Calculating Defensible Space and Ignition Resistant Construction

Follow the steps below to determine the amount of defensible space and level of ignition resistant construction. This information is also available on pages 12-15 in the Building Permit Application (PDF).

STEP 1: IDENTIFY THE FIRE HAZARD SEVERITY AT THE SITE

Start by accessing the County’s Interactive Map and loading the “Local Wildfire Hazard” layer, also see the “Landscape Wildfire Hazard” layer.  The map can be found at: https://www.gunnisoncounty.org/325/Map-Viewer

• The Local-level Wildfire Hazard shows where fires are likely to burn at high intensity, but does not indicate the likelihood of a fire occurring. This layer informs about the relative worst-case (hottest, driest, windiest days during a fire season) wildfire exposure (radiant, convective, and ember) that can be expected where development exists or is planned. 
• The Landscape-level Wildfire Hazard map shows the likelihood (probability) of a fire occurring and the intensity of the fire at the landscape level based on the inherent landscape characteristics, including broad existing vegetation, biophysical settings, fire regimes, and fire histories. Rankings at this scale are difficult to change at the local/parcel level. Mitigation affecting change at this scale is typically done by large-scale disturbances such as insect mortality, fire, or landscape-level mitigation. This layer informs on the general areas where fires are most likely to occur and where collaborative, multi-agency, large-scale fire management planning and mitigation are necessary. 

STEP 2: DETERMINE CONSTRUCTION MATERIALS

Refer to Table 503.1 - Ignition-Resistant Construction of the International Wildland Urban Interface code to determine the Ignition-Resistant Construction level necessary for the proposed development. 

Class 1 (IR1) is the highest level of protection 
Class 2 (IR2) is the next step down in the level of protection 
Class 3 (IR3) is the minimum level of protection 

If you are unsure about whether the water supply is “conforming” or “nonconforming” please contact the Building Office for assistance – 970.641.0360

For more information about classes of ignition resistant construction, refer to the “Home Hardening” tab.

STEP 3: DEVELOP THE DEFENSIBLE SPACE PLAN 

Refer to the Defensible Space Table below to determine the amount of defensible space required based on the hazard severity level. The IWUIC requirements for defensible space can be followed or the Colorado State Forest Service’s guide “The Home Ignition Zone”, can be used to create the defensible space plan. Additionally, mitigations techniques are provided in Table 4  that are based on vegetation type, slope, and potential for crown fire. 

The defensible space requirements of the IWUIC generally require that the horizontal distance between crowns of adjacent trees and crowns of trees and structures, overhead electrical facilities or unmodified fuel is not less than 10 feet.  This requirement may not apply to all trees, such as aspens. Additionally, trees within the defensible space need to be pruned to remove limbs located less than 6 feet above the ground surface. Ground cover, such as grasses and shrubs, shall not form a means of transmitting fire from the growth to any structure.  The area within 5 feet of the structure shall be a hardened zone designed to prevent flames from coming in contact with the structure. Use nonflammable, hard surface materials in this zone, such as rock, gravel, sand, cement, bare earth or stone/concrete pavers. For more information, navigate to the “Defensible Space” tab. 

The numbers shown in the Defensible Space Table correspond to the Mitigation Difficulty Classes and Descriptions (Table 4) . These categories take into account vegetation type, slope, and potential for crown fire. Click on the photo below to access the table.

STEP 4: SPECIFY MATERIALS

Make sure to clearly specify the required ignition resistant materials on an itemized list, showing how the materials meet the required ignition resistance level. Additionally, have the defensible space plan details on the site plan before submitting the application for a building permit. Please contact the Community Development Department by email or phone if you would like to schedule a preapplication or design phase discussion of the requirements and options.  

What is Wildfire Risk?

According to the Community Planning Assistance for Wildfire (CPAW) Final Recommendations for Gunnison County, CO 2019, wildfire risk can be visualized as a triangle consisting of three components:

1. Likelihood of a wildfire occurring based on topography, weather, and ignition patterns; this can also include ignition sources from hazardous land uses (e.g., sawmills or propane storage facilities);
2. Predicted intensity of a wildfire (usually measured in flame length) based on vegetation type and weather conditions;
3. Susceptibility of values (for land use planning purposes, values consist of communities, structures, and infrastructure).

To determine the fire hazard severity at a specific site, start by accessing the County’s Interactive Map and loading the “Local Wildfire Hazard” layer, also see the “Landscape Wildfire Hazard” layer.  The map site is found at the following web address: https://www.gunnisoncounty.org/325/Map-Viewer. You can also view the Colorado State Forest Service "Wildfire Risk" map to help determine the wildfire hazard at the site. 

• The Local-level Wildfire Hazard layer shows where fires are likely to burn at high intensity, but does not indicate the likelihood of a fire occurring. This layer informs about the relative worst-case (hottest, driest, windiest days during a fire season) wildfire exposure (radiant, convective, and ember) that can be expected where development exists or is planned.
• The Landscape-level Wildfire Hazard layer shows the likelihood (probability) of a fire occurring and the intensity of the fire at the landscape level based on the inherent landscape characteristics, including broad existing vegetation, biophysical settings, fire regimes, and fire histories. Rankings at this scale are difficult to change at the local/parcel level. Mitigation affecting change at this scale is typically done by large-scale disturbances such as insect mortality, fire, or landscape-level mitigation. This layer informs on the general areas where fire are most likely to occur and where collaborative, multi-agency, large-scale fire management planning and mitigation are necessary.

Additional Links: 

Reducing Wildfire Risk – Headwaters Economics

Gunnison County’s Interactive Map 

CPAW Final Recommendations for Gunnison County, CO 2019 

Structure Ignitability

Is the likelihood the materials in and on your home will ignite during a wildfire. The practice of reducing structural ignitability is commonly called “home hardening”

What Is The Home Ignition Zone?

The Home Ignition Zone (HIZ) is the home itself and its immediate surroundings, including the defensible space. The HIZ considers both the potential structure ignitability and the quality of defensible space around it. The Home Ignition Zone is a great resource from the Colorado State Forest Service (CSFS) to better understand the HIZ and how to better prepare your home for wildfire.

Check out THIS VIDEO from the Insurance Institute for Business and Home Safety about how defensible space and ignition resistant materials are important factors in protecting your home from a wildfire. Additionally, visit Headwaters Economics 'Building for Wildfire' page for a video explaining the importance of defensible space and ignition resistant materials.

Ignition Resistant Materials

CLICK HERE to download presentation

Case Studies

CLICK HERE to download presentation

As outlined in the Building Permit Application, refer to Table 503.1 – Ignition-Resistant Construction to determine minimum level of ignition-resistant construction required for each project.

Review the information sheet below titled "WUI Code Concepts For The Home" by CPAW

Additional Links

The Home Ignition Zone – Colorado State Forest Service

Hardening Your Home – Wildfire Prepared

FireWise Construction – Colorado State Forest Service

Home Survival in Wildfire-Prone Areas: Building Materials and Design Considerations

Building Wildfire Resistant Homes: Codes and Costs – Headwaters Economics

Step 1: Build A House. Step 2: Set It On Fire – NPR

FireWise Decks - FireWise

Ignition Resistant Landscaping – CSU Extension

Fire Resistant plants – Sunset Magazine

Fire Resistant Plants – Landscaping Network

What is defensible space?

As defined in the 2021 International Wildland Urban Interface Code (IWUIC), Defensible space is:

“An area either natural or man-made, where material capable of allowing a fire to spread unchecked has been treated, cleared or modified to slow the rate and intensity of an advancing wildfire and to create an area for fire suppression operations to occur.”

It goes on to say that defensible space provides two important functions:

1. The defensible space has a lower fuel content, so the fire is slowed as it approaches a structure. The defensible space itself is a level of protection to reduce the fire impact on structures.
2. The defensible space provides an area surrounding structures where fire fighters can operate to defend against an approaching wildfire.

What is the home ignition zone?

The Home Ignition Zone (HIZ) is the home itself and its immediate surroundings, including the defensible space. The HIZ considers both the potential structure ignitability and the quality of defensible space around it. The Home Ignition Zone is a great resource from the Colorado State Forest Service (CSFS) to better understand the HIZ and best prepare your home (Could move this to “home hardening” section, or duplicate it there)

Who is responsible for creating and maintaining effective defensible space?

Each and every homeowner is personally responsible for creating and maintaining effective defensible space.

Avoiding Adverse Impacts

When creating your defensible space plan, take care to avoid adverse impacts on the land. The intent when creating a defensible space plan is to minimize the hazard effectively with minimal adverse impacts on the property. Some examples of adverse impacts include: soil erosion, removing excess vegetation, overly thinning trees (i.e. clear cutting), or negatively impacting wildlife.

How to Determine Defensible Space?

As outlined in the Building Permit Application, refer to the Defensible Space Table to determine minimum distance of defensible space required for each project based on the hazard level.

The numbers shown in the Defensible Space Table correspond to the Mitigation Difficulty Classes and Descriptions (Table 4) . These categories take into account vegetation type, slope, and potential for crown fire. Click on the photo below to access the table.

Example: Defensible Space By Zones

Image above from: NFPA - Preparing homes for wildfire

Example: Tree Thinning*

**Note - Tree spacing and ladder fuel guidelines do not apply to mature stands of aspen trees.

Image above from: NFPA - Preparing homes for wildfire

Best Practices

Trim branches that overhang your home, porch, and deck. Keep plants, trees and branches at least 10 ft. from your home.

Clear leaves and branches from roofs, gutters, porches, and decks. Remove dead plants, leaves and branches at least 10 feet from your home 

Remove ladder fuels from your yard

Keep trees limbed up to 10 feet from the ground and keep 10 feet between tree branches

Additional Links

Defensible Space – Wildfire Prepared

Preparing homes for wildfire – National Fire Protection Association

The Home Ignition Zone – Colorado State Forest Service

Register

For Emergency Alerts through Gunnison County “Notify Me”

Check

Current fire restrictions and find additional resources at Gunnison County’s Emergency Management Wildfire Information Page

General

https://headwaterseconomics.org/

https://www.nfpa.org/

https://firewise2000.com/

https://csfs.colostate.edu/

https://www.fs.usda.gov/

https://cpaw.headwaterseconomics.org/

https://fireadaptedco.org/

WUI & Wildfire Risk

WUI Report Presentation – FEMA & USFA

Living with Wildfire Video – Headwaters Economics

Wildfire & Insurance handout – Colorado State Forest Service

Risk Management in the WUI – US Forest Service

Land Use Planning in the WUI – Community Wildfire Planning Center

Home Hardening & Defensible Space

How To Prepare Your Home For Wildfires - FireWise

The Home Ignition Zone – Colorado State Forest Service

Wildfire Prepared Home - Homepage - Wildfire Prepared Home - A Program of IBHS

Resources | Community Wildfire Planning Center

Defensible Space - WILDFIRE PREPARED

NFPA - Preparing homes for wildfire

Hardening Your Home – Wildfire Prepared

FireWise Construction – Colorado State Forest Service

Home Survival in Wildfire-Prone Areas: Building Materials and Design Considerations

 Building Wildfire Resistant Homes: Codes and Costs – Headwaters Economics

Step 1: Build A House. Step 2: Set It On Fire – NPR

Ignition Resistant Landscaping – CSU Extension

Fire Resistant plants – Sunset

Fire Resistant Plants – Landscaping Network

Colorado State Forest Service Materials:

List of Publications

Defensible Space

FireWise Construction

FireWise Plant Materials