Wildfire season now reaches places that never used to burn, and building codes are catching up. The wildland-urban interface, where homes meet forest and grassland, is the fastest-growing land use type in the contiguous United States, and states are responding with stricter construction rules. Those rules make homes more survivable, but they also restrict materials, raise costs, and generate sharp disagreement about what belongs on the outside of a house. The debate now playing out in Colorado, where a new wildfire resiliency code bans natural wood siding and decking, shows builders what happens when fire policy meets material science. Pushback is a normal part of code adoption; the pattern of utility pushback over rooftop solar systems shaped that technology’s code path, and builders who understand the process can shape the outcome.
What the Wildland-Urban Interface Means for Builders
WUI stands for wildland-urban interface: the zone where structures and human development meet or intermingle with undeveloped wildlands and vegetative fuels. The definition matters because fire behavior in the WUI differs from fire behavior in open country or city blocks. In the WUI, the fuel is often the landscaping and the structure itself rather than the forest alone, and windblown embers can travel a mile ahead of the main flame front to ignite a deck or an eave.
Population growth is pushing development deeper into these zones. New housing in mountain valleys, foothills, and prairie edges puts more structures in the path of seasonal fires, and the WUI is now the fastest-growing land use type in the lower 48 states. Builders in these areas are learning that the code book has changed: modern residential codes bundle seismic, flood, wildfire, and energy requirements into a single approval process, and wildfire provisions are no longer optional extras.
The Fastest-Growing Land Use Type
- Development at the urban fringe grows faster than suburban infill.
- Fire risk rises with drought cycles and longer fire seasons.
- Local governments respond with zoning, defensible space rules, and building codes.
For builders, the takeaway is that wildfire-resilient construction is becoming a baseline requirement rather than a premium option, and planning for it early beats retrofitting later.
How Colorado’s 2025 Wildfire Resiliency Code Changed the Rules
Colorado adopted the 2025 Colorado Wildfire Resiliency Code through the state legislature, and the code’s most controversial provision bans natural wood products for exterior uses such as siding and decking. That single rule eliminates long-standing options: cedar and redwood, both of which carry a natural Class B flame spread rating, would no longer be allowed on new construction in covered areas.
Colorado’s building community has a strong record of pushing for better performance, and groups such as the Colorado Green Building Guild track new projects and policy through events like the Colorado on the Rise showcase. The same community now finds itself debating a code that trades material choice for fire performance.
What the Code Restricts
- Natural wood siding on exterior walls.
- Wood decking on balconies, porches, and ground-level decks.
- Some wood trim and fascia applications in high-risk zones.
Supporters argue the restrictions cut ignition risk. Critics respond that the code goes further than the fire science requires, and that blanket bans ignore the performance differences between species and treatments.
The Housing Cost Question
Cost is the sharpest edge of the debate. Replacing cedar and redwood with fire-rated alternatives raises material prices on every exterior, and in a state already short on affordable housing, higher construction costs translate into fewer units built. The industry’s position is that the code would lead to higher housing costs, fewer material options, and a deeper housing shortage, all without a measurable safety gain over Class B materials used correctly.
Flame Spread Ratings and Natural Wood Exteriors
The technical core of the dispute is the flame spread rating, a number that measures how fast a flame travels across a material’s surface under standardized test conditions. The rating determines which class a material falls into, and building codes use those classes to decide what can go on the outside of a house. Cedar and redwood naturally rate as Class B, which places them in the middle of the scale: safer than untreated commodity lumber and allowed for exterior use in many fire hazard zones.
Effective wildfire mitigation and building practices recognize that no single material guarantees survival; the envelope, the roof, the deck, and the defensible space around the house all matter together.
| Class | Flame spread index | Common materials | Typical exterior use |
|---|---|---|---|
| Class A | 0 to 25 | Fiber cement, metal, brick, stucco | Siding and roofs in high-risk zones |
| Class B | 26 to 75 | Cedar, redwood, some treated woods | Siding and decking in moderate zones |
| Class C | 76 to 200 | Some plywoods and untreated lumber | Limited exterior use |
Reading a Flame Spread Rating
The index comes from standardized panel testing, and lower numbers mean slower flame travel. A Class B material is not fireproof, but it is measurably better than commodity lumber, which is why the California approach allows it in specific zones rather than banning it outright. Builders who understand the scale can have a productive conversation with code officials; those who do not end up arguing opinions instead of data.
California Chapter 7A: A Two-Decade Test Case
California faced the same material question in 2008 and answered it with California Building Code Chapter 7A, Materials and Construction Methods for Exterior Wildfire Exposure. Chapter 7A approves redwood siding and decking for use in California’s WUI fire hazard severity zones, and nearly two decades of enforcement have demonstrated that the provisions reduce wildfire losses in new construction.
The California experience has become the reference point for fire-resilient material specifications; when Los Angeles rebuilt after its 2025 fires, the material selection process leaned on the same testing and classification framework.
What Chapter 7A Allows
- Approved natural wood products in designated fire hazard severity zones.
- Prescriptive assemblies for walls, roofs, eaves, and decks.
- Clear paths for alternative materials that meet the same test standards.
Chapter 7A has proven effective enough that California folded its provisions into the new 2025 California Wildland-Urban Interface Code. The wood products industry argues that Colorado could adopt the same proven framework instead of a blanket ban.
Why the Precedent Matters
The California record is the strongest evidence in the debate because it is real operating history rather than theory. Two decades of new construction, fire seasons, and insurance claims produced the data, and the industry’s recommendation to Colorado is to incorporate the same successful WUI provisions rather than write a stricter, untested rule.
How Stakeholder Input Shapes Code Development
Codes are written by boards and ratified by legislatures, but the process depends on public comment from the people who build, insure, and live in the buildings. In Colorado, the wood products industry submitted technical comments through the American Wood Council, and stakeholders have questioned whether the Wildfire Resiliency Code Board adequately incorporated those comments during development. The letters recommending California’s provisions went unanswered before finalization, which is why the future of natural wood decking and siding in Colorado remains uncertain.
The code debate also continues long after a fire. One year after the L.A. fires, construction strategies for building professionals still center on material choice, defensible space, and insurance requirements, the same issues Colorado is wrestling with now.
The Public Comment Process
- Draft codes are published with a comment window before board adoption.
- Technical groups submit data-backed recommendations and letters.
- Boards hold hearings where stakeholders can testify.
- Legislatures ratify or amend the final document.
- Building departments implement and enforce the adopted text.
The process only works when stakeholders participate at every step. After adoption, the remaining lever is the legislature that wrote the enabling law, which is why industry groups are urging builders, designers, and homeowners in Colorado to contact their state senators, representatives, and local building department officials.
What Builders and Homeowners Can Do Now
Until Colorado’s board responds, builders in covered areas have to plan around the code as written. That means pricing fire-rated alternatives, checking the current adopted text before specifying materials, and documenting compliance choices early in design. It also means participating in the political process, because a code written without material options is harder to fix after it is in force.
The practical side of wildfire work is logistics as much as materials. Crews in burn zones and remote WUI parcels depend on vehicles that can handle rough terrain, and factory-engineered off-road performance in trucks such as the Chevrolet Colorado ZR2 Bison gives construction pros a dependable way to reach sites that standard pickups cannot.
Practical Steps Before the Next Fire Season
- Verify which code version applies to the project jurisdiction.
- Price at least two material systems for every exterior assembly.
- Design defensible space into the lot plan from the start.
- Document material certifications for the building official.
- Track code amendments and comment windows for future cycles.
Wildfire codes will keep evolving, and the builders who follow the science, the process, and the market will be the ones who can still build when the next season arrives.