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Engineering Laboratory / Fire Research Division

Wildland-Urban Interface Fire Group

WUI Definitions

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Traditional WUI Definitions

The common conceptual definition of wildland-urban interface (WUI) is the geographical area where human development, including structures and other infrastructure, meets or intermixes with undeveloped wildlands. 

The WUI is often further distinguished into two WUI types based on housing densities:

  • Interface: High-density development adjacent to undeveloped wildland vegetation.
  • Intermix: Lower-density housing mingled with undeveloped wildland vegetation.

While these definitions can be useful to identify general areas, they do not focus on the parcel-level details that characterize parcel-to-parcel and structure-to-structure fire spread that is characteristic of many WUI fire disasters.
 

Category

Definition Component

Federal Register

Common Implementation

CAL FIRE

Interface

Conceptual

There is a clear line of demarcation between residential, business, and public structures and wildland fuels; wildland fuels do not generally continue into the developed area

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

High-density development adjacent to undeveloped wildland vegetation

Housing density

≥ 3 structure/ac (741 structure/km2)

≥ 1 HU/40 ac (6.18 HU/km2)

>1 HU/20 ac (12.4 HU/km2) in Moderate, High, or Very High FHSZ

Population density

≥ 250 people/mi2 (96 people/km2)

 

 

Vegetation cover

Structures directly abut wildland fuels

< 50 %

Not dominated by wildland vegetation

Buffer from wildland

Up to 1.5 mi (2.4 km) from community border

< 1.5 mi (2.4 km) from land with > 75 % vegetative cover

Wildfire susceptible vegetation up to 1.5 mi (2.4 km) from interface

Infrastructure

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

 

 

Intermix

Conceptual

There is no clear line of demarcation; wildland fuels are continuous outside of and within the developed area

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

Lower-density housing mingled with undeveloped wildland vegetation

Housing density

≥ 1 structure/40 ac (6.18 structure/km2)

≥ 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) when dominated by wildland vegetation, in Moderate, High, or Very High FHSZ

Population density

(28 to 250) people/mi2 [(11 to 96) people/km2]

 

 

Vegetation cover

Structures are scattered throughout a wildland area

> 50 %

Dominated by wildland vegetation

Buffer from wildland

 

 

Wildfire susceptible vegetation up to 1.5 mi from intermix

Infrastructure

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

 

 

Note: HU = housing units

 

WUI Definition by SSD and Parcel Size

More specific WUI definitions are established in the HMM by explicitly adding the concept of structure separation distance (SSD). These definitions provide perspective and can help develop effective parcel-level mitigation strategies based on housing density and potential fire exposures.

 

WUI Type 1 and Type 2:

High-density interface communities with an SSD as small as 6 ft and parcel size of less than 0.5 ac. Type 1 is characterized by Perimeter and Type 2 as Interior WUI community. Significant portions of communities can get consumed in fire even with small number of ignitions in such settings.
 

WUI Type 3, Type 4, and Type 5:

Medium-density construction with SSD ranging from 30 ft to 100 ft with parcel size in excess of 1 ac. Type 3 and Type 4 are characterized as interface and Type 5 as intermix communities.

WUI Type 6 and Type 7:

Low structure density at the interface (Type 6) and intermix (Type 7).

alt text here
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.
alt text here
Scenario B, 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 blue presents reduced impact on residential structures. The lower figures show before and after fire imagery with the actual fire spread pathway highlighted.
alt text here
WUI Type 6. Low Density Intermix, The Trails, Rancho Bernardo, CA. Note irrigated 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.

 

WUI Types classified by structure separation distance (SSD) and typical parcel size.
 

Type #

WUI Type Name

SSD (ft)

Typical Parcel Size (ac)

Typical Housing Density (struct/ac)

1

High Density Interface – Perimeter

6a to 30

< 0.5

2 to 8 +

2

High Density Interface – Interior b

6a to 30

< 0.5

2 to 8 +

3

Medium Density Interface – Perimeter

30 to 100

0.5 to 1+

< 2

4

Medium Density Interface – Interior b

30 to 100

0.5 to 1+

< 2

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 ha
a representative of parcels with a 3 ft setback (common for new construction of sprinklered residences)
b interior of community defined as > 0.25 mi (400 m) from wildlands

 

Created July 6, 2023, Updated August 25, 2023