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How Density Impacts Community
The effectiveness of protecting a home from direct flame exposures
is dependent on the density of structures in the community.
The effectiveness of structure hardening is dependent on the potential fire exposure from neighboring structures, wildlands, and other fire sources. Fire hardening can be an effective mitigation method in certain spatial situations where structures are located far enough apart that exposures are moderated and fuel removal or displacement is not possible. Protecting a home from direct flame exposures can be difficult and expensive, and is likely ineffective for the highest exposures, such as from a burning neighboring home.
- High-density communities - must not have any structures ignite as risk of entire community loss is very high due to structure-to-structure fire spread.
- Medium-density communities - greatest impact on structure ignition resistance by hardening for fire exposures from neighboring structures.
- Low-density communities - lower potential losses from individual structure ignitions, low potential for urban conflagration.
The table below outlines the probability of structure survival for potential exposures (fire and ember) for various WUI types. Hover your mouse over each area of the table for specific details.
Click here to view the table as an image.
Structure and parcel hardening effectiveness
# |
WUI Type Density |
WUI Type |
Probability of Structure Survivability if Neighboring Structure Ignites |
Potential Firea Exposure from Burning Neighboring Structure |
Exposure from Other Parcels |
Exposureb from Wildlands |
Impact of Structure Ignition on Fire Spread in Community |
Likely Effectiveness of Partial Structure/Parcel Hardening |
Community/ Neighborhood Participation |
|---|---|---|---|---|---|---|---|---|---|
|
1 |
High |
Interface – Perimeter |
f (fuels, di= st.)c |
Variable |
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|
2 |
High |
Interface – Interior |
f (fuels, dist.)c |
Low |
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|
3 |
Medium |
Interface – Perimeter |
f (fuels, dist.)e |
Variable |
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|
4 |
Medium |
Interface – Interior |
f (fuels, dist.)e |
Low |
|||||
|
5 |
Medium |
Intermix |
f (fuels, dist.)e |
Variable |
|||||
|
6 |
Low |
Interface |
f (fuels, dist.)e |
Variable |
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|
7 |
Low |
Intermix |
f (fuels, dist.)e |
Variable |
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| HD = high density, MD = medium density, LD = low density f (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) a flames and radiation b based on fire history, fuel loading, wind, and topography/aspect; wildland fuel treatments may not be at the control of the community c parcel-level mitigation will have limited impact if nearby upwind structures catch on fire d would be a function of wildland fuel treatment AND hardening of most/all perimeter structures and parcels e parcel-level mitigation, including wildland fuel treatment, together with home hardening, will enhance structure ignition resistance f ignitions due to embers from burning residential structures have been observed as far as 200 ft to 300 ft downwind |
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