What is WUI (Wildland Urban Interface) and How It Works

Learn what the Wildland-Urban Interface (WUI) is, how it works, why it’s prone to wildfires, and how communities can reduce risks, from the team at WFCA.

WUI (Wildland Urban Interface) Explained

Wildland Urban Interface (WUI) is the area where land that has been developed by humans, meets land that is unoccupied and undeveloped.¹ It exists in all regions of the United States, in any place where humans live among or adjacent to vegetation.⁵ Expanding human development into areas of WUI has led to a rise in the risk of wildland fires, which can be challenging and complicated to fight.¹⁸ Currently there are over 60,000 communities at risk of fires in areas of WUI, and every year the amount of WUI area grows by an estimated 2,000,000 acres.¹

Different Types of WUI

There are two distinguishable types of WUI: Interface and Intermix. The difference between the two types is based on density of housing.² Housing density has a significant effect on the spread of WUI fires, with higher density housing enabling fire to spread more easily.^4,6

Interface: WUI occurs in areas where high-density development is next to undeveloped wildland. The population in these areas is greater than or equal to 250 people per square mile, and human structures are directly next to wildland. The housing density is greater than or equal to 3 structures per acre.²

Intermix: WUI exists in areas between lower-density housing and adjacent undeveloped wildland. In these regions, the population density is between 28 and 250 people per square mile. Physical structures are spread throughout an area of wildland, with a housing density of greater than or equal to 1 structure per 30 acres.²

How Fire Spreads in WUI Areas

Fires spread through WUI areas by means of radiant exposure, ember exposure, and direct flame contact. Radiant exposure can happen when flames from wildland fires are next to exposed structures, and thermal radiation increases. Direct flame contact takes place when flames from vegetation fires or other sources come into contact with adjacent structures. Ember exposures, which are the primary cause of structural loss from WUI fires, can happen in different ways. Embers can ignite structures directly, they can smolder and later transition to flames, they can enter structures through openings and ignite materials within, they may ignite vegetation, and they can cause fire to spread long after the “fire front” has moved on.3 In areas of higher housing density, a fire that has moved from wildland to a structure can then spread rapidly between buildings, creating a “domino effect.”⁶

How Residents Can Reduce WUI Fire Risks

There are many ways residents can reduce their risk of WUI fires. Homeowners should follow any local guidance or requirements to prepare their structure and surrounding property. This information may be available through a city or county fire marshal or building code office.

Utilizing defensible spaces and home hardening practices can help mitigate your risk. Defensible space is created by removing, reducing, or relocating fuels around a structure. Home hardening changes the physical structure of a home itself. The following are recommended practices to reduce risk to your home:

• Remove tree branches that are below 15 feet high or within 10 feet of any chimney or stove openings.
• Provide space around the structure that is free from vegetation, mulch, bark, or other landscaping.
• Ensure porches, decks, roofs, and gutters are free of leaves, needles, and other organic materials.
• Maintain a 30 foot distance between the structure and any fuel tanks, firewood, or heavy brush.
• Water and mow lawns regularly and dispose of cuttings properly.
• Place metal mesh screening on vents leading to attics or crawl spaces.⁷

Below are some additional resources from WFCA to help protect your home and property:

• How to Create a Firewise® Defensible Space for Your Home
• Home Hardening Tips and Best Practices
• Top 8 Fire-Resistant Building Materials for New Homes

In the event of a nearby wildfire, residents can take additional actions to prepare themselves and their home:

• Remove any flammable outdoor items such as door mats, cushions, or propane tanks.
• Prepare an emergency bag in case evacuation is needed.
• Identify at least two entrances and exits routes at their house.
• Make a plan for any pets or livestock.
• Identify two exit routes out of the neighborhood and a place to meet other family members.
• Take any steps required to receive emergency alerts or community notices.⁷

What Fire Departments Are Doing to Reduce WUI Risks

Fire departments and other fire service organizations are taking an active role in reducing the risk for WUI fires. From communicating information on state and local guidance, to offering direct resources that assist homeowners and communities in preparing for wildland risk, fire service organizations are a vital part of prevention and mitigation.

In areas more prone to WUI fires, fire departments can be the first point of contact for residents and homeowners. These departments offer a variety of helpful resources for the public, including self-assessments, inspections, state and county guidelines, and informational videos.¹⁰ Other online services provided by fire departments include tools to help prevent wildland fires, such as defensible space guides and wildfire preparedness checklists, virtual webinars, brochures, and links to other relevant resources. Community-based services can include in-person speakers, and trained community leaders to further educate people and help them prepare for wildfires.^{13, 14}

Wildland Urban Interface Map

Percentage of houses in the WUI relative to the total houses in the state.¹

Future Predictions of WUI

Among fire disasters in the U.S. in recent years, fires in WUI areas have caused the most devastation. These fires can result in substantial property damage and loss of life.¹⁵ One notable example is the 2018 Camp Fire in Paradise, California, which caused 82 fatalities and destroyed 19,558 structures.¹⁹ Globally, fires in WUI have become a greater proportion of overall fires over the last two decades. From 2005 to 2020, the percentage of fires in WUI areas compared to all fire events increased by 23%, and the amount of area burned by fires in WUI areas compared to all fires increased by 35%.¹⁷ These increases are a result of WUI area expansion. The percentage of land burned by fires in WUI areas is predicted to increase over the next 15 years, as the amount of WUI itself increases.¹⁷

On local, state, and national levels, organizations have been working to prepare for and mitigate the impact of potential wildland fires. Current studies and projects aim to examine fire dynamics, fire spread, risk assessment, and fire safety in the WUI.¹⁵ Such studies will expand the knowledge and tools available for fighting these fires. In 2024, the U.S. Fire Administration created two new tools to increase awareness of WUI areas. These tools assist local fire departments and community members in preparing for and reducing the risk of fires in WUI areas.¹⁶ The U.S. Forest Service also provides an interactive Wildland Urban Interface Map, which allows users to assess proximity to WUI for communities around the country. Research, education, and practical tools like these can assist organizations and individuals in preparing for wildland fires in the future.