principles

66

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 (item burning) directly 

impacts a target, in this case a residence or commercial structure. Indirect exposures occur 

when a source ignites a secondary fuel which then impacts the target.

9. Fire exposure increases via fuels agglomeration. Increased fire exposures can then 

potentially impact the residence/commercial structure. Increased exposure also negatively 

impacts defensible space, making it more hazardous for first responders to conduct 

firefighting 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.

NIST TN 2205 (HMM), Section 8.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Summary Of Technical Principles

NIST TN 2205 (HMM), Section 8. 67

Parcel/Community

12. The parcel size and placement of structure(s) will determine 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.

15. Even if certain parcel-level combustibles attributes are seen as expendable/disposable in 

the context of parcel hardening, the impact of these attributes must be considered in the 

context 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.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Summary Of Technical Principles

68

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 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 density communities.

4. High fire exposures can readily cause direct ignition of exposed combustibles.

5. The ember hardening and structure survivability relationship in high ember exposures can be conceptualized 

using the “80/20 Rule”—80 % ember hardening will nominally provide 20 % ember ignition protection. While 

the actual values will vary based on local conditions and specifics of hardening, the general rule 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 frequently be the critical path to reducing the ignition of the residential/commercial structure.

NIST TN 2205 (HMM), Section 8.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles
Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles

Summary Of Technical Principles

69

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 low-density community.

2. Partial community compliance, in the form of incomplete structure and parcel hardening, has 

moderate impacts beyond the partially hardened properties in a moderate-density community.

3. Partial community compliance, in the form of incomplete structure and parcel hardening, has

very significant impacts across the entire community in high- density communities. Just a few 

partially hardened properties can jeopardize an entire high-density community.

NIST TN 2205 (HMM), Section 8.

Introduction

WUI Definitions

WUI Fire 

Exposures

Fire Spread 

Pathways

Structure 

Hardening

Multi-Parcel 

Layout Examples

Structure 

Survivability

Summary

Listing of Key 

Technical 

Principles